diff --git a/app/Actions/CreateDefaultCategories.php b/app/Actions/CreateDefaultCategories.php
index c1dff91a..eead2ea5 100644
--- a/app/Actions/CreateDefaultCategories.php
+++ b/app/Actions/CreateDefaultCategories.php
@@ -5,6 +5,7 @@ namespace App\Actions;
 use App\Enums\CategoryCashflowDirection;
 use App\Enums\CategoryType;
 use App\Models\Category;
+use App\Models\Space;
 use App\Models\User;
 use Illuminate\Support\Arr;
 use Illuminate\Support\Str;
@@ -13,14 +14,16 @@ class CreateDefaultCategories
 {
     /**
      * Create default categories for a newly registered user, nesting child
-     * categories under their configured parent.
+     * categories under their configured parent. Categories live in a space, so
+     * seeding targets the given space (defaulting to the user's active one).
      */
-    public function handle(User $user): void
+    public function handle(User $user, ?Space $space = null): void
     {
+        $space ??= $user->activeSpace();
         $locale = $user->locale ?? app()->getLocale();
         $defaultCategories = self::getDefaultCategories($locale);
 
-        $existingCategories = $user->categories()
+        $existingCategories = $space->categories()
             ->whereIn('name', array_column($defaultCategories, 'name'))
             ->pluck('id', 'name');
 
@@ -32,6 +35,7 @@ class CreateDefaultCategories
                 'cashflow_direction' => $category['cashflow_direction'] ?? CategoryCashflowDirection::Hidden->value,
                 'id' => (string) Str::uuid(),
                 'user_id' => $user->id,
+                'space_id' => $space->id,
                 'created_at' => $now,
                 'updated_at' => $now,
             ]);
diff --git a/app/Console/Commands/BackfillSpaces.php b/app/Console/Commands/BackfillSpaces.php
new file mode 100644
index 00000000..8392f40c
--- /dev/null
+++ b/app/Console/Commands/BackfillSpaces.php
@@ -0,0 +1,82 @@
+<?php
+
+namespace App\Console\Commands;
+
+use App\Models\Account;
+use App\Models\AutomationRule;
+use App\Models\BankingConnection;
+use App\Models\Budget;
+use App\Models\Category;
+use App\Models\Label;
+use App\Models\SavedFilter;
+use App\Models\Space;
+use App\Models\Transaction;
+use App\Models\User;
+use Illuminate\Console\Command;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Support\Facades\DB;
+
+class BackfillSpaces extends Command
+{
+    protected $signature = 'spaces:backfill {--chunk=500 : Users processed per batch}';
+
+    protected $description = 'Provision a personal space for every user and stamp their existing rows with it';
+
+    /**
+     * Owned tables to backfill. Each row inherits the personal space of its
+     * user_id. Idempotent: only rows with a null space_id are touched.
+     *
+     * @var list<class-string<Model>>
+     */
+    private array $models = [
+        Account::class,
+        BankingConnection::class,
+        Transaction::class,
+        Category::class,
+        Label::class,
+        Budget::class,
+        AutomationRule::class,
+        SavedFilter::class,
+    ];
+
+    public function handle(): int
+    {
+        $chunk = (int) $this->option('chunk');
+
+        // Soft-deleted users are included: their accounts/transactions still
+        // exist and must be stamped too, so a restored account keeps its data
+        // and the column can eventually go NOT NULL.
+        $this->info('Provisioning personal spaces…');
+        User::withTrashed()->whereNull('current_space_id')->chunkById($chunk, function ($users): void {
+            foreach ($users as $user) {
+                $user->provisionPersonalSpace();
+            }
+        });
+
+        $this->info('Backfilling space_id on owned rows…');
+        Space::query()->where('personal', true)->chunkById($chunk, function ($spaces): void {
+            foreach ($spaces as $space) {
+                foreach ($this->models as $model) {
+                    $this->stamp($model, $space->owner_id, $space->id);
+                }
+            }
+        });
+
+        $this->info('Done.');
+
+        return self::SUCCESS;
+    }
+
+    /**
+     * @param  class-string<Model>  $model
+     */
+    private function stamp(string $model, string $ownerId, string $spaceId): void
+    {
+        // Go through the query builder rather than Eloquent so soft-deleted rows
+        // are stamped too (no soft-delete global scope to work around).
+        DB::table((new $model)->getTable())
+            ->whereNull('space_id')
+            ->where('user_id', $ownerId)
+            ->update(['space_id' => $spaceId]);
+    }
+}
diff --git a/app/Models/Account.php b/app/Models/Account.php
index fbc7536f..3d96c7ca 100644
--- a/app/Models/Account.php
+++ b/app/Models/Account.php
@@ -3,6 +3,7 @@
 namespace App\Models;
 
 use App\Enums\AccountType;
+use App\Models\Concerns\BelongsToSpace;
 use Database\Factories\AccountFactory;
 use Illuminate\Database\Eloquent\Casts\Attribute;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
@@ -19,10 +20,11 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class Account extends Model
 {
     /** @use HasFactory<AccountFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'name',
         'name_iv',
         'bank_id',
@@ -40,6 +42,7 @@ class Account extends Model
     /** @var list<string> */
     protected $hidden = [
         'user_id',
+        'space_id',
         'bank_id',
         'iban',
         'position',
diff --git a/app/Models/AutomationRule.php b/app/Models/AutomationRule.php
index c9abc248..08153311 100644
--- a/app/Models/AutomationRule.php
+++ b/app/Models/AutomationRule.php
@@ -3,6 +3,7 @@
 namespace App\Models;
 
 use App\Enums\RuleOrigin;
+use App\Models\Concerns\BelongsToSpace;
 use Database\Factories\AutomationRuleFactory;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
@@ -19,10 +20,11 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class AutomationRule extends Model
 {
     /** @use HasFactory<AutomationRuleFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'title',
         'priority',
         'origin',
@@ -32,6 +34,11 @@ class AutomationRule extends Model
         'action_note_iv',
     ];
 
+    /** @var list<string> */
+    protected $hidden = [
+        'space_id',
+    ];
+
     protected function casts(): array
     {
         return [
diff --git a/app/Models/BankingConnection.php b/app/Models/BankingConnection.php
index d390cd8b..eea236cb 100644
--- a/app/Models/BankingConnection.php
+++ b/app/Models/BankingConnection.php
@@ -4,6 +4,7 @@ namespace App\Models;
 
 use App\Enums\BankingConnectionStatus;
 use App\Enums\BankingProvider;
+use App\Models\Concerns\BelongsToSpace;
 use Carbon\Carbon;
 use Database\Factories\BankingConnectionFactory;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
@@ -28,10 +29,11 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class BankingConnection extends Model
 {
     /** @use HasFactory<BankingConnectionFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'provider',
         'authorization_id',
         'state_token',
@@ -52,6 +54,7 @@ class BankingConnection extends Model
     ];
 
     protected $hidden = [
+        'space_id',
         'api_token',
         'api_secret',
         'pending_accounts_data',
diff --git a/app/Models/Budget.php b/app/Models/Budget.php
index 13f07b7e..cad0a537 100644
--- a/app/Models/Budget.php
+++ b/app/Models/Budget.php
@@ -4,6 +4,7 @@ namespace App\Models;
 
 use App\Enums\BudgetPeriodType;
 use App\Enums\RolloverType;
+use App\Models\Concerns\BelongsToSpace;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
@@ -18,10 +19,11 @@ use Illuminate\Database\Eloquent\SoftDeletes;
  */
 class Budget extends Model
 {
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'name',
         'period_type',
         'period_start_day',
@@ -32,6 +34,7 @@ class Budget extends Model
     /** @var list<string> */
     protected $hidden = [
         'period_duration',
+        'space_id',
     ];
 
     protected function casts(): array
diff --git a/app/Models/Category.php b/app/Models/Category.php
index d735df4b..7326d3c6 100644
--- a/app/Models/Category.php
+++ b/app/Models/Category.php
@@ -4,6 +4,7 @@ namespace App\Models;
 
 use App\Enums\CategoryCashflowDirection;
 use App\Enums\CategoryType;
+use App\Models\Concerns\BelongsToSpace;
 use Database\Factories\CategoryFactory;
 use Illuminate\Database\Eloquent\Builder;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
@@ -26,7 +27,7 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class Category extends Model
 {
     /** @use HasFactory<CategoryFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     /**
      * Maximum allowed nesting depth (a root counts as level 1).
@@ -40,12 +41,14 @@ class Category extends Model
         'type',
         'cashflow_direction',
         'user_id',
+        'space_id',
         'parent_id',
     ];
 
     /** @var list<string> */
     protected $hidden = [
         'user_id',
+        'space_id',
         'created_at',
         'updated_at',
         'deleted_at',
diff --git a/app/Models/Concerns/BelongsToSpace.php b/app/Models/Concerns/BelongsToSpace.php
new file mode 100644
index 00000000..2345453f
--- /dev/null
+++ b/app/Models/Concerns/BelongsToSpace.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace App\Models\Concerns;
+
+use App\Models\Space;
+use App\Models\User;
+use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * Marks a model as belonging to a Space (the tenant). On create, `space_id` is
+ * filled from the row's own `user_id` (its owner's current space) unless it was
+ * set explicitly — so factories and existing single-owner creation paths keep
+ * working untouched, while multi-space callers pass `space_id` themselves.
+ *
+ * @property ?string $space_id
+ */
+trait BelongsToSpace
+{
+    public static function bootBelongsToSpace(): void
+    {
+        static::creating(function ($model): void {
+            if ($model->space_id === null) {
+                $model->space_id = $model->resolveDefaultSpaceId();
+            }
+        });
+    }
+
+    /** @return BelongsTo<Space, $this> */
+    public function space(): BelongsTo
+    {
+        return $this->belongsTo(Space::class);
+    }
+
+    /**
+     * Restrict a query to a single space.
+     *
+     * @param  Builder<static>  $query
+     * @return Builder<static>
+     */
+    public function scopeForSpace(Builder $query, Space|string $space): Builder
+    {
+        return $query->where($this->getTable().'.space_id', $space instanceof Space ? $space->id : $space);
+    }
+
+    /**
+     * The space a new row defaults to when none was set explicitly: the current
+     * space of the user that owns the row. Models with a stronger anchor (e.g. a
+     * transaction inheriting its account's space) override this.
+     */
+    protected function resolveDefaultSpaceId(): ?string
+    {
+        return $this->spaceIdFromUser();
+    }
+
+    /**
+     * The current space of the user that owns this row, if any.
+     */
+    protected function spaceIdFromUser(): ?string
+    {
+        $userId = $this->getAttribute('user_id');
+
+        if ($userId === null) {
+            return null;
+        }
+
+        return User::query()->whereKey($userId)->value('current_space_id');
+    }
+}
diff --git a/app/Models/Label.php b/app/Models/Label.php
index da3f425c..92282596 100644
--- a/app/Models/Label.php
+++ b/app/Models/Label.php
@@ -2,6 +2,7 @@
 
 namespace App\Models;
 
+use App\Models\Concerns\BelongsToSpace;
 use Database\Factories\LabelFactory;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
@@ -13,12 +14,13 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class Label extends Model
 {
     /** @use HasFactory<LabelFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     protected $fillable = [
         'name',
         'color',
         'user_id',
+        'space_id',
     ];
 
     /**
@@ -29,6 +31,7 @@ class Label extends Model
      */
     protected $hidden = [
         'pivot',
+        'space_id',
     ];
 
     /** @return BelongsTo<User, $this> */
diff --git a/app/Models/SavedFilter.php b/app/Models/SavedFilter.php
index 261dae15..9719e9db 100644
--- a/app/Models/SavedFilter.php
+++ b/app/Models/SavedFilter.php
@@ -3,6 +3,7 @@
 namespace App\Models;
 
 use App\Enums\AnalysisMode;
+use App\Models\Concerns\BelongsToSpace;
 use Database\Factories\SavedFilterFactory;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
@@ -12,10 +13,11 @@ use Illuminate\Database\Eloquent\Relations\BelongsTo;
 class SavedFilter extends Model
 {
     /** @use HasFactory<SavedFilterFactory> */
-    use HasFactory, HasUuids;
+    use BelongsToSpace, HasFactory, HasUuids;
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'name',
         'filters',
         'analysis_days',
@@ -25,6 +27,7 @@ class SavedFilter extends Model
     /** @var list<string> */
     protected $hidden = [
         'user_id',
+        'space_id',
         'created_at',
         'updated_at',
     ];
diff --git a/app/Models/Space.php b/app/Models/Space.php
new file mode 100644
index 00000000..f2533874
--- /dev/null
+++ b/app/Models/Space.php
@@ -0,0 +1,134 @@
+<?php
+
+namespace App\Models;
+
+use Database\Factories\SpaceFactory;
+use Illuminate\Database\Eloquent\Concerns\HasUuids;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+use Illuminate\Database\Eloquent\Relations\HasMany;
+
+/**
+ * @property string $id
+ * @property string $owner_id
+ * @property string $name
+ * @property bool $personal
+ */
+class Space extends Model
+{
+    /** @use HasFactory<SpaceFactory> */
+    use HasFactory, HasUuids;
+
+    protected $fillable = [
+        'owner_id',
+        'name',
+        'personal',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'personal' => 'boolean',
+        ];
+    }
+
+    /** @return BelongsTo<User, $this> */
+    public function owner(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'owner_id');
+    }
+
+    /**
+     * Members invited into the space (excludes the owner, who is implicit).
+     *
+     * @return BelongsToMany<User, $this>
+     */
+    public function members(): BelongsToMany
+    {
+        return $this->belongsToMany(User::class, 'space_user')
+            ->withPivot('role')
+            ->withTimestamps();
+    }
+
+    /** @return HasMany<SpaceInvitation, $this> */
+    public function invitations(): HasMany
+    {
+        return $this->hasMany(SpaceInvitation::class);
+    }
+
+    /** @return HasMany<Account, $this> */
+    public function accounts(): HasMany
+    {
+        return $this->hasMany(Account::class);
+    }
+
+    /** @return HasMany<BankingConnection, $this> */
+    public function bankingConnections(): HasMany
+    {
+        return $this->hasMany(BankingConnection::class);
+    }
+
+    /** @return HasMany<Transaction, $this> */
+    public function transactions(): HasMany
+    {
+        return $this->hasMany(Transaction::class);
+    }
+
+    /** @return HasMany<Category, $this> */
+    public function categories(): HasMany
+    {
+        return $this->hasMany(Category::class);
+    }
+
+    /** @return HasMany<Label, $this> */
+    public function labels(): HasMany
+    {
+        return $this->hasMany(Label::class);
+    }
+
+    /** @return HasMany<Budget, $this> */
+    public function budgets(): HasMany
+    {
+        return $this->hasMany(Budget::class);
+    }
+
+    /** @return HasMany<AutomationRule, $this> */
+    public function automationRules(): HasMany
+    {
+        return $this->hasMany(AutomationRule::class);
+    }
+
+    /** @return HasMany<SavedFilter, $this> */
+    public function savedFilters(): HasMany
+    {
+        return $this->hasMany(SavedFilter::class);
+    }
+
+    /**
+     * Whether the given user owns or is a member of this space. The owner check
+     * short-circuits without a query for the common (personal-space) case.
+     */
+    public function hasMember(User $user): bool
+    {
+        if ($this->owner_id === $user->id) {
+            return true;
+        }
+
+        return $this->members()->whereKey($user->id)->exists();
+    }
+
+    /**
+     * The role the user holds in this space: owner, the pivot role, or null when
+     * they have no access.
+     */
+    public function roleFor(User $user): ?string
+    {
+        if ($this->owner_id === $user->id) {
+            return 'owner';
+        }
+
+        return $this->members()->whereKey($user->id)->value('role');
+    }
+}
diff --git a/app/Models/SpaceInvitation.php b/app/Models/SpaceInvitation.php
new file mode 100644
index 00000000..71b41027
--- /dev/null
+++ b/app/Models/SpaceInvitation.php
@@ -0,0 +1,63 @@
+<?php
+
+namespace App\Models;
+
+use Carbon\Carbon;
+use Illuminate\Database\Eloquent\Concerns\HasUuids;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * @property string $id
+ * @property string $space_id
+ * @property ?string $invited_by_id
+ * @property string $email
+ * @property string $role
+ * @property string $token
+ * @property ?Carbon $expires_at
+ * @property ?Carbon $accepted_at
+ */
+class SpaceInvitation extends Model
+{
+    use HasUuids;
+
+    protected $fillable = [
+        'space_id',
+        'invited_by_id',
+        'email',
+        'role',
+        'token',
+        'expires_at',
+        'accepted_at',
+    ];
+
+    protected function casts(): array
+    {
+        return [
+            'expires_at' => 'datetime',
+            'accepted_at' => 'datetime',
+        ];
+    }
+
+    /** @return BelongsTo<Space, $this> */
+    public function space(): BelongsTo
+    {
+        return $this->belongsTo(Space::class);
+    }
+
+    /** @return BelongsTo<User, $this> */
+    public function invitedBy(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'invited_by_id');
+    }
+
+    public function isExpired(): bool
+    {
+        return $this->expires_at !== null && $this->expires_at->isPast();
+    }
+
+    public function isAccepted(): bool
+    {
+        return $this->accepted_at !== null;
+    }
+}
diff --git a/app/Models/Transaction.php b/app/Models/Transaction.php
index f937ebb1..fb9dccdd 100644
--- a/app/Models/Transaction.php
+++ b/app/Models/Transaction.php
@@ -9,6 +9,7 @@ use App\Enums\TransactionSource;
 use App\Events\TransactionCreated;
 use App\Events\TransactionDeleted;
 use App\Events\TransactionUpdated;
+use App\Models\Concerns\BelongsToSpace;
 use App\Services\CategoryTree;
 use Carbon\Carbon;
 use Database\Factories\TransactionFactory;
@@ -35,7 +36,7 @@ use Illuminate\Database\Eloquent\SoftDeletes;
 class Transaction extends Model
 {
     /** @use HasFactory<TransactionFactory> */
-    use HasFactory, HasUuids, SoftDeletes;
+    use BelongsToSpace, HasFactory, HasUuids, SoftDeletes;
 
     /** @var array<string, class-string> */
     protected $dispatchesEvents = [
@@ -46,6 +47,7 @@ class Transaction extends Model
 
     protected $fillable = [
         'user_id',
+        'space_id',
         'account_id',
         'category_id',
         'category_source',
@@ -77,6 +79,7 @@ class Transaction extends Model
      * @var list<string>
      */
     protected $hidden = [
+        'space_id',
         'original_description',
         'external_transaction_id',
         'dedup_fingerprint',
@@ -111,6 +114,26 @@ class Transaction extends Model
         return $this->belongsTo(Account::class);
     }
 
+    /**
+     * A transaction always lives in its account's space (the account is the
+     * tenant anchor), so bank-sync inserts land in the right space regardless of
+     * whichever space the syncing user is currently viewing.
+     */
+    protected function resolveDefaultSpaceId(): ?string
+    {
+        $accountId = $this->getAttribute('account_id');
+
+        if ($accountId !== null) {
+            $spaceId = Account::query()->whereKey($accountId)->value('space_id');
+
+            if ($spaceId !== null) {
+                return $spaceId;
+            }
+        }
+
+        return $this->spaceIdFromUser();
+    }
+
     /** @return BelongsTo<Category, $this> */
     public function category(): BelongsTo
     {
diff --git a/app/Models/User.php b/app/Models/User.php
index e128f2af..27d4a66b 100644
--- a/app/Models/User.php
+++ b/app/Models/User.php
@@ -10,8 +10,11 @@ use Database\Factories\UserFactory;
 use Illuminate\Contracts\Auth\MustVerifyEmail;
 use Illuminate\Contracts\Translation\HasLocalePreference;
 use Illuminate\Database\Eloquent\Builder;
+use Illuminate\Database\Eloquent\Collection;
 use Illuminate\Database\Eloquent\Concerns\HasUuids;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
 use Illuminate\Database\Eloquent\Relations\HasMany;
 use Illuminate\Database\Eloquent\Relations\HasOne;
 use Illuminate\Database\Eloquent\SoftDeletes;
@@ -50,6 +53,7 @@ class User extends Authenticatable implements HasLocalePreference, MustVerifyEma
         'currency_code',
         'locale',
         'timezone',
+        'current_space_id',
     ];
 
     /**
@@ -89,6 +93,18 @@ class User extends Authenticatable implements HasLocalePreference, MustVerifyEma
         ];
     }
 
+    /**
+     * Memoized active space for the current request lifecycle.
+     */
+    protected ?Space $resolvedActiveSpace = null;
+
+    protected static function booted(): void
+    {
+        static::created(function (User $user): void {
+            $user->provisionPersonalSpace();
+        });
+    }
+
     public function isOnboarded(): bool
     {
         return $this->onboarded_at !== null;
@@ -139,6 +155,97 @@ class User extends Authenticatable implements HasLocalePreference, MustVerifyEma
             });
     }
 
+    /** @return BelongsTo<Space, $this> */
+    public function currentSpace(): BelongsTo
+    {
+        return $this->belongsTo(Space::class, 'current_space_id');
+    }
+
+    /** @return HasMany<Space, $this> */
+    public function ownedSpaces(): HasMany
+    {
+        return $this->hasMany(Space::class, 'owner_id');
+    }
+
+    /**
+     * The one personal space every user owns (created on registration).
+     *
+     * @return HasOne<Space, $this>
+     */
+    public function personalSpace(): HasOne
+    {
+        return $this->hasOne(Space::class, 'owner_id')->where('personal', true);
+    }
+
+    /**
+     * Spaces the user was invited into (excludes the ones they own).
+     *
+     * @return BelongsToMany<Space, $this>
+     */
+    public function memberSpaces(): BelongsToMany
+    {
+        return $this->belongsToMany(Space::class, 'space_user')
+            ->withPivot('role')
+            ->withTimestamps();
+    }
+
+    /**
+     * Every space the user can access: the ones they own plus the ones they were
+     * invited into, ordered with the personal space first.
+     *
+     * @return Collection<int, Space>
+     */
+    public function accessibleSpaces(): Collection
+    {
+        return Space::query()
+            ->where('owner_id', $this->id)
+            ->orWhereHas('members', fn (Builder $query) => $query->whereKey($this->id))
+            ->orderByDesc('personal')
+            ->orderBy('name')
+            ->get();
+    }
+
+    /**
+     * Idempotently ensure the user has a personal space and points at it.
+     */
+    public function provisionPersonalSpace(): Space
+    {
+        $space = $this->ownedSpaces()->firstOrCreate(
+            ['personal' => true],
+            ['name' => 'Personal'],
+        );
+
+        if ($this->current_space_id === null) {
+            $this->forceFill(['current_space_id' => $space->id])->saveQuietly();
+        }
+
+        return $space;
+    }
+
+    /**
+     * The space the user is currently working in. Falls back to (and repairs
+     * towards) the personal space when the pointer is missing or points at a
+     * space the user can no longer access — e.g. after a membership is revoked or
+     * a Business subscription lapses.
+     */
+    public function activeSpace(): Space
+    {
+        if ($this->resolvedActiveSpace !== null) {
+            return $this->resolvedActiveSpace;
+        }
+
+        $space = $this->current_space_id !== null
+            ? Space::query()->find($this->current_space_id)
+            : null;
+
+        if ($space === null || ! $space->hasMember($this)) {
+            $space = $this->provisionPersonalSpace();
+            $this->forceFill(['current_space_id' => $space->id])->saveQuietly();
+        }
+
+        return $this->resolvedActiveSpace = $space;
+    }
+
     /** @return HasMany<AutomationRule, $this> */
     public function automationRules(): HasMany
     {
diff --git a/app/Services/Banking/TransactionSyncService.php b/app/Services/Banking/TransactionSyncService.php
index ef05684f..6a7c85f6 100644
--- a/app/Services/Banking/TransactionSyncService.php
+++ b/app/Services/Banking/TransactionSyncService.php
@@ -181,6 +181,7 @@ class TransactionSyncService
         try {
             $account->transactions()->create([
                 'user_id' => $account->user_id,
+                'space_id' => $account->space_id,
                 'description' => $formatted['description'],
                 'description_iv' => null,
                 'original_description' => $formatted['original_description'],
diff --git a/app/Services/Banking/WiseTransactionSyncService.php b/app/Services/Banking/WiseTransactionSyncService.php
index a04c449d..a280ccc3 100644
--- a/app/Services/Banking/WiseTransactionSyncService.php
+++ b/app/Services/Banking/WiseTransactionSyncService.php
@@ -166,6 +166,7 @@ class WiseTransactionSyncService
         try {
             $account->transactions()->create([
                 'user_id' => $account->user_id,
+                'space_id' => $account->space_id,
                 'description' => $parsed['description'],
                 'description_iv' => null,
                 'original_description' => $parsed['description'],
diff --git a/database/factories/SpaceFactory.php b/database/factories/SpaceFactory.php
new file mode 100644
index 00000000..b48b372e
--- /dev/null
+++ b/database/factories/SpaceFactory.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Database\Factories;
+
+use App\Models\Space;
+use App\Models\User;
+use Illuminate\Database\Eloquent\Factories\Factory;
+
+/**
+ * @extends Factory<Space>
+ */
+class SpaceFactory extends Factory
+{
+    /**
+     * @return array<string, mixed>
+     */
+    public function definition(): array
+    {
+        return [
+            'owner_id' => User::factory(),
+            'name' => fake()->company(),
+            'personal' => false,
+        ];
+    }
+
+    public function personal(): static
+    {
+        return $this->state(fn (array $attributes): array => [
+            'personal' => true,
+            'name' => 'Personal',
+        ]);
+    }
+}
diff --git a/database/migrations/2026_07_06_120000_create_spaces_table.php b/database/migrations/2026_07_06_120000_create_spaces_table.php
new file mode 100644
index 00000000..32694dc6
--- /dev/null
+++ b/database/migrations/2026_07_06_120000_create_spaces_table.php
@@ -0,0 +1,26 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('spaces', function (Blueprint $table) {
+            $table->uuid('id')->primary();
+            $table->foreignUuid('owner_id')->constrained('users')->cascadeOnDelete();
+            $table->string('name');
+            $table->boolean('personal')->default(false);
+            $table->timestamps();
+
+            $table->index('owner_id');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('spaces');
+    }
+};
diff --git a/database/migrations/2026_07_06_120001_create_space_user_table.php b/database/migrations/2026_07_06_120001_create_space_user_table.php
new file mode 100644
index 00000000..1dc3907d
--- /dev/null
+++ b/database/migrations/2026_07_06_120001_create_space_user_table.php
@@ -0,0 +1,26 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('space_user', function (Blueprint $table) {
+            $table->uuid('id')->primary();
+            $table->foreignUuid('space_id')->constrained('spaces')->cascadeOnDelete();
+            $table->foreignUuid('user_id')->constrained('users')->cascadeOnDelete();
+            $table->string('role')->default('member');
+            $table->timestamps();
+
+            $table->unique(['space_id', 'user_id']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('space_user');
+    }
+};
diff --git a/database/migrations/2026_07_06_120002_create_space_invitations_table.php b/database/migrations/2026_07_06_120002_create_space_invitations_table.php
new file mode 100644
index 00000000..108cf32f
--- /dev/null
+++ b/database/migrations/2026_07_06_120002_create_space_invitations_table.php
@@ -0,0 +1,30 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::create('space_invitations', function (Blueprint $table) {
+            $table->uuid('id')->primary();
+            $table->foreignUuid('space_id')->constrained('spaces')->cascadeOnDelete();
+            $table->foreignUuid('invited_by_id')->nullable()->constrained('users')->nullOnDelete();
+            $table->string('email');
+            $table->string('role')->default('member');
+            $table->string('token', 64)->unique();
+            $table->timestamp('expires_at')->nullable();
+            $table->timestamp('accepted_at')->nullable();
+            $table->timestamps();
+
+            $table->index(['space_id', 'email']);
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('space_invitations');
+    }
+};
diff --git a/database/migrations/2026_07_06_120003_add_current_space_id_to_users_table.php b/database/migrations/2026_07_06_120003_add_current_space_id_to_users_table.php
new file mode 100644
index 00000000..1e1d01f6
--- /dev/null
+++ b/database/migrations/2026_07_06_120003_add_current_space_id_to_users_table.php
@@ -0,0 +1,22 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            $table->foreignUuid('current_space_id')->nullable()->constrained('spaces')->nullOnDelete();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('users', function (Blueprint $table) {
+            $table->dropConstrainedForeignId('current_space_id');
+        });
+    }
+};
diff --git a/database/migrations/2026_07_06_120004_add_space_id_to_owned_tables.php b/database/migrations/2026_07_06_120004_add_space_id_to_owned_tables.php
new file mode 100644
index 00000000..e1367c46
--- /dev/null
+++ b/database/migrations/2026_07_06_120004_add_space_id_to_owned_tables.php
@@ -0,0 +1,49 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Tables whose rows belong to a space (the tenant). We add a plain indexed
+     * column rather than a foreign key: adding an FK to large tables such as
+     * `transactions` triggers a validating table scan/lock on Postgres, which is
+     * exactly what a phased, zero-downtime backfill needs to avoid. Referential
+     * integrity for space deletion is enforced in application code instead.
+     *
+     * ponytail: indexed column, no FK. Add the FK + NOT NULL once prod has been
+     * backfilled and space deletion reassigns rows (see spaces:backfill command).
+     *
+     * @var list<string>
+     */
+    private array $tables = [
+        'accounts',
+        'banking_connections',
+        'transactions',
+        'categories',
+        'labels',
+        'budgets',
+        'automation_rules',
+        'saved_filters',
+    ];
+
+    public function up(): void
+    {
+        foreach ($this->tables as $table) {
+            Schema::table($table, function (Blueprint $blueprint) {
+                $blueprint->uuid('space_id')->nullable()->index();
+            });
+        }
+    }
+
+    public function down(): void
+    {
+        foreach ($this->tables as $table) {
+            Schema::table($table, function (Blueprint $blueprint) {
+                $blueprint->dropColumn('space_id');
+            });
+        }
+    }
+};
diff --git a/database/migrations/2026_07_06_120005_backfill_spaces.php b/database/migrations/2026_07_06_120005_backfill_spaces.php
new file mode 100644
index 00000000..a1333236
--- /dev/null
+++ b/database/migrations/2026_07_06_120005_backfill_spaces.php
@@ -0,0 +1,24 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\Artisan;
+
+return new class extends Migration
+{
+    /**
+     * Stamp every existing user and their rows with a personal space before the
+     * space-scoped reads go live in the same release, so no data disappears in
+     * the window between adding the columns and switching the queries. Reuses the
+     * idempotent, chunked `spaces:backfill` command (a no-op on a fresh install).
+     */
+    public function up(): void
+    {
+        Artisan::call('spaces:backfill');
+    }
+
+    public function down(): void
+    {
+        // ponytail: irreversible — spaces stay; dropping the columns (previous
+        // migration) is what unwinds this.
+    }
+};
diff --git a/tests/Feature/Spaces/SpaceFoundationTest.php b/tests/Feature/Spaces/SpaceFoundationTest.php
new file mode 100644
index 00000000..c06f5b6a
--- /dev/null
+++ b/tests/Feature/Spaces/SpaceFoundationTest.php
@@ -0,0 +1,92 @@
+<?php
+
+use App\Actions\CreateDefaultCategories;
+use App\Models\Account;
+use App\Models\Category;
+use App\Models\Space;
+use App\Models\Transaction;
+use App\Models\User;
+
+it('provisions a personal space when a user is created', function () {
+    $user = User::factory()->create();
+
+    expect($user->personalSpace)->not->toBeNull()
+        ->and($user->personalSpace->personal)->toBeTrue()
+        ->and($user->current_space_id)->toBe($user->personalSpace->id)
+        ->and($user->activeSpace()->id)->toBe($user->personalSpace->id);
+});
+
+it('stamps owned rows with the owner\'s current space by default', function () {
+    $user = User::factory()->create();
+
+    $account = Account::factory()->for($user)->create();
+
+    expect($account->space_id)->toBe($user->current_space_id);
+});
+
+it('inherits a transaction\'s space from its account, not the acting user', function () {
+    $owner = User::factory()->create();
+    $account = Account::factory()->for($owner)->create();
+
+    // A transaction created with a different acting user still lands in the
+    // account's space (the account is the tenant anchor).
+    $other = User::factory()->create();
+    $transaction = Transaction::factory()->for($account)->create(['user_id' => $other->id]);
+
+    expect($transaction->space_id)->toBe($account->space_id);
+});
+
+it('seeds default categories into the given space', function () {
+    $user = User::factory()->create();
+
+    app(CreateDefaultCategories::class)->handle($user);
+
+    expect(Category::where('space_id', $user->current_space_id)->count())->toBeGreaterThan(0)
+        ->and(Category::whereNull('space_id')->count())->toBe(0);
+});
+
+it('resolves active space back to personal when the pointer is stale', function () {
+    $user = User::factory()->create();
+    $foreign = Space::factory()->create();
+
+    // Point the user at a space they cannot access.
+    $user->forceFill(['current_space_id' => $foreign->id])->saveQuietly();
+    $user->refresh();
+
+    expect($user->activeSpace()->personal)->toBeTrue()
+        ->and($user->fresh()->current_space_id)->toBe($user->personalSpace->id);
+});
+
+it('backfills legacy rows that predate spaces', function () {
+    $user = User::factory()->create();
+    $account = Account::factory()->for($user)->create();
+
+    // Simulate a pre-migration state: no space, dangling pointers.
+    Account::query()->where('id', $account->id)->update(['space_id' => null]);
+    User::query()->where('id', $user->id)->update(['current_space_id' => null]);
+    Space::query()->where('owner_id', $user->id)->delete();
+
+    $this->artisan('spaces:backfill')->assertSuccessful();
+
+    $user->refresh();
+    expect($user->current_space_id)->not->toBeNull()
+        ->and($account->fresh()->space_id)->toBe($user->current_space_id);
+});
+
+it('backfills soft-deleted users and their rows', function () {
+    $user = User::factory()->create();
+    $account = Account::factory()->for($user)->create();
+
+    // Simulate a pre-migration, soft-deleted user with dangling data.
+    Account::query()->where('id', $account->id)->update(['space_id' => null]);
+    User::query()->where('id', $user->id)->update(['current_space_id' => null]);
+    Space::query()->where('owner_id', $user->id)->delete();
+    $user->delete();
+
+    $this->artisan('spaces:backfill')->assertSuccessful();
+
+    $trashed = User::withTrashed()->find($user->id);
+    expect($trashed->trashed())->toBeTrue()
+        ->and($trashed->current_space_id)->not->toBeNull()
+        ->and($account->fresh()->space_id)->toBe($trashed->current_space_id);
+});