diff --git a/src/Foundation/InstalledSite.php b/src/Foundation/InstalledSite.php
index c811e32cf..3aa8ca415 100644
--- a/src/Foundation/InstalledSite.php
+++ b/src/Foundation/InstalledSite.php
@@ -69,6 +69,11 @@ class InstalledSite implements SiteInterface
      */
     protected $config;
 
+    /**
+     * @var \Flarum\Extend\ExtenderInterface[]
+     */
+    protected $extenders = [];
+
     public function __construct($basePath, $publicPath, array $config)
     {
         $this->basePath = $basePath;
@@ -100,6 +105,17 @@ class InstalledSite implements SiteInterface
         return $this;
     }
 
+    /**
+     * @param \Flarum\Extend\ExtenderInterface[] $extenders
+     * @return InstalledSite
+     */
+    public function extendWith(array $extenders): self
+    {
+        $this->extenders = $extenders;
+
+        return $this;
+    }
+
     protected function bootLaravel(): Application
     {
         $laravel = new Application($this->basePath, $this->publicPath);
@@ -156,6 +172,10 @@ class InstalledSite implements SiteInterface
 
         $laravel->boot();
 
+        foreach ($this->extenders as $extension) {
+            $extension($laravel);
+        }
+
         return $laravel;
     }
 
diff --git a/src/Foundation/Site.php b/src/Foundation/Site.php
index dc58cb593..00de21ae1 100644
--- a/src/Foundation/Site.php
+++ b/src/Foundation/Site.php
@@ -35,11 +35,11 @@ class Site
         date_default_timezone_set('UTC');
 
         if (static::hasConfigFile($paths['base'])) {
-            return new InstalledSite(
+            return (new InstalledSite(
                 $paths['base'],
                 $paths['public'],
                 static::loadConfig($paths['base'])
-            );
+            ))->extendWith(static::loadExtenders($paths['base']));
         } else {
             return new UninstalledSite($paths['base'], $paths['public']);
         }
@@ -60,4 +60,15 @@ class Site
 
         return $config;
     }
+
+    private static function loadExtenders($basePath): array
+    {
+        $extenderFile = "$basePath/extend.php";
+
+        if (!file_exists($extenderFile)) {
+            return [];
+        }
+
+        return array_flatten(require $extenderFile);
+    }
 }