diff --git a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
index 0f1f25bb91..739d617741 100644
--- a/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
+++ b/core/lib/Drupal/Core/Installer/Form/SiteSettingsForm.php
@@ -67,6 +67,8 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
     $settings_file = './' . $this->sitePath . '/settings.php';
 
     $form['#title'] = $this->t('Database configuration');
@@ -155,6 +157,9 @@ public function buildForm(array $form, FormStateInterface $form_state) {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
     $driver = $form_state->getValue('driver');
     $database = $form_state->getValue($driver);
     $drivers = drupal_get_database_types();
@@ -236,6 +241,9 @@ public static function getDatabaseErrorsTemplate(array $errors) {
   public function submitForm(array &$form, FormStateInterface $form_state) {
     global $install_state;
 
+    // Make sure the install API is available.
+    include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
     // Update global settings array and save.
     $settings = [];
     $database = $form_state->get('database');
diff --git a/core/modules/system/tests/modules/install_form_test/install_form_test.info.yml b/core/modules/system/tests/modules/install_form_test/install_form_test.info.yml
new file mode 100644
index 0000000000..fb0457d495
--- /dev/null
+++ b/core/modules/system/tests/modules/install_form_test/install_form_test.info.yml
@@ -0,0 +1,5 @@
+name: Install Form Test
+type: module
+description: Test the SiteSettingsForm can be extended.
+package: Testing
+version: VERSION
diff --git a/core/modules/system/tests/modules/install_form_test/install_form_test.routing.yml b/core/modules/system/tests/modules/install_form_test/install_form_test.routing.yml
new file mode 100644
index 0000000000..9d5ec44479
--- /dev/null
+++ b/core/modules/system/tests/modules/install_form_test/install_form_test.routing.yml
@@ -0,0 +1,7 @@
+install_form_test.test-form:
+  path: /test-form
+  defaults:
+    _form: \Drupal\install_form_test\Form\ExtendedForm
+    _title: Extended Site Settings Form
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/install_form_test/src/Form/ExtendedForm.php b/core/modules/system/tests/modules/install_form_test/src/Form/ExtendedForm.php
new file mode 100644
index 0000000000..613519ead3
--- /dev/null
+++ b/core/modules/system/tests/modules/install_form_test/src/Form/ExtendedForm.php
@@ -0,0 +1,11 @@
+<?php
+
+namespace Drupal\install_form_test\Form;
+
+use Drupal\Core\Installer\Form\SiteSettingsForm;
+
+/**
+ * Extends the site setting form.
+ */
+class ExtendedForm extends SiteSettingsForm {
+}
diff --git a/core/tests/Drupal/FunctionalTests/Installer/SiteSettingsFormTest.php b/core/tests/Drupal/FunctionalTests/Installer/SiteSettingsFormTest.php
new file mode 100644
index 0000000000..58fd60ab81
--- /dev/null
+++ b/core/tests/Drupal/FunctionalTests/Installer/SiteSettingsFormTest.php
@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\FunctionalTests\Installer;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the extension of the site settings form.
+ *
+ * @group Installer
+ */
+class SiteSettingsFormTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['install_form_test'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Confirms that the form is extensible.
+   */
+  public function testSiteSettingsForm() {
+    // Test that the form page can be loaded without errors.
+    $this->drupalGet('test-form');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
+}