diff --git a/core/lib/Drupal/Core/Updater/Module.php b/core/lib/Drupal/Core/Updater/Module.php
index 06e1c1e..6c1c50a 100644
--- a/core/lib/Drupal/Core/Updater/Module.php
+++ b/core/lib/Drupal/Core/Updater/Module.php
@@ -20,7 +20,7 @@ class Module extends Updater implements UpdaterInterface {
    *
    * If the module is already installed, drupal_get_path() will return
    * a valid path and we should install it there (although we need to use an
-   * absolute path, so we prepend DRUPAL_ROOT). If we're installing a new
+   * absolute path, so we prepend the root path). If we're installing a new
    * module, we always want it to go into /modules, since that's
    * where all the documentation recommends users install their modules, and
    * there's no way that can conflict on a multi-site installation, since
@@ -35,9 +35,16 @@ public function getInstallDirectory() {
       $relative_path = dirname($relative_path);
     }
     else {
-      $relative_path = 'modules';
+      $relative_path = $this->getRootDirectoryRelativePath();
     }
-    return DRUPAL_ROOT . '/' . $relative_path;
+    return $this->root . '/' . $relative_path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getRootDirectoryRelativePath() {
+    return 'modules';
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Updater/Theme.php b/core/lib/Drupal/Core/Updater/Theme.php
index b379325..03a03d9 100644
--- a/core/lib/Drupal/Core/Updater/Theme.php
+++ b/core/lib/Drupal/Core/Updater/Theme.php
@@ -20,7 +20,7 @@ class Theme extends Updater implements UpdaterInterface {
    *
    * If the theme is already installed, drupal_get_path() will return
    * a valid path and we should install it there (although we need to use an
-   * absolute path, so we prepend DRUPAL_ROOT). If we're installing a new
+   * absolute path, so we prepend the root path). If we're installing a new
    * theme, we always want it to go into /themes, since that's
    * where all the documentation recommends users install their themes, and
    * there's no way that can conflict on a multi-site installation, since
@@ -35,9 +35,16 @@ public function getInstallDirectory() {
       $relative_path = dirname($relative_path);
     }
     else {
-      $relative_path = 'themes';
+      $relative_path = $this->getRootDirectoryRelativePath();
     }
-    return DRUPAL_ROOT . '/' . $relative_path;
+    return $this->root . '/' . $relative_path;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getRootDirectoryRelativePath() {
+    return 'themes';
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Updater/Updater.php b/core/lib/Drupal/Core/Updater/Updater.php
index af14805..5e5fff9 100644
--- a/core/lib/Drupal/Core/Updater/Updater.php
+++ b/core/lib/Drupal/Core/Updater/Updater.php
@@ -24,13 +24,25 @@ class Updater {
   public $source;
 
   /**
+   * The root directory under which new projects will be copied.
+   *
+   * @var string
+   */
+  protected $root;
+
+  /**
    * Constructs a new updater.
    *
    * @param string $source
    *   Directory to install from.
+   * @param string $root
+   *   The root directory under which the project will be copied to if it's a
+   *   new project. Usually this is the app root (the directory in which the
+   *   Drupal site is installed).
    */
-  public function __construct($source) {
+  public function __construct($source, $root) {
     $this->source = $source;
+    $this->root = $root;
     $this->name = self::getProjectName($source);
     $this->title = self::getProjectTitle($source);
   }
@@ -43,20 +55,24 @@ public function __construct($source) {
    *
    * @param string $source
    *   Directory of a Drupal project.
+   * @param string $root
+   *   The root directory under which the project will be copied to if it's a
+   *   new project. Usually this is the app root (the directory in which the
+   *   Drupal site is installed).
    *
    * @return \Drupal\Core\Updater\Updater
    *   A new Drupal\Core\Updater\Updater object.
    *
    * @throws \Drupal\Core\Updater\UpdaterException
    */
-  public static function factory($source) {
+  public static function factory($source, $root) {
     if (is_dir($source)) {
       $updater = self::getUpdaterFromDirectory($source);
     }
     else {
       throw new UpdaterException(t('Unable to determine the type of the source directory.'));
     }
-    return new $updater($source);
+    return new $updater($source, $root);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Updater/UpdaterInterface.php b/core/lib/Drupal/Core/Updater/UpdaterInterface.php
index 50a1409..438c3a4 100644
--- a/core/lib/Drupal/Core/Updater/UpdaterInterface.php
+++ b/core/lib/Drupal/Core/Updater/UpdaterInterface.php
@@ -35,7 +35,7 @@ public function isInstalled();
   public static function getProjectName($directory);
 
   /**
-   * Returns the path to the default install location.
+   * Returns the path to the default install location for the current project.
    *
    * @return string
    *   An absolute path to the default install location.
@@ -43,6 +43,14 @@ public static function getProjectName($directory);
   public function getInstallDirectory();
 
   /**
+   * Returns the name of the root directory under which projects will be copied.
+   *
+   * @return string
+   *   A relative path to the root directory.
+   */
+  public static function getRootDirectoryRelativePath();
+
+  /**
    * Determines if the Updater can handle the project provided in $directory.
    *
    * @todo Provide something more rational here, like a project spec file.
diff --git a/core/modules/update/src/Form/UpdateManagerInstall.php b/core/modules/update/src/Form/UpdateManagerInstall.php
index b3e2deb..750e1c6 100644
--- a/core/modules/update/src/Form/UpdateManagerInstall.php
+++ b/core/modules/update/src/Form/UpdateManagerInstall.php
@@ -27,7 +27,7 @@ class UpdateManagerInstall extends FormBase {
   protected $moduleHandler;
 
   /**
-   * The app root.
+   * The root location under which installed projects will be saved.
    *
    * @var string
    */
@@ -37,7 +37,7 @@ class UpdateManagerInstall extends FormBase {
    * Constructs a new UpdateManagerInstall.
    *
    * @param string $root
-   *   The app root.
+   *   The root location under which installed projects will be saved.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The module handler.
    */
@@ -58,7 +58,7 @@ public function getFormID() {
    */
   public static function create(ContainerInterface $container) {
     return new static(
-      $container->get('app.root'),
+      $container->get('update.root'),
       $container->get('module_handler')
     );
   }
@@ -180,7 +180,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
 
     $project_location = $directory . '/' . $project;
     try {
-      $updater = Updater::factory($project_location);
+      $updater = Updater::factory($project_location, $this->root);
     }
     catch (\Exception $e) {
       drupal_set_message($e->getMessage(), 'error');
diff --git a/core/modules/update/src/Form/UpdateReady.php b/core/modules/update/src/Form/UpdateReady.php
index 8e106fe..729265c 100644
--- a/core/modules/update/src/Form/UpdateReady.php
+++ b/core/modules/update/src/Form/UpdateReady.php
@@ -21,7 +21,7 @@
 class UpdateReady extends FormBase {
 
   /**
-   * The app root.
+   * The root location under which updated projects will be saved.
    *
    * @var string
    */
@@ -45,7 +45,7 @@ class UpdateReady extends FormBase {
    * Constructs a new UpdateReady object.
    *
    * @param string $root
-   *   The app root.
+   *   The root location under which updated projects will be saved.
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   The object that manages enabled modules in a Drupal installation.
    * @param \Drupal\Core\State\StateInterface $state
@@ -69,7 +69,7 @@ public function getFormID() {
    */
   public static function create(ContainerInterface $container) {
     return new static(
-      $container->get('app.root'),
+      $container->get('update.root'),
       $container->get('module_handler'),
       $container->get('state')
     );
@@ -128,7 +128,7 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       $project_real_location = NULL;
       foreach ($projects as $project => $url) {
         $project_location = $directory . '/' . $project;
-        $updater = Updater::factory($project_location);
+        $updater = Updater::factory($project_location, $this->root);
         $project_real_location = drupal_realpath($project_location);
         $updates[] = array(
           'project' => $project,
diff --git a/core/modules/update/src/Tests/UpdateTestBase.php b/core/modules/update/src/Tests/UpdateTestBase.php
index 3d36aac..3306182 100644
--- a/core/modules/update/src/Tests/UpdateTestBase.php
+++ b/core/modules/update/src/Tests/UpdateTestBase.php
@@ -21,6 +21,7 @@
 
 namespace Drupal\update\Tests;
 
+use Drupal\Core\DrupalKernel;
 use Drupal\Core\Url;
 use Drupal\simpletest\WebTestBase;
 
@@ -29,6 +30,29 @@
  */
 abstract class UpdateTestBase extends WebTestBase {
 
+  protected function setUp() {
+    parent::setUp();
+
+    // Change the root path which Update Manager uses to install and update
+    // projects to be inside the testing site directory. See
+    // \Drupal\updateUpdateRootFactory::get() for equivalent changes to the
+    // test child site.
+    $request = \Drupal::request();
+    $update_root = $this->container->get('update.root') . '/' . DrupalKernel::findSitePath($request);
+    $this->container->set('update.root', $update_root);
+    \Drupal::setContainer($this->container);
+
+    // Create the directories within the root path within which the Update
+    // Manager will install projects.
+    foreach (drupal_get_updaters() as $updater_info) {
+      $updater = $updater_info['class'];
+      $install_directory = $update_root . '/' . $updater::getRootDirectoryRelativePath();
+      if (!is_dir($install_directory)) {
+        mkdir($install_directory);
+      }
+    }
+  }
+
   /**
    * Refreshes the update status based on the desired available update scenario.
    *
diff --git a/core/modules/update/src/Tests/UpdateUploadTest.php b/core/modules/update/src/Tests/UpdateUploadTest.php
index 0565ff0..8f09736 100644
--- a/core/modules/update/src/Tests/UpdateUploadTest.php
+++ b/core/modules/update/src/Tests/UpdateUploadTest.php
@@ -45,6 +45,7 @@ public function testUploadModule() {
     // This also checks that the correct archive extensions are allowed.
     $this->drupalPostForm('admin/modules/install', $edit, t('Install'));
     $this->assertText(t('Only files with the following extensions are allowed: @archive_extensions.', array('@archive_extensions' => archiver_get_extensions())),'Only valid archives can be uploaded.');
+    $this->assertUrl('admin/modules/install');
 
     // Check to ensure an existing module can't be reinstalled. Also checks that
     // the archive was extracted since we can't know if the module is already
@@ -55,6 +56,25 @@ public function testUploadModule() {
     );
     $this->drupalPostForm('admin/modules/install', $edit, t('Install'));
     $this->assertText(t('@module_name is already installed.', array('@module_name' => 'AAA Update test')), 'Existing module was extracted and not reinstalled.');
+    $this->assertUrl('admin/modules/install');
+
+    // Ensure that a new module can be extracted and installed.
+    $updaters = drupal_get_updaters();
+    $moduleUpdater = $updaters['module']['class'];
+    $installedInfoFilePath = $this->container->get('update.root') . '/' . $moduleUpdater::getRootDirectoryRelativePath() . '/update_test_new_module/update_test_new_module.info.yml';
+    $this->assertFalse(file_exists($installedInfoFilePath), 'The new module does not exist in the filesystem before it is installed with the Update Manager.');
+    $validArchiveFile = drupal_get_path('module', 'update') . '/tests/update_test_new_module.tar.gz';
+    $edit = array(
+      'files[project_upload]' => $validArchiveFile,
+    );
+    $this->drupalPostForm('admin/modules/install', $edit, t('Install'));
+    // Check that submitting the form takes the user to authorize.php.
+    // @todo Remove <none> from the expected URL once that bug is fixed.
+    $this->assertUrl('core/authorize.php/<none>');
+    // Check for a success message on the page, and check that the installed
+    // module now exists in the expected place in the filesystem.
+    $this->assertRaw(t('Installed %project_name successfully', array('%project_name' => 'update_test_new_module')));
+    $this->assertTrue(file_exists($installedInfoFilePath), 'The new module exists in the filesystem after it is installed with the Update Manager.');
   }
 
   /**
diff --git a/core/modules/update/src/UpdateRootFactory.php b/core/modules/update/src/UpdateRootFactory.php
new file mode 100644
index 0000000..42a6d7c
--- /dev/null
+++ b/core/modules/update/src/UpdateRootFactory.php
@@ -0,0 +1,74 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\update\UpdateRootFactory.
+ */
+
+namespace Drupal\update;
+
+use Drupal\Core\DrupalKernelInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Gets the root path used by the Update Manager to install or update projects.
+ */
+class UpdateRootFactory {
+
+  /**
+   * The Drupal kernel.
+   *
+   * @var \Drupal\Core\DrupalKernelInterface
+   */
+  protected $drupalKernel;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * Constructs an UpdateRootFactory instance.
+   *
+   * @param \Drupal\Core\DrupalKernelInterface $drupal_kernel
+   *   The Drupal kernel.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *  The request stack.
+   */
+  public function __construct(DrupalKernelInterface $drupal_kernel, RequestStack $request_stack) {
+    $this->drupalKernel = $drupal_kernel;
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * Gets the root path under which projects are installed or updated.
+   *
+   * The Update Manager will ensure that project files can only be copied to
+   * specific subdirectories of this root path.
+   *
+   * @return string
+   */
+  public function get() {
+    // Normally the Update Manager's root path is the same as the app root (the
+    // directory in which the Drupal site is installed).
+    $root_path = $this->drupalKernel->getAppRoot();
+
+    // When running in a test site, change the root path to be the testing site
+    // directory. This ensures that it will always be writable by the webserver
+    // (thereby allowing the actual extraction and installation of projects by
+    // the Update Manager to be tested) and also ensures that new project files
+    // added there won't be visible to the parent site and will be properly
+    // cleaned up once the test finishes running. See also
+    // \Drupal\update\Tests\UpdateTestBase::setUp().
+    if (DRUPAL_TEST_IN_CHILD_SITE) {
+      $kernel = $this->drupalKernel;
+      $request = $this->requestStack->getCurrentRequest();
+      $root_path .= '/' . $kernel::findSitePath($request);
+    }
+
+    return $root_path;
+  }
+
+}
diff --git a/core/modules/update/tests/update_test_new_module.tar.gz b/core/modules/update/tests/update_test_new_module.tar.gz
new file mode 100644
index 0000000..fba2143
--- /dev/null
+++ b/core/modules/update/tests/update_test_new_module.tar.gz
@@ -0,0 +1 @@
+��w�T��AO�0���+z#�0ڲ�Ƙ�A^4�JV`�m�֪�{;�"&�!A��}�B�ulog���յ�m���[�ApH�S*nF�b�9v."ŤJ��y$�8��A���IEi�V��s�O���۞���0+�e���]�	X�hg�B�����2��~疿:���$����ʛ6yS�7m���4~��FR]O��ج,�)#gLY��"��mߚn��,+fa��d���F��y�U�����at{G&e���wr�rb��;�{�U�����l��_(�}��d���fn������4�=��]�/7��.3]�Q��f]�Ch,��}�\��(
\ No newline at end of file
diff --git a/core/modules/update/update.authorize.inc b/core/modules/update/update.authorize.inc
index d6a7066..9ff8d02 100644
--- a/core/modules/update/update.authorize.inc
+++ b/core/modules/update/update.authorize.inc
@@ -147,7 +147,7 @@ function update_authorize_batch_copy_project($project, $updater_name, $local_url
     return;
   }
 
-  $updater = new $updater_name($local_url);
+  $updater = new $updater_name($local_url, \Drupal::getContainer()->get('update.root'));
 
   try {
     if ($updater->isInstalled()) {
diff --git a/core/modules/update/update.services.yml b/core/modules/update/update.services.yml
index fe01f47..3286b2a 100644
--- a/core/modules/update/update.services.yml
+++ b/core/modules/update/update.services.yml
@@ -13,3 +13,12 @@ services:
   update.fetcher:
     class: Drupal\update\UpdateFetcher
     arguments: ['@config.factory', '@http_client']
+  update.root:
+    class: SplString
+    factory_service: 'update.root.factory'
+    factory_method: 'get'
+    tags:
+      - { name: parameter_service }
+  update.root.factory:
+    class: Drupal\update\UpdateRootFactory
+    arguments: ['@kernel', '@request_stack']