diff --git a/core/modules/layout_builder/layout_builder.services.yml b/core/modules/layout_builder/layout_builder.services.yml
index b2ee1fe13e..98740eb9b5 100644
--- a/core/modules/layout_builder/layout_builder.services.yml
+++ b/core/modules/layout_builder/layout_builder.services.yml
@@ -58,3 +58,8 @@ services:
     class: Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController
     public: false
     arguments: ['@layout_builder.controller.entity_form.inner']
+  layout_builder.element.prepare_layout:
+    class: Drupal\layout_builder\EventSubscriber\PrepareLayout
+    arguments: ['@layout_builder.tempstore_repository', '@messenger']
+    tags:
+      - { name: event_subscriber }
diff --git a/core/modules/layout_builder/src/Element/LayoutBuilder.php b/core/modules/layout_builder/src/Element/LayoutBuilder.php
index 80dcff90f7..f24c56314b 100644
--- a/core/modules/layout_builder/src/Element/LayoutBuilder.php
+++ b/core/modules/layout_builder/src/Element/LayoutBuilder.php
@@ -3,18 +3,18 @@
 namespace Drupal\layout_builder\Element;
 
 use Drupal\Core\Ajax\AjaxHelperTrait;
-use Drupal\Core\Messenger\MessengerInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Plugin\PluginFormInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\Element\RenderElement;
 use Drupal\Core\Url;
 use Drupal\layout_builder\Context\LayoutBuilderContextTrait;
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
 use Drupal\layout_builder\LayoutBuilderHighlightTrait;
-use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
-use Drupal\layout_builder\OverridesSectionStorageInterface;
 use Drupal\layout_builder\SectionStorageInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
+use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 
 /**
  * Defines a render element for building the Layout Builder UI.
@@ -31,18 +31,11 @@ class LayoutBuilder extends RenderElement implements ContainerFactoryPluginInter
   use LayoutBuilderHighlightTrait;
 
   /**
-   * The layout tempstore repository.
+   * The Event Dispatcher.
    *
-   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   * @var \Symfony\Component\EventDispatcher\EventDispatcherInterface
    */
-  protected $layoutTempstoreRepository;
-
-  /**
-   * The messenger service.
-   *
-   * @var \Drupal\Core\Messenger\MessengerInterface
-   */
-  protected $messenger;
+  protected $eventDispatcher;
 
   /**
    * Constructs a new LayoutBuilder.
@@ -53,15 +46,24 @@ class LayoutBuilder extends RenderElement implements ContainerFactoryPluginInter
    *   The plugin ID for the plugin instance.
    * @param mixed $plugin_definition
    *   The plugin implementation definition.
-   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
-   *   The layout tempstore repository.
-   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
-   *   The messenger service.
+   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher service.
+   * @param \Drupal\Core\Messenger\MessengerInterface|NULL $messenger
+   *   The messenger service. This is left optional for BC reasons and
+   *   will be deprecated in drupal:9.1.0.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, $event_dispatcher, $messenger = NULL) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
-    $this->layoutTempstoreRepository = $layout_tempstore_repository;
-    $this->messenger = $messenger;
+
+    if (!($event_dispatcher instanceof EventDispatcherInterface)) {
+      @trigger_error('The event_dispatcher service should be passed to LayoutBuilder::__construct() instead of the layout_builder.tempstore_repository service since 9.1.0. This will be required in Drupal 10.0.0.', E_USER_DEPRECATED);
+      $event_dispatcher = \Drupal::service('event_dispatcher');
+    }
+    $this->eventDispatcher = $event_dispatcher;
+
+    if ($messenger) {
+      @trigger_error('Calling LayoutBuilder::__construct() with the $messenger argument is deprecated in drupal:9.1.0 and will be removed in drupal:10.0.0', E_USER_DEPRECATED);
+    }
   }
 
   /**
@@ -72,8 +74,8 @@ public static function create(ContainerInterface $container, array $configuratio
       $configuration,
       $plugin_id,
       $plugin_definition,
-      $container->get('layout_builder.tempstore_repository'),
-      $container->get('messenger')
+      $container->get('event_dispatcher'),
+      NULL
     );
   }
 
@@ -145,19 +147,8 @@ protected function layout(SectionStorageInterface $section_storage) {
    *   The section storage.
    */
   protected function prepareLayout(SectionStorageInterface $section_storage) {
-    // If the layout has pending changes, add a warning.
-    if ($this->layoutTempstoreRepository->has($section_storage)) {
-      $this->messenger->addWarning($this->t('You have unsaved changes.'));
-    }
-    // If the layout is an override that has not yet been overridden, copy the
-    // sections from the corresponding default.
-    elseif ($section_storage instanceof OverridesSectionStorageInterface && !$section_storage->isOverridden()) {
-      $sections = $section_storage->getDefaultSectionStorage()->getSections();
-      foreach ($sections as $section) {
-        $section_storage->appendSection($section);
-      }
-      $this->layoutTempstoreRepository->set($section_storage);
-    }
+    $event = new PrepareLayoutEvent($section_storage);
+    $this->eventDispatcher->dispatch(LayoutBuilderEvents::PREPARE_LAYOUT, $event);
   }
 
   /**
diff --git a/core/modules/layout_builder/src/Event/PrepareLayoutEvent.php b/core/modules/layout_builder/src/Event/PrepareLayoutEvent.php
new file mode 100644
index 0000000000..bd42e8b2df
--- /dev/null
+++ b/core/modules/layout_builder/src/Event/PrepareLayoutEvent.php
@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\layout_builder\Event;
+
+use Drupal\layout_builder\SectionStorageInterface;
+use Symfony\Component\EventDispatcher\Event;
+
+/**
+ * Event fired when during preRender() of a LayoutBuilder Element.
+ *
+ * Subscribers to this event should manipulate the cacheability object and the
+ * build array in this event.
+ *
+ * @see \Drupal\layout_builder\LayoutBuilderEvents::PREPARE_LAYOUT
+ */
+class PrepareLayoutEvent extends Event {
+
+  /**
+   * The Section Storage Plugin.
+   *
+   * @var \Drupal\layout_builder\SectionStorageInterface
+   */
+  protected $sectionStorage;
+
+  /**
+   * LayoutPrepareEvent constructor.
+   *
+   * @param \Drupal\layout_builder\SectionStorageInterface $section_storage
+   *   The section storage preparing the Layout.
+   */
+  public function __construct(SectionStorageInterface $section_storage) {
+    $this->sectionStorage = $section_storage;
+  }
+
+  /**
+   * Gets the section storage.
+   *
+   * @return \Drupal\layout_builder\SectionStorageInterface
+   *   The section storage.
+   */
+  public function getSectionStorage() {
+    return $this->sectionStorage;
+  }
+
+}
diff --git a/core/modules/layout_builder/src/EventSubscriber/PrepareLayout.php b/core/modules/layout_builder/src/EventSubscriber/PrepareLayout.php
new file mode 100644
index 0000000000..2c6b6a17ba
--- /dev/null
+++ b/core/modules/layout_builder/src/EventSubscriber/PrepareLayout.php
@@ -0,0 +1,79 @@
+<?php
+
+namespace Drupal\layout_builder\EventSubscriber;
+
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\OverridesSectionStorageInterface;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * An event subscriber that prepares a LayoutBuilder element for preRender.
+ */
+class PrepareLayout implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * PrepareLayout constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    $events[LayoutBuilderEvents::PREPARE_LAYOUT][] = ['onPrepareLayout', 10];
+    return $events;
+  }
+
+  /**
+   * Prepares a layout for use in the UI.
+   *
+   * @param \Drupal\layout_builder\Event\PrepareLayoutEvent $event
+   *   The prepare layout event.
+   */
+  public function onPrepareLayout(PrepareLayoutEvent $event) {
+    $section_storage = $event->getSectionStorage();
+
+    // If the layout has pending changes, add a warning.
+    if ($this->layoutTempstoreRepository->has($section_storage)) {
+      $this->messenger->addWarning($this->t('You have unsaved changes.'));
+    }
+    // If the layout is an override that has not yet been overridden, copy the
+    // sections from the corresponding default.
+    elseif ($section_storage instanceof OverridesSectionStorageInterface && !$section_storage->isOverridden()) {
+      $sections = $section_storage->getDefaultSectionStorage()->getSections();
+      foreach ($sections as $section) {
+        $section_storage->appendSection($section);
+      }
+      $this->layoutTempstoreRepository->set($section_storage);
+    }
+  }
+
+}
diff --git a/core/modules/layout_builder/src/LayoutBuilderEvents.php b/core/modules/layout_builder/src/LayoutBuilderEvents.php
index 5b19e1c198..19b5c2a541 100644
--- a/core/modules/layout_builder/src/LayoutBuilderEvents.php
+++ b/core/modules/layout_builder/src/LayoutBuilderEvents.php
@@ -26,4 +26,19 @@ final class LayoutBuilderEvents {
    */
   const SECTION_COMPONENT_BUILD_RENDER_ARRAY = 'section_component.build.render_array';
 
+  /**
+   * Name of the event fired when preparing a LayoutBuilder Element.
+   *
+   * This event allows modules to collabourate on creating the sections used in
+   * a LayoutBuilder Element during PreRender().
+   *
+   * @EVENT
+   *
+   * @see \Drupal\layout_builder\Event\PrepareLayoutEvent
+   * @see \Drupal\layout_builder\Element\LayoutBuilder
+   *
+   * @var string
+   */
+  const PREPARE_LAYOUT = 'prepare_layout';
+
 }
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.info.yml b/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.info.yml
new file mode 100644
index 0000000000..8d92f61a10
--- /dev/null
+++ b/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.info.yml
@@ -0,0 +1,8 @@
+name: 'Layout Builder element test'
+type: module
+description: 'Support module for testing the layout builder element.'
+package: Testing
+version: VERSION
+core: 8.x
+dependencies:
+  - drupal:layout_builder
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.services.yml b/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.services.yml
new file mode 100644
index 0000000000..ce75a2e892
--- /dev/null
+++ b/core/modules/layout_builder/tests/modules/layout_builder_element_test/layout_builder_element_test.services.yml
@@ -0,0 +1,6 @@
+services:
+  layout_builder_element_test.prepare_layout:
+    class: Drupal\layout_builder_element_test\EventSubscriber\TestPrepareLayout
+    arguments: ['@layout_builder.tempstore_repository', '@messenger']
+    tags:
+      - { name: event_subscriber }
diff --git a/core/modules/layout_builder/tests/modules/layout_builder_element_test/src/EventSubscriber/TestPrepareLayout.php b/core/modules/layout_builder/tests/modules/layout_builder_element_test/src/EventSubscriber/TestPrepareLayout.php
new file mode 100644
index 0000000000..3b50dc0716
--- /dev/null
+++ b/core/modules/layout_builder/tests/modules/layout_builder_element_test/src/EventSubscriber/TestPrepareLayout.php
@@ -0,0 +1,118 @@
+<?php
+
+namespace Drupal\layout_builder_element_test\EventSubscriber;
+
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\layout_builder\Event\PrepareLayoutEvent;
+use Drupal\layout_builder\LayoutBuilderEvents;
+use Drupal\layout_builder\LayoutTempstoreRepositoryInterface;
+use Drupal\layout_builder\Section;
+use Drupal\layout_builder\SectionComponent;
+use Symfony\Component\EventDispatcher\EventSubscriberInterface;
+
+/**
+ * An event subscriber that prepares a LayoutBuilder element for preRender.
+ */
+class TestPrepareLayout implements EventSubscriberInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The layout tempstore repository.
+   *
+   * @var \Drupal\layout_builder\LayoutTempstoreRepositoryInterface
+   */
+  protected $layoutTempstoreRepository;
+
+  /**
+   * The messenger service.
+   *
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * PrepareLayout constructor.
+   *
+   * @param \Drupal\layout_builder\LayoutTempstoreRepositoryInterface $layout_tempstore_repository
+   *   The tempstore repository.
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The messenger service.
+   */
+  public function __construct(LayoutTempstoreRepositoryInterface $layout_tempstore_repository, MessengerInterface $messenger) {
+    $this->layoutTempstoreRepository = $layout_tempstore_repository;
+    $this->messenger = $messenger;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getSubscribedEvents() {
+    // Act before LB subscriber.
+    $events[LayoutBuilderEvents::PREPARE_LAYOUT][] = ['onBeforePrepareLayout', 20];
+    // Act after LB subscriber.
+    $events[LayoutBuilderEvents::PREPARE_LAYOUT][] = ['onAfterPrepareLayout', -10];
+    return $events;
+  }
+
+  /**
+   * Subscriber to test acting before the LB subscriber.
+   *
+   * @param \Drupal\layout_builder\Event\PrepareLayoutEvent $event
+   *   The prepare layout event.
+   */
+  public function onBeforePrepareLayout(PrepareLayoutEvent $event) {
+    $section_storage = $event->getSectionStorage();
+    $context = $section_storage->getContextValues();
+
+    if (!empty($context['entity'])) {
+      /** @var \Drupal\Core\Entity\EntityInterface $entity */
+      $entity = $context['entity'];
+
+      // Node 1 or 2: Append a block to the layout.
+      if (in_array($entity->id(), ['1', '2'])) {
+        $section = new Section('layout_onecol');
+        $section->appendComponent(new SectionComponent('fake-uuid', 'content', [
+          'id' => 'static_block',
+          'label' => 'Test static block title',
+          'label_display' => 'visible',
+        ]));
+        $section_storage->appendSection($section);
+      }
+
+      // Node 2: Stop event propagation.
+      if ($entity->id() === '2') {
+        $event->stopPropagation();
+      }
+    }
+  }
+
+  /**
+   * Subscriber to test acting after the LB subscriber.
+   *
+   * @param \Drupal\layout_builder\Event\PrepareLayoutEvent $event
+   *   The prepare layout event.
+   */
+  public function onAfterPrepareLayout(PrepareLayoutEvent $event) {
+    $section_storage = $event->getSectionStorage();
+    $context = $section_storage->getContextValues();
+
+    if (!empty($context['entity'])) {
+      /** @var \Drupal\Core\Entity\EntityInterface $entity */
+      $entity = $context['entity'];
+
+      // Node 1, 2, or 3: Append a block to the layout.
+      if (in_array($entity->id(), ['1', '2', '3'])) {
+        $section = new Section('layout_onecol');
+        $section->appendComponent(new SectionComponent('fake-uuid', 'content', [
+          'id' => 'static_block_two',
+          'label' => 'Test second static block title',
+          'label_display' => 'visible',
+        ]));
+        $section_storage->appendSection($section);
+      }
+    }
+  }
+
+}
diff --git a/core/modules/layout_builder/tests/src/Functional/LayoutBuilderPrepareLayoutTest.php b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderPrepareLayoutTest.php
new file mode 100644
index 0000000000..95c5b2e24c
--- /dev/null
+++ b/core/modules/layout_builder/tests/src/Functional/LayoutBuilderPrepareLayoutTest.php
@@ -0,0 +1,119 @@
+<?php
+
+namespace Drupal\Tests\layout_builder\Functional;
+
+use Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay;
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests the ability to alter a layout builder element while preparing.
+ *
+ * @group layout_builder
+ */
+class LayoutBuilderPrepareLayoutTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'layout_builder',
+    'node',
+    'layout_builder_element_test',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $this->createContentType(['type' => 'bundle_with_section_field']);
+    LayoutBuilderEntityViewDisplay::load('node.bundle_with_section_field.default')
+      ->enableLayoutBuilder()
+      ->setOverridable()
+      ->save();
+
+    $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The first node title',
+      'body' => [
+        [
+          'value' => 'The first node body',
+        ],
+      ],
+    ]);
+    $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The second node title',
+      'body' => [
+        [
+          'value' => 'The second node body',
+        ],
+      ],
+    ]);
+    $this->createNode([
+      'type' => 'bundle_with_section_field',
+      'title' => 'The third node title',
+      'body' => [
+        [
+          'value' => 'The third node body',
+        ],
+      ],
+    ]);
+  }
+
+  /**
+   * Tests that we can alter a Layout Builder element while preparing.
+   *
+   * @see \Drupal\layout_builder_element_test\EventSubscriber\TestPrepareLayout;
+   */
+  public function testAlterPrepareLayout() {
+    $assert_session = $this->assertSession();
+    $page = $this->getSession()->getPage();
+
+    $this->drupalLogin($this->drupalCreateUser([
+      'access content',
+      'configure any layout',
+      'administer node display',
+      'configure all bundle_with_section_field node layout overrides',
+    ]));
+
+    // Add a block to the defaults.
+    $this->drupalGet('admin/structure/types/manage/bundle_with_section_field/display/default');
+    $page->clickLink('Manage layout');
+    $page->clickLink('Add block');
+    $page->clickLink('Powered by Drupal');
+    $page->fillField('settings[label]', 'Default block title');
+    $page->checkField('settings[label_display]');
+    $page->pressButton('Add block');
+    $page->pressButton('Save layout');
+
+    // Chack the block is on the node page.
+    $this->drupalGet('node/1');
+    $assert_session->pageTextContains('Default block title');
+
+    // When we edit the layout, it get's the static blocks.
+    $this->drupalGet('node/1/layout');
+    $assert_session->pageTextContains('Test static block title');
+    $assert_session->pageTextNotContains('Default block title');
+    $assert_session->pageTextContains('Test second static block title');
+
+    // When we edit the second node, only the first event fires.
+    $this->drupalGet('node/2/layout');
+    $assert_session->pageTextContains('Test static block title');
+    $assert_session->pageTextNotContains('Default block title');
+    $assert_session->pageTextNotContains('Test second static block title');
+
+    // When we edit the third node, the default exists PLUS our static block.
+    $this->drupalGet('node/3/layout');
+    $assert_session->pageTextNotContains('Test static block title');
+    $assert_session->pageTextContains('Default block title');
+    $assert_session->pageTextContains('Test second static block title');
+  }
+
+}