diff --git a/core/Object.php b/core/Object.php
index e5130c4e0..f04629a7f 100755
--- a/core/Object.php
+++ b/core/Object.php
@@ -446,11 +446,18 @@ abstract class Object {
 	 * Keep in mind that the extension will only be applied to new
 	 * instances, not existing ones (including all instances created through {@link singleton()}).
 	 *
+	 * @deprecated 3.2 Use YAML config instead, see 
+	 *             http://doc.silverstripe.org/framework/en/trunk/reference/dataextension
 	 * @param string $class Class that should be extended - has to be a subclass of {@link Object}
 	 * @param string $extension Subclass of {@link Extension} with optional parameters 
 	 *  as a string, e.g. "Versioned" or "Translatable('Param')"
 	 */
 	public static function add_extension($classOrExtension, $extension = null) {
+		Deprecation::notice(
+			'3.2', 
+			'Use YAML config instead, see http://doc.silverstripe.org/framework/en/trunk/reference/dataextension'
+		);
+
 		if(func_num_args() > 1) {
 			$class = $classOrExtension;
 		} else {
diff --git a/docs/en/changelogs/3.1.0.md b/docs/en/changelogs/3.1.0.md
index 7484f90a5..d788eff3b 100644
--- a/docs/en/changelogs/3.1.0.md
+++ b/docs/en/changelogs/3.1.0.md
@@ -450,3 +450,4 @@ you can enable those warnings and future-proof your code already.
  * Hard limit displayed pages in the CMS tree to `500`, and the number of direct children to `250`,
    to avoid excessive resource usage. Configure through `Hierarchy.node_threshold_total` and `
    Hierarchy.node_threshold_leaf`.  Set to `0` to show tree unrestricted.
+ * Deprecated `Object::add_extension()`, use YAML config instead, see [docs](http://doc.silverstripe.org/framework/en/trunk/reference/dataextension)
\ No newline at end of file