diff --git a/documentation/dev-manual/building.rst b/documentation/dev-manual/building.rst
index 04c1500943..60ad11f52f 100644
--- a/documentation/dev-manual/building.rst
+++ b/documentation/dev-manual/building.rst
@@ -757,6 +757,11 @@ to point to that directory::
 
    EXTERNALSRC_BUILD:pn-myrecipe = "path-to-your-source-tree"
 
+.. note::
+
+   The values of :term:`EXTERNALSRC` and :term:`EXTERNALSRC_BUILD`
+   must be absolute paths.
+
 Replicating a Build Offline
 ===========================