diff --git a/documentation/dev-manual/building.rst b/documentation/dev-manual/building.rst
index 04c150094..8d00d908e 100644
--- a/documentation/dev-manual/building.rst
+++ b/documentation/dev-manual/building.rst
@@ -736,6 +736,13 @@ are the statements to put in your ``local.conf`` file::
    INHERIT += "externalsrc"
    EXTERNALSRC:pn-myrecipe = "path-to-your-source-tree"
 
+.. note::
+
+   When you inherit :ref:`ref-classes-externalsrc` class globally make
+   sure to set the :term:`EXTERNALSRC` and :term:`EXTERNALSRC_BUILD`
+   variables to specific recipes, not doing that will redirect
+   all recipes to one external source directory.
+
 This next example shows how to accomplish the same thing by setting
 :term:`EXTERNALSRC` in the recipe itself or in the recipe's append file::