Index: trunk/data/validator/geometry.mapcss
===================================================================
--- trunk/data/validator/geometry.mapcss	(revision 15931)
+++ trunk/data/validator/geometry.mapcss	(revision 15940)
@@ -165,4 +165,10 @@
 area[building][building!~/no|entrance/] {
   throwWarning: tr("Building inside building");
+}
+
+/* Building overlapping building (spatial test) */
+area[building][building!~/no|entrance/][any(tag("layer"),"0") = any(parent_tag("layer"),"0")] ⧉
+area[building][building!~/no|entrance/] {
+  throwWarning: tr("Overlapping buildings");
 }