Index: trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java	(revision 4408)
+++ trunk/src/org/openstreetmap/josm/data/osm/QuadBuckets.java	(revision 4409)
@@ -527,9 +527,4 @@
     }
 
-    public void unsupported()
-    {
-        System.out.println("unsupported operation");
-        throw new UnsupportedOperationException();
-    }
     public boolean retainAll(Collection<?> objects)
     {
Index: trunk/src/org/openstreetmap/josm/data/validation/OsmValidator.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/validation/OsmValidator.java	(revision 4408)
+++ trunk/src/org/openstreetmap/josm/data/validation/OsmValidator.java	(revision 4409)
@@ -26,4 +26,5 @@
 import org.openstreetmap.josm.data.projection.Lambert;
 import org.openstreetmap.josm.data.projection.Mercator;
+import org.openstreetmap.josm.data.validation.tests.BuildingInBuilding;
 import org.openstreetmap.josm.data.validation.tests.Coastlines;
 import org.openstreetmap.josm.data.validation.tests.CrossingWays;
@@ -96,4 +97,5 @@
         TurnrestrictionTest.class, // ID  1801 ..  1899
         DuplicateRelation.class, // ID 1901 .. 1999
+        BuildingInBuilding.class, // ID 2001 .. 2099
     };
 
Index: trunk/src/org/openstreetmap/josm/data/validation/tests/BuildingInBuilding.java
===================================================================
--- trunk/src/org/openstreetmap/josm/data/validation/tests/BuildingInBuilding.java	(revision 4409)
+++ trunk/src/org/openstreetmap/josm/data/validation/tests/BuildingInBuilding.java	(revision 4409)
@@ -0,0 +1,86 @@
+package org.openstreetmap.josm.data.validation.tests;
+
+import static org.openstreetmap.josm.tools.I18n.tr;
+
+import java.util.Collection;
+import java.util.LinkedList;
+import java.util.List;
+import org.openstreetmap.josm.data.osm.Node;
+import org.openstreetmap.josm.data.osm.OsmPrimitive;
+import org.openstreetmap.josm.data.osm.QuadBuckets;
+import org.openstreetmap.josm.data.osm.Way;
+import org.openstreetmap.josm.data.validation.Severity;
+import org.openstreetmap.josm.data.validation.Test;
+import org.openstreetmap.josm.data.validation.TestError;
+import org.openstreetmap.josm.tools.FilteredCollection;
+import org.openstreetmap.josm.tools.Geometry;
+import org.openstreetmap.josm.tools.Predicate;
+
+public class BuildingInBuilding extends Test {
+
+    protected static int BUILDING_INSIDE_BUILDING = 2001;
+    protected List<OsmPrimitive> primitivesToCheck = new LinkedList<OsmPrimitive>();
+    protected QuadBuckets<Way> index = new QuadBuckets<Way>();
+
+    public BuildingInBuilding() {
+        super(tr("Building inside building"));
+    }
+
+    @Override
+    public void visit(Node n) {
+        if (n.isUsable() && isBuilding(n)) {
+            primitivesToCheck.add(n);
+        }
+    }
+
+    @Override
+    public void visit(Way w) {
+        if (w.isUsable() && w.isClosed() && isBuilding(w)) {
+            primitivesToCheck.add(w);
+            index.add(w);
+        }
+    }
+
+    private static boolean isInPolygon(Node n, List<Node> polygon) {
+        return Geometry.nodeInsidePolygon(n, polygon);
+    }
+
+    private static boolean isInPolygon(Way w, List<Node> polygon) {
+        for (Node n : w.getNodes()) {
+            if (!isInPolygon(n, polygon)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    @Override
+    public void endTest() {
+        for (final OsmPrimitive p : primitivesToCheck) {
+            Collection<Way> outer = index.search(p.getBBox());
+            outer = new FilteredCollection<Way>(outer, new Predicate<Way>() {
+
+                @Override
+                public boolean evaluate(Way object) {
+                    if (p.equals(object)) {
+                        return false;
+                    } else if (p instanceof Node) {
+                        return isInPolygon((Node) p, object.getNodes()) || object.getNodes().contains((Node) p);
+                    } else if (p instanceof Way) {
+                        return isInPolygon((Way) p, object.getNodes());
+                    } else {
+                        return false;
+                    }
+                }
+            });
+            if (!outer.isEmpty()) {
+                errors.add(new TestError(this, Severity.WARNING,
+                        tr("Building inside building"), BUILDING_INSIDE_BUILDING, p));
+            }
+        }
+    }
+
+    private static boolean isBuilding(OsmPrimitive p) {
+        return "yes".equals(p.get("building"));
+    }
+}