Index: src/org/openstreetmap/josm/plugins/buildings_tools/Building.java
===================================================================
--- src/org/openstreetmap/josm/plugins/buildings_tools/Building.java	(revision 34720)
+++ src/org/openstreetmap/josm/plugins/buildings_tools/Building.java	(working copy)
@@ -9,6 +9,7 @@
 import java.awt.Point;
 import java.awt.geom.GeneralPath;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedList;
@@ -36,6 +37,7 @@
 import org.openstreetmap.josm.data.osm.Way;
 import org.openstreetmap.josm.gui.MainApplication;
 import org.openstreetmap.josm.gui.MapView;
+import org.openstreetmap.josm.tools.Geometry;
 
 class Building {
     private final EastNorth[] en = new EastNorth[4];
@@ -396,11 +398,72 @@
 
         addAddress(w);
 
+        // calculate BBox which is slightly larger than the new building
+        BBox searchBox = new BBox();
+        final double maxDist = 0.001;
+        List<Node> wayNodes = w.getNodes();
+        for (Node n : nodes) {
+            LatLon l = eastNorth2latlon(n.getEastNorth());
+            searchBox.add(new BBox(l.lon() - 0.0000001, l.lat() - 0.0000001,
+                    l.lon() + 0.0000001, l.lat() + 0.0000001));
+        }
+        // find the ways which might be snapped to the new building
+        List<Way> others = ds.searchWays(searchBox);
+        // add nodes of existing buildings to the new one
+        for (Way other : others) {
+            if (other.get("building") == null)
+                continue;
+            snapToWay(wayNodes, other.getNodes(), maxDist);
+            w.setNodes(wayNodes);
+        }
+        // add new nodes to existing buildings
+        for (Way other : others) {
+            if (other.get("building") == null)
+                continue;
+            List<Node> otherNodes = other.getNodes();
+            snapToWay(otherNodes, Arrays.asList(nodes), maxDist);
+            if (otherNodes.size() != other.getNodesCount()) {
+                Way newWay = new Way(other);
+                newWay.setNodes(otherNodes);
+                cmds.add(new ChangeCommand(other, newWay));
+            }
+        }
+
         Command c = new SequenceCommand(tr("Create building"), cmds);
         UndoRedoHandler.getInstance().add(c);
         return w;
     }
 
+    /**
+     * Add all nodes in otherNodes to wayNodes that are within maxDist to the
+     * segments described by wayNodes
+     *
+     * @param wayNodes
+     *            List of nodes that might be changed
+     * @param otherNodes
+     *            other nodes
+     * @param maxDist
+     *            maximum Distance in
+     */
+    private static void snapToWay(List<Node> wayNodes, Collection<Node> otherNodes, double maxDist) {
+        for (int i = 0; i < wayNodes.size(); i++) {
+            Node n0 = wayNodes.get(i);
+            Node n1 = wayNodes.get(i + 1 == wayNodes.size() ? 0 : i + 1);
+            for (Node n2 : otherNodes) {
+                if (n2 == n0 || n2 == n1)
+                    continue;
+                EastNorth x = Geometry.closestPointToSegment(n0.getEastNorth(), n1.getEastNorth(),
+                        n2.getEastNorth());
+                if (x.distanceSq(n2.getEastNorth()) < maxDist) {
+                    wayNodes.add(i + 1, n2);
+                    i--; // we may add multiple nodes to one segment, so repeat
+                         // it
+                    break;
+                }
+            }
+        }
+    }
+
     private void addAddress(Way w) {
         if (ToolSettings.PROP_USE_ADDR_NODE.get()) {
             Node addrNode = getAddressNode();