diff --git a/src/Mod/Arch/Building.py b/src/Mod/Arch/Building.py
index d04d99af9d..adc2f4b335 100644
--- a/src/Mod/Arch/Building.py
+++ b/src/Mod/Arch/Building.py
@@ -51,25 +51,21 @@ class _CommandBuilding:
         
     def Activated(self):
         sel = FreeCADGui.Selection.getSelection()
-        transmode = True
-        if not sel:
-            transmode = False
-        for obj in sel:
-            if not (Draft.getType(obj) in ["Cell","Site","Floor"]):
-                transmode = False
-        if transmode:
-            FreeCAD.ActiveDocument.openTransaction("Type conversion")
-            for obj in sel:
+        ok = False
+        if (len(sel) == 1):
+            if Draft.getType(sel[0]) in ["Cell","Site","Floor"]:
+                FreeCAD.ActiveDocument.openTransaction("Type conversion")
                 nobj = makeBuilding()
-                Commands.copyProperties(obj,nobj)
-                FreeCAD.ActiveDocument.removeObject(obj.Name)
-            FreeCAD.ActiveDocument.commitTransaction()
-        else:
+                Commands.copyProperties(sel[0],nobj)
+                FreeCAD.ActiveDocument.removeObject(sel[0].Name)
+                FreeCAD.ActiveDocument.commitTransaction()
+                ok = True
+        if not ok:
             FreeCAD.ActiveDocument.openTransaction("Building")
             makeBuilding(sel)
             FreeCAD.ActiveDocument.commitTransaction()
-        FreeCAD.ActiveDocument.recompute()
-
+            FreeCAD.ActiveDocument.recompute()
+        
 class _Building(Cell._Cell):
     "The Building object"
     def __init__(self,obj):
diff --git a/src/Mod/Arch/Cell.py b/src/Mod/Arch/Cell.py
index 7cdeaa5bdc..5dac629013 100644
--- a/src/Mod/Arch/Cell.py
+++ b/src/Mod/Arch/Cell.py
@@ -53,24 +53,20 @@ class _CommandCell:
         
     def Activated(self):
         sel = FreeCADGui.Selection.getSelection()
-        transmode = True
-        if not sel:
-            transmode = False
-        for obj in sel:
-            if not (Draft.getType(obj) in ["Floor","Site","Building"]):
-                transmode = False
-        if transmode:
-            FreeCAD.ActiveDocument.openTransaction("Type conversion")
-            for obj in sel:
+        ok = False
+        if (len(sel) == 1):
+            if Draft.getType(sel[0]) in ["Floor","Site","Building"]:
+                FreeCAD.ActiveDocument.openTransaction("Type conversion")
                 nobj = makeCell()
-                Commands.copyProperties(obj,nobj)
-                FreeCAD.ActiveDocument.removeObject(obj.Name)
-            FreeCAD.ActiveDocument.commitTransaction()
-        else:
+                Commands.copyProperties(sel[0],nobj)
+                FreeCAD.ActiveDocument.removeObject(sel[0].Name)
+                FreeCAD.ActiveDocument.commitTransaction()
+                ok = True
+        if not ok:
             FreeCAD.ActiveDocument.openTransaction("Cell")
             makeCell(sel)
             FreeCAD.ActiveDocument.commitTransaction()
-        FreeCAD.ActiveDocument.recompute()
+            FreeCAD.ActiveDocument.recompute()
 
 class _Cell(Component.Component):
     "The Cell object"
diff --git a/src/Mod/Arch/Commands.py b/src/Mod/Arch/Commands.py
index b0dccb6989..48d4983230 100644
--- a/src/Mod/Arch/Commands.py
+++ b/src/Mod/Arch/Commands.py
@@ -223,7 +223,33 @@ def removeShape(objs,mark=True):
         else:
             if mark:
                 obj.ViewObject.ShapeColor = (1.0,0.0,0.0,1.0)
-        
+
+def mergeCells(objectslist):
+    '''mergeCells(objectslist): merges the objects in the given list
+    into one. All objects must be of the same type and based on the Cell
+    object (cells, floors, buildings, or sites).'''
+    if not objectslist:
+        return None
+    if not isinstance(objectslist,list):
+        return None
+    if len(objectslist) < 2:
+        return None
+    typ = Draft.getType(objectslist[0])
+    if not(typ in ["Cell","Floor","Building","Site"]):
+               return None
+    for o in objectslist:
+        if Draft.getType(o) != typ:
+            return None
+    base = objectslist.pop(0)
+    for o in objectslist:
+        l = base.Components
+        for c in o.Components:
+            if not c in l:
+                l.append(c)
+        base.Components = l
+        FreeCAD.ActiveDocument.removeObject(o.Name)
+    FreeCAD.ActiveDocument.recompute()
+    return base
     
 # command definitions ###############################################
                        
@@ -242,11 +268,11 @@ class _CommandAdd:
         
     def Activated(self):
         sel = FreeCADGui.Selection.getSelection()
-        host = sel.pop()
         FreeCAD.ActiveDocument.openTransaction("Grouping")
-        addComponents(sel,host)
+        if not mergeCells(sel):
+            host = sel.pop()
+            addComponents(sel,host)
         FreeCAD.ActiveDocument.commitTransaction()
-
         
 class _CommandRemove:
     "the Arch Add command definition"
diff --git a/src/Mod/Arch/Floor.py b/src/Mod/Arch/Floor.py
index e0217b1f3a..ac256f950d 100644
--- a/src/Mod/Arch/Floor.py
+++ b/src/Mod/Arch/Floor.py
@@ -52,24 +52,20 @@ class _CommandFloor:
         
     def Activated(self):
         sel = FreeCADGui.Selection.getSelection()
-        transmode = True
-        if not sel:
-            transmode = False
-        for obj in sel:
-            if not (Draft.getType(obj) in ["Cell","Site","Building"]):
-                transmode = False
-        if transmode:
-            FreeCAD.ActiveDocument.openTransaction("Type conversion")
-            for obj in sel:
+        ok = False
+        if (len(sel) == 1):
+            if Draft.getType(sel[0]) in ["Cell","Site","Building"]:
+                FreeCAD.ActiveDocument.openTransaction("Type conversion")
                 nobj = makeFloor()
-                Commands.copyProperties(obj,nobj)
-                FreeCAD.ActiveDocument.removeObject(obj.Name)
-            FreeCAD.ActiveDocument.commitTransaction()
-        else:
+                Commands.copyProperties(sel[0],nobj)
+                FreeCAD.ActiveDocument.removeObject(sel[0].Name)
+                FreeCAD.ActiveDocument.commitTransaction()
+                ok = True
+        if not ok:
             FreeCAD.ActiveDocument.openTransaction("Floor")
             makeFloor(sel)
             FreeCAD.ActiveDocument.commitTransaction()
-        FreeCAD.ActiveDocument.recompute()
+            FreeCAD.ActiveDocument.recompute()
         
 class _Floor(Cell._Cell):
     "The Cell object"
diff --git a/src/Mod/Arch/Site.py b/src/Mod/Arch/Site.py
index 5cd59cf52d..8ab58052f8 100644
--- a/src/Mod/Arch/Site.py
+++ b/src/Mod/Arch/Site.py
@@ -49,25 +49,21 @@ class _CommandSite:
         
     def Activated(self):
         sel = FreeCADGui.Selection.getSelection()
-        transmode = True
-        if not sel:
-            transmode = False
-        for obj in sel:
-            if not (Draft.getType(obj) in ["Cell","Building","Floor"]):
-                transmode = False
-        if transmode:
-            FreeCAD.ActiveDocument.openTransaction("Type conversion")
-            for obj in sel:
+        ok = False
+        if (len(sel) == 1):
+            if Draft.getType(sel[0]) in ["Cell","Building","Floor"]:
+                FreeCAD.ActiveDocument.openTransaction("Type conversion")
                 nobj = makeSite()
-                Commands.copyProperties(obj,nobj)
-                FreeCAD.ActiveDocument.removeObject(obj.Name)
-            FreeCAD.ActiveDocument.commitTransaction()
-        else:
+                Commands.copyProperties(sel[0],nobj)
+                FreeCAD.ActiveDocument.removeObject(sel[0].Name)
+                FreeCAD.ActiveDocument.commitTransaction()
+                ok = True
+        if not ok:
             FreeCAD.ActiveDocument.openTransaction("Site")
             makeSite(sel)
             FreeCAD.ActiveDocument.commitTransaction()
-        FreeCAD.ActiveDocument.recompute()
-
+            FreeCAD.ActiveDocument.recompute()
+        
 class _Site(Cell._Cell):
     "The Site object"
     def __init__(self,obj):