diff --git a/src/db/db/dbCell.cc b/src/db/db/dbCell.cc
index 83acc1182..bda9d71f8 100644
--- a/src/db/db/dbCell.cc
+++ b/src/db/db/dbCell.cc
@@ -92,7 +92,8 @@ Cell::box_type Cell::ms_empty_box = Cell::box_type ();
 
 Cell::Cell (cell_index_type ci, db::Layout &l) 
   : db::Object (l.manager ()), 
-    m_cell_index (ci), mp_layout (&l), m_instances (this), m_prop_id (0), m_hier_levels (0), m_bbox_needs_update (false), m_ghost_cell (false), 
+    m_cell_index (ci), mp_layout (&l), m_instances (this), m_prop_id (0), m_hier_levels (0),
+    m_instances_need_sort (false), m_instances_need_sort_box_tree (false), m_bbox_needs_update (false), m_ghost_cell (false),
     mp_last (0), mp_next (0)
 {
   //  .. nothing yet 
@@ -131,6 +132,10 @@ Cell::operator= (const Cell &d)
     m_prop_id = d.m_prop_id;
     m_bbox_needs_update = d.m_bbox_needs_update;
 
+    //  need to update the instances inst_by_cell_index pointers
+    m_instances_need_sort = true;
+    m_instances_need_sort_box_tree = true;
+
   }
   return *this;
 }
@@ -613,6 +618,8 @@ Cell::invalidate_insts ()
 {
   mp_layout->invalidate_hier ();  //  HINT: must come before the change is done!
   mp_layout->invalidate_bboxes (std::numeric_limits<unsigned int>::max ());
+  m_instances_need_sort = true;
+  m_instances_need_sort_box_tree = true;
   m_bbox_needs_update = true;
 }
 
@@ -700,7 +707,10 @@ Cell::clear_parent_insts (size_t sz)
 void 
 Cell::sort_child_insts ()
 {
-  m_instances.sort_child_insts ();
+  if (m_instances_need_sort) {
+    m_instances.sort_child_insts ();
+  }
+  m_instances_need_sort = false;
 }
 
 std::pair<bool, db::pcell_id_type> 
@@ -744,9 +754,12 @@ Cell::change_pcell_parameters (const instance_type &ref, const std::vector<tl::V
 }
 
 void 
-Cell::sort_inst_tree ()
+Cell::sort_inst_tree (bool force)
 {
-  m_instances.sort_inst_tree (mp_layout);
+  if (force || m_instances_need_sort_box_tree) {
+    m_instances.sort_inst_tree (mp_layout);
+  }
+  m_instances_need_sort_box_tree = false;
 
   //  update the number of hierarchy levels
   m_hier_levels = count_hier_levels ();
diff --git a/src/db/db/dbCell.h b/src/db/db/dbCell.h
index 2d04bc0ba..162b7fe49 100644
--- a/src/db/db/dbCell.h
+++ b/src/db/db/dbCell.h
@@ -1076,7 +1076,9 @@ private:
   db::properties_id_type m_prop_id;
 
   // packed fields
-  unsigned int m_hier_levels : 29;
+  unsigned int m_hier_levels : 28;
+  bool m_instances_need_sort : 1;
+  bool m_instances_need_sort_box_tree : 1;
   bool m_bbox_needs_update : 1;
   bool m_ghost_cell : 1;
 
@@ -1147,9 +1149,9 @@ private:
    *  This will sort the cell instance list. As a prerequesite
    *  the cell's bounding boxes must have been computed.
    *
-   *  @param layers The maximum number of layers in the child cells
+   *  @param force Force sorting, even if not strictly needed
    */
-  void sort_inst_tree ();
+  void sort_inst_tree (bool force);
 };
 
 /**
diff --git a/src/db/db/dbLayout.cc b/src/db/db/dbLayout.cc
index 1790caadc..dc550432c 100644
--- a/src/db/db/dbLayout.cc
+++ b/src/db/db/dbLayout.cc
@@ -1799,8 +1799,9 @@ Layout::do_update ()
       for (bottom_up_iterator c = begin_bottom_up (); c != end_bottom_up (); ++c) {
         ++*pr;
         cell_type &cp (cell (*c));
-        if (hier_dirty () || dirty_parents.find (*c) != dirty_parents.end ()) {
-          cp.sort_inst_tree ();
+        bool force_sort_inst_tree = dirty_parents.find (*c) != dirty_parents.end ();
+        if (hier_dirty () || force_sort_inst_tree) {
+          cp.sort_inst_tree (force_sort_inst_tree);
         }
         if (cp.layers () > layers) {
           layers = cp.layers ();