diff --git a/city_model_structure/city.py b/city_model_structure/city.py
index c7a169c5..02b095f0 100644
--- a/city_model_structure/city.py
+++ b/city_model_structure/city.py
@@ -3,7 +3,7 @@ from typing import List, Union
 
 
 class City:
-  def __init__(self, lower_corner, upper_corner, city_objects, srs_name):
+  def __init__(self, lower_corner, upper_corner, srs_name, city_objects=None):
     self._city_objects = None
     self._name = None
     self._lower_corner = lower_corner
@@ -12,7 +12,7 @@ class City:
     self._srs_name = srs_name
 
   @property
-  def city_objects(self) -> List[CityObject]:
+  def city_objects(self) -> Union[List[CityObject], None]:
     return self._city_objects
 
   @property
@@ -29,6 +29,16 @@ class City:
         return c
     return None
 
+  def add_city_object(self, new_city_object):
+    if self.city_objects is None:
+      self._city_objects = []
+    for city_object in self.city_objects:
+      # ToDo: Check for shared walls.
+      for surface in city_object.surfaces:
+        for surface2 in new_city_object.surfaces:
+          surface.shared(surface2)
+    self._city_objects.append(new_city_object)
+
   @property
   def srs_name(self):
     return self._srs_name
@@ -40,4 +50,3 @@ class City:
   @name.setter
   def name(self, value):
     self._name = value
-
diff --git a/city_model_structure/city_object.py b/city_model_structure/city_object.py
index 67af8767..0e9ca95a 100644
--- a/city_model_structure/city_object.py
+++ b/city_model_structure/city_object.py
@@ -29,12 +29,15 @@ class CityObject:
     self._storeys_above_ground = None
     self._foot_print = None
     self._usage_zones = []
+    # ToDo: this need to be changed when we have other city_objects beside "buildings"
+    self._type = 'building'
 
     # ToDo: Check this for LOD4
     self._thermal_zones = []
     if self.lod < 8:
       # for lod under 4 is just one thermal zone
       self._thermal_zones.append(ThermalZone(self.surfaces))
+
     for t in self._thermal_zones:
       t.bounded = [ThermalBoundary(s, [t]) for s in t.surfaces]
     surface_id = 0
@@ -47,8 +50,11 @@ class CityObject:
     return self._usage_zones
 
   @usage_zones.setter
-  def usage_zones(self, value):
-    self._usage_zones = value
+  def usage_zones(self, values):
+    # ToDo: this is only valid for one usage zone need to be revised for multiple usage zones.
+    self._usage_zones = values
+    for thermal_zone in self.thermal_zones:
+      thermal_zone.usage_zones = [(100, usage_zone) for usage_zone in values]
 
   @property
   def terrains(self) -> List[Surface]:
@@ -186,6 +192,10 @@ class CityObject:
       self._foot_print = Surface(points, remove_last=False, is_projected=True)
     return self._foot_print
 
+  @property
+  def type(self):
+    return self._type
+
   @property
   def max_height(self):
     if self._polyhedron is None:
diff --git a/city_model_structure/surface.py b/city_model_structure/surface.py
index 522ad438..10b17e76 100644
--- a/city_model_structure/surface.py
+++ b/city_model_structure/surface.py
@@ -25,6 +25,7 @@ class Surface:
     self._parent = None
     self._shapely = None
     self._projected_surface = None
+    self._shared_surfaces = None
     self._global_irradiance_hour = np.zeros(8760)
     self._global_irradiance_month = np.zeros(12)
 
@@ -76,7 +77,10 @@ class Surface:
       self._area = self.polygon.get_area()
     return self._area
 
-  def is_almost_same_terrain(self, terrain_points, ground_points):
+  def _is_shared(self, surface):
+    return False
+
+  def _is_almost_same_terrain(self, terrain_points, ground_points):
     equal = 0
     for t in terrain_points:
       for g in ground_points:
@@ -85,9 +89,9 @@ class Surface:
     return equal == len(terrain_points)
 
   @property
-  def is_terrain(self):
+  def _is_terrain(self):
     for t_points in self._parent.terrains:
-      if len(t_points) == len(self.points) and self.is_almost_same_terrain(t_points, self.points):
+      if len(t_points) == len(self.points) and self._is_almost_same_terrain(t_points, self.points):
         return True
     return False
 
@@ -101,7 +105,7 @@ class Surface:
   def area_below_ground(self):
     if self._area_below_ground is None:
       self._area_below_ground = 0.0
-      if self.is_terrain:
+      if self._is_terrain:
         self._area_below_ground = self.area
     return self._area_below_ground
 
@@ -126,7 +130,6 @@ class Surface:
       self._inclination = np.arccos(self.normal[2])
     return self._inclination
 
-  # todo: share wall
   @property
   def type(self):
     if self._type is None:
@@ -139,6 +142,13 @@ class Surface:
         self._type = 'Roof'
     return self._type
 
+  def shared(self, surface):
+    if self.type is not 'Wall':
+      return
+    if self._is_shared(surface):
+      self._shared_surfaces.append((100, surface))
+      surface.shared(self)
+
   @property
   def global_irradiance_hour(self):
     return self._global_irradiance_hour
diff --git a/city_model_structure/thermal_zone.py b/city_model_structure/thermal_zone.py
index 2d33ad02..c9749db0 100644
--- a/city_model_structure/thermal_zone.py
+++ b/city_model_structure/thermal_zone.py
@@ -85,11 +85,11 @@ class ThermalZone:
   def infiltration_rate_system_off(self, value):
     self._infiltration_rate_system_off = value
 
-  # todo: is it well defined?
-  def set_usage_zones(self, usage_zones_list):
-    if self._usage_zones is None:
-      self._usage_zones = []
-      for usage_zone in usage_zones_list:
-        self._usage_zones.append((usage_zone.percentage, usage_zone.type))
+  @property
+  def usage_zones(self):
     return self._usage_zones
 
+  @usage_zones.setter
+  def usage_zones(self, values):
+    self._usage_zones = values
+
diff --git a/geometry/geometry_feeders/city_gml.py b/geometry/geometry_feeders/city_gml.py
index 00e78a1a..57ee84f6 100644
--- a/geometry/geometry_feeders/city_gml.py
+++ b/geometry/geometry_feeders/city_gml.py
@@ -42,7 +42,7 @@ class CityGml:
   @property
   def city(self):
     if self._city is None:
-      city_objects = []
+      self._city = City(self._lower_corner, self._upper_corner, self._srs_name)
       for o in self._gml['CityModel']['cityObjectMember']:
         lod = 0
         if 'lod1Solid' in o['Building']:
@@ -98,9 +98,7 @@ class CityGml:
           year_of_construction = o['Building']['yearOfConstruction']
         if 'function' in o['Building']:
           function = o['Building']['function']
-
-        city_objects.append(CityObject(name, lod, surfaces, terrains, year_of_construction, function))
-      self._city = City(self._lower_corner, self._upper_corner, city_objects, self._srs_name)
+        self._city.add_city_object(CityObject(name, lod, surfaces, terrains, year_of_construction, function))
     return self._city