Two churches, one Protestant and one Catholic, have been erected in one building on the central square of a new district of the city of Freiburg, Germany. The building is designed in fair-faced concrete both on the inside and out. The design of the multi-aisle structure makes it possible to move the limiting inner walls of both churches to the side and then use the resulting area as an ecumenical church space.The building, with an overall height of 13 metres, is partly a single-storey structure, progressing to four storeys at times. The length of the building is limited by two 40-metre long walls in a free geometrical form: the walls are slightly “folded“ at irregular intervals and are inclined inwards and outwards. In this geometry, the west wall is double-walled with a wall thickness of two metres. Together with the inclined embrasures, the thickness of the wall creates a marked light for the church area. All the walls, including the interior walls which can be moved on rails, are made from fair-faced concrete. The moveable walls are approx. eight metres high and are moved by electric motors. Each wall weighs approx. twenty-two tons. These four walls were first constructed outside the actual building. With the aid of heavy-duty armatures, they were then hoisted by crane over the outer walls and set on the rails. All the outer walls are made monolithically from lightweight aggregate concrete to avoid any necessary thermal insulating layer. The walls are approx. 40 centimetres thick to achieve the heat insulation values required.The construction firm contracted had the task of creating an almost non-porous surface homogenous in colour despite the high viscosity of the lightweight aggregate concrete and the inclined form panels. Another requirement was to design the breaks in the walls in a single, continuous line over the overall height of the building.