An incised extension: Singe-family residence near Paris

The house in question, an old one picturesquely situated at the top of a 2,400-square-metre sloping site, is located not far from the historic town centre of Germain-en-Laye in the western environs of Paris. The buyer needed more than the 100 square metres of living space that the building could provide for his large family, yet the grounds, which include a huge old lime tree at their heart, were not designated for development. The task of the architects was thus to resolve this conflict.

Architects: Hertweck Devernois Architectes Urbanistes, Versailles
Location: Saint Germain-en-Laye, Paris, France

The planners came up with a design that practically disappears into the earth, and thus managed to fulfil the client's wish while reconciling it with the need to preserve nature and keep the property unbuilt.  The information that it would barely be possible to see the house extension from public areas or on Google Earth swayed the local planning authority, and a building permit was duly granted.

The design was realised as put forward in the plan. As a result, a single clean incision sweeps along the slope, the earth rising towards the top and falling to the bottom to form a narrow fissure closed by a continuous band of windows.  The glazed façade zigzags in a semi-circle around the old tree for a kind of protected green courtyard situation. The south-west alignment is ideal for the cave-like building in solar heat gain terms while the covering in earth protects its interior from undue heat and cold.

The weight of the reinforced concrete ceiling and the 60 cm of earth above it are borne by 25 slender steel pillars that have been integrated into the 32-metre-long glass façade.  Despite free spans up to 8 metres in length, no load-carrying intermediate walls or further pillars were required.

The entrance to the extension is at street level and dips into the earth next to the garage, leading downwards via a staircase behind which a bathroom, toilet and technical installations room are situated – the only rooms without daylight illumination. The rest of the building is a fluid series of spaces with cooking, eating, living and playing facilities. A staircase at the eastern end leads up to the main house, which is where the bedrooms and further bathrooms of the five-member family are located. 

Project data Origian usable space: 100 m²
Usable space of extension: 320 m²
Structural engineer: Bollinger Grohmann Ingenieure, Paris
Completion: 2013
Costs without VAT: Euro 600,000