Onslow Village, Guildford, Surrey

Narrow front to back detached house with sloping, 160 foot rear garden. Large new build basement required under garden with 2 storey extension above, complex roof, new garage, suspended and waterproofed concrete deck and basement view out over the sweeping garden.

The extension alone doubled the size of the property but it was decided that a huge basement could be built to take advantage of the large sloping garden. Built on a hill on chalk soil, the ground was well drained but did suffer from periodic ground water when the runoff from the hill was in full flow. The whole rear of the house had to be completely underpinned and reinforced concrete retaining walls cast to form the basement level. The huge concrete roof to the basement was cast and structurally reinforced to allow the maximum open spans in the basement below. The structural engineering on this job was critical and the great foundation chalk supplies for this type of build definitely helped, but such a dense material expands greatly when it is excavated filling many more skips and trucks than softer materials and the actual excavation process is much harder initially.

Fortunately, there was access for large machinery and this made better work of the tough ground.

Head height ranged from 2.5m to 2.8m and open aspect of the basement to the rear was amazing. The client was a keen gardener and, as an add-on, some spoil was used to re-landscape the garden vista which still sloped down to a fantastic organic vegetable garden.

Everyone pitched in and the client in his spare time helped throughout the build to move tons of materials to the rear of the building site, so the technicians could build the structure up quickly.

Garden basements are a great idea and, where the ground slopes, the iceberg effect of having more below the surface works really well from a planning point of view.

The end use of this basement was as a stand-alone flat that transformed into a cinema and play room.