The Two and a Half Storey House is a masterwork of three-dimensional imagination in timber and an ingenious solution to a common problem: how to create an extra bedroom in a two-storey terrace house. In the case of the conventional terrace house with a doublepitched roof, a loft extension would have been the obvious answer, but this was not a conventional terrace but part of a two-storey two-bedroom 70s terrace in a former council housing estate in Stoke Newington, London.

The front brick wall rises above the first floor where it is clad with vertical tile-hanging and supports a mono-pitched tiled roof sloping down to the rear wall. The local planners would not permit any loft extension which rose above the highest point of this existing roof and the owners had already received two planning refusals for proposals for a single-storey loft extension which exceeded this height.

The owners, a couple with a young child and a second baby on the way, needed more space but were unable to afford a threebedroom house in the same area. They asked the practice Bradley Van Der Straeten to investigate what could be done to fit another bedroom within these restrictions. The architect approached the design as an interlocking jigsaw, as George Bradley explains: We knew the half height of the loft was fixed so the design was all about creating two interlocking floor levels in the space of one and half floors. We may have given less footprint but we created more volume and an additional bedroom by using it creatively. The whole design of the project hinged on using the ceiling of the bedroom below as a bed platform for the bedroom above, which is spacious and light due to effective use of roof windows. Integrating the bedframe into the fabric of the design allowed space to be freed up for other things such as the communal circulation spaces.

Timber, in particular Finnish birch-faced plywood, was the key material in the achievement of a design which was dependent on very close attention to detail. The timber frame structure is stiffened and lined with birch-faced plywood to create wall linings and ingenious storage spaces. The birch-faced plywood panels link the new loft bedroom with the updated first floor bedrooms and the warm and tactile nature of the birch finish enhances the spaces.

The key to the achievement of the new loft bedroom was the creation of a series of interlocking volumes designed very precisely to achieve the necessary headroom heights on each floor. The section through the upper floors shows the original roof (as a dotted line) sloping down to the rear of the house. The new loft bedroom extends with a very gently sloped roof from front to rear, terminating in a dormer window which looks west over the garden and the surrounding roofscape. Directly below the dormer window is the bed platform, raised 600mm above the floor and rising to a ceiling with exposed timber joists and a headroom height of 1450mm. The bed platform is lined on all three sides with birch-faced plywood and built-in plywood shelves and storage spaces. Other walls of the loft bedroom are lined with shelves and cupboards of birch-faced plywood with exposed edges and the radiator is screened by a set of plywood strip louvres.

The floor to ceiling height of the loft bedroom is 2150mm, a height which has been achieved by lowering the floor immediately below it to a height of 1500mm. This lowered section on the first floor is used for a variety of storage spaces; a large storage room with double doors opening off the corridor, another storage cupboard opening off the half landing to the staircase and a wardrobe which runs the full length of the internal wall to the second bedroom.

At one side of the loft bedroom is a raised plinth which accommodates the 2100mm headroom height to the first floor corridor immediately below it. The plinth is utilised as a bench seat, the base of a built-in wardrobe and as a bookshelf.

The construction of the loft bedroom consists of a highly insulated timber frame with a single ply membrane roof covering and a dormer window clad with roof tiles. Two raised roof windows in the roof, one above the staircase and another above the bed, flood the interior with light. The new timber staircase was built directly above the original staircase flight and is flanked by a solid balustrade of paired 24mm plywood panels. The loft bedroom is separated from the staircase by a fire-rated door and a panel of 8mm toughened fire-rated glass; it gives a visual link to the floor below, allowing parents to keep an eye on their eldest son playing in his new bedroom.

On the first floor the original bathroom has been retained and a new en-suite bathroom alongside has been installed with access from the master bedroom. The first floor cupboards have a timber frame with birch-faced plywood panels and fire-resistant plasterboard to achieve 30-minute fire resistant construction. The ground floor of the house has also been opened up to create a generous living, dining and kitchen area overlooking the garden, with a large sliding glass door which leads out onto an area of timber decking.