Semi leaner

Semi-detached suburbia never looked like this before. Twin pitched roofs without their familiar eaves, the same tile cladding stretching up the walls and over the roof, and angled floor plans that encourage occupants to meet neighbours on the front doorstep. These are just the most obvious features that set these houses apart from their antecedents on Acacia Avenue.

Unphased by their innovation, planners have given approval this month for a pair of these semis to be built alongside more conventional-looking new homes on a site in Stevenage, Hertfordshire. They could be the first of many as The William Sutton Trust hopes to repeat the design on other sites if the prototypes prove a winner.

The non-identical pair of a two-bed unit and a three-bedder were designed by London architect Sergison Bates, in response to a competition by the housing association which invited young architects to reinvent the semi. What made Sergison Bates' design the housing association's choice was its innovation while still respecting the public's nostalgic perceptions of semi-detached suburbia.

"The first thing we did was to look at the typical semi-detached house and ask how it should be improved upon, like, if two families are under one roof how could that be articulated more strongly? says Jonathan Sergison, partner with Sergison Bates. Hence the roof itself has a double pitch, while the houses are angled in plan rather than straight.

"The houses are still house-like. Brick and tile are traditions that people understand. We've just contemporised them," says partner Stephen Bates.

Prefabrication plays a big part in bringing the semi up to date. The structure is made entirely from timber panels - the walls should arrive on site fitted with pre-glazed timber windows. The base of the houses' front and side elevations and the ground floor of the rear elevation are clad in brickwork - not traditionally laid but brick slips mounted on panels. The brick slips themselves, however, will be hand-made so the homes retain a crafted feel.

Above the brickwork, walls and roof are clad in the same fibre-cement tiles, each house identified by a different shade of tile. The tiles are fixed on battens, fitted to the timber panels in the factory, and site-fitted counterbattens. The houses have no eaves for the simple reason, Bates explains that, "we don't need overhangs."

On the inside, angled walls and raised ceilings differentiate the design from standard housing association housetypes. At first floor, the two main bedrooms extend upwards into the roofspace with a sharply sloping ceiling; the remainder of the first floor has a lower, flatter ceiling so that some attic storage space can be provided for tenants. "We are giving rooms their own volume and character. There is variety," says Bates. General floor to ceiling height for the semis is 2.7 m but could be up to 3.2 m in the bedrooms.

For the competition entry Sergison Bates worked with structural engineer Ove Arup & Partners, environmental engineer Max Fordham & Partners and cost consultant Capita Beard Dove. Since the project went live last September, the architect has been in a formal partnership arrangement with the client, project manager GHM Project Management, structural engineer The Baldock Quick Partnership, quantity surveyor Philip Pank Partnership, Max Fordham & Partners and contractor Willmott Dixon.

Budget and build methods are being thoroughly examined in the run up to the scheduled summer start of construction. The Trust set a total budget of £130 000 for the project, "slightly more than we'd usually expect," according to chief executive, Michael Morris.

Trade contracts have not been awarded yet, but fabrication and delivery of the timber wall, floor and roof panels, and the unlined internal stud walls, is budgeted to cost £27 000 for the two houses. The budget for site erection is £5500.

For contractor Willmott Dixon, one of the biggest challenges thrown up by the job could be finding roof tilers whose work also looks good on walls.