Reciprocal House

Like a game of architectural whispers, Reciprocal House is a response to a response. In 1969, Norman Foster extended a Victorian mews cottage, tucked behind a pub in Hampstead. It was one of his first built projects – a lightweight single-storey structure of steel lattice beams, concrete blocks and large-span glazing. “The owner used it largely as a space to entertain and host parties,” says architect Gianni Botsford. “After he died, the new owner wanted to update the house and appointed us to explore a range of different options.”

Botsford’s response has been to replace the cottage – which was by now fairly rundown – with a two- storey structure within the same volume and footprint, connected to the Foster extension. The new house’s minimal industrial palette – including exposed concrete walls and soffits – is the main “reciprocal” gesture, echoing Foster’s stripped- back aesthetic. But ghostly traces also remain of the original house, not just in the Victorian bricks, which have been repurposed as paving, but also in the angled elevations – a reference to the cottage’s mansard roof.

Botsford began with a forensic analysis of the site, which is surrounded by tall trees and back gardens. The client needed a space that would not be overlooked, while also allowing long views out, and sun and daylight in. “We wanted nature to lead the design process,” says Botsford.

Through a series of physical study models, a formal response began to emerge. The sloping elevations would counterbalance the powerful presence of the Foster structure, while framing views and leaning back from the canopy of the trees. It also became clear that this trapezoidal form should be built from in-situ concrete. “It worked from a structural point of view, because of the angles, and was practical because the site was difficult to access. And it has a very nice relationship with the blockwork of the extension.”

Perhaps counter-intuitively, concrete also helped to draw the natural landscape into the home: “All the windows are oriented very carefully in the skylights, and concrete tempers that light so well. In this climate, it’s also an ideal thermal buffer. It all came together as a solution that made sense.”

Botsford worked closely with structural engineer TALL to optimise the frame. The largest floorplate, which spans 8m across the ground floor, is 300mm deep, but the two upper floors and external walls are just 150mm. Because of the angled pours, they chose a self-compacting mix, with a test carried out first on a 2m x 1m section of the ground floor wall.

The concrete is exposed throughout the interior, and has a quietly crafted appearance. “We wanted it to be a background. There was no attempt to make it legible, or express the bolt holes or formwork,” says Botsford. GGBS has lightened the colour and phenolic-faced plywood has left a smooth finish. Apart from circular downlights in the soffits, there are barely any penetrations in the concrete – sockets are located in floor boxes and wall lights are placed on partitions.

The exposed structure is complemented by bespoke aluminium furniture and fittings, including a central spiral stair. “The house has no corridors. The staircase connects everything.” The staircase and the concrete spaces that it connects are also at the heart of the nature-led heating and ventilation design. The house is all-electric and uses an air source heat pump for heating, but also works in tune with the seasons.

In winter, sunlight pierces through the bare trees to passively warm the structure, slowly releasing that heat into the home overnight. In summer, with the house shaded by branches in full leaf, the spiral becomes a funnel for passing breezes, which circulate over the cool concrete before rising up to the skylight. “With concrete, you expose it to open windows and fresh air, and it just works in a very natural way.”

Interview by Nick Jones

Photos Schnepp Renou