From the street, it's a listed Victorian warehouse. But up on the roof, a weird £250,000 extension is being built to showcase the latest building technologies.
A small extension to a top-floor flat in central London is an unlikely place to trial new technology. Particularly when it is on the roof of a listed Victorian warehouse bang in the middle of a Southwark conservation area. But for multidisciplinary design group B Consultants, the £250,000 scheme has provided an ideal opportunity to experiment.

From the street below, a curved polycarbonate facade will peep over the building's parapet wall, giving the extension a bubble-like appearance. The polycarbonate will act like a mirror, giving pedestrians a reflected view of the sky, "which is the reason the project got planning permission", explains Tom Barker, a director at B Consultants.

A steel structure beneath the flat's floor will transfer the weight of the extension to the building's party walls, and a T-shaped beam projecting upwards from the stairwell supports the weight of the roof. The result: there are no loadbearing walls, a fact Barker has exploited on the flat's rear elevation by using an aluminium-clad cardboard wall panel. Barker describes this as a "cheap and thermally efficient solution".

The cardboard wall is only one part of B Consultants' environmental strategy. Inside the flat, mounted on the chimneybreast, are three large tanks filled with eutectic balls. These will help keep the flat warm in winter and cool in summer. "It's an intelligently programmed storage heater," Barker says.

Construction is set to start in May, and the neighbours have given the proposal a mixed response. "Architects occupy the buildings each side," says Barker. "One wrote to the planning officer describing the extension as an exciting piece of avant-garde architecture, the other wrote saying it was vandalism."

Double-skin coated polycarbonate facade

The facade curves away from the street to minimise the extension’s visual impact from the street below. Polycarbonate was used to keep weight to a minimum, avoiding the need for major structural alterations, and its Saphir coating will protect the plastic from abrasion. Adjustible aluminium shading will help shut out morning sun on the east-facing facade; a roller-shutter will provide ventilation and allow access for cleaning the glazing. T-frame roof support
This will transfer the weight of the roof directly to the stairwell to allow flexibility in the layout of internal partitions. Roof and skylight
A ventilated south-facing skylight will allow the sun to heat the eutectic balls in winter. Aluminium staircase
The staircase that links the extension to the flat is made of 15 mm thick plates of water-cut aluminium. These are slotted together using a mortise-and-tenon jointing system to form a free-standing structure. Rendered mansard walls built on existing party walls
The walls were originally designed to finish perpendicular to the roof, but after a meeting with the planning officer they were redesigned with a chamfered edge to be less obtrusive from the street.

Breathable cardboard wall

The wall is made from a 75 mm thick sheet of corrugated cardboard impregnated with water-resistant solution and clad with a 0.1 mm thick high-grade stainless steel rainscreen. “We put the money into the bit that matters,” says Barker. A twin-walled polycarbonate window will give a view onto the world outside.

Eutectic tanks

Three water-filled transparent aquariums are stocked with polypropylene spheres. These contain a solution that freezes at 16°C and melts at 23°C. This means that the latent heat effect can be used to store surplus energy, making the flat cooler in the summer and warmer in the winter.