This block of tiny flats in north London was once the trendiest address in 1930s Britain. Agatha Christie, Walter Gropius and Marcel Breuer all lived here. In the 1990s, only the pigeons called it home. We report on the restoration of a modernist gem
Architectural students across the land are delighted that a derelict block of microflats in Hampstead, north London, is being refurbished. So much so that coachloads of them regularly turn up outside the job in the hope of getting a closer look. The designers of the refurb, Fiona Lamb and Keyvan Lankarani of Avanti Architect, made the pilgrimage themselves when they were at college.

The big attraction is that the block is a grade I-listed modernist gem that goes by the name of the Isokon flats. It was one of the UK's first modernist buildings – and a social experiment in minimal, high-density living. "When you consider the new small pods they are putting in central London, it was 70 years ahead of its time," says Lamb.

Despite its fame, the building was in desperate need of refurbishment. "When we arrived the roof was leaking, water was dribbling down to the ground floor, the concrete frame had failed and there were pigeons nesting everywhere," says Lamb. "It was an awful place."

In 1998, a consortium that included Notting Hill Housing Group, contractor Makers and Avanti Architects won a competitive bid to refurbish the flats. The building's owner, Camden council, was won over by the consortium's proposal to provide housing for key workers while restoring the historic building to its original design. This prospectus, however, presented a challenge, because the compact design of the building's studios and one-bedroom flats relied on custom-made 1930s fittings, whereas the refurbished units would have to accommodate the toasters and home cinema systems that are essential to 21st-century life.

The bareness of the accommodation was part of the philosophy of the original developer–client Jack Pritchard and architect Wells Coates. The flats – marketed as "minimalist flats" – were as small as possible. They were intended as monastic pieds-à-terre for professionals. As many items as possible were built in to maintain the streamlined, minimalistic look – the fittings were designed by Coates and made by Pritchard's plywood factory Venestra.

The sparseness of the individual flats were offset by the lavishness of the communal facilities. Isokon residents enjoyed a whole range of services, such as laundry, shoe-shining and food preparation.

This new approach to living proved very successful, and the Lawn Road Flats, as they were known at the time, became a magnet for artists, designers and writers. Agatha Christie was one famous resident; another was Walter Gropius, founder of the Bauhaus movement, who took up residence after fleeing Nazi Germany. Architect Marcel Breuer, who designed the famous Isokon chair, also lived there and designed the Isobar, a communal kitchen and bar for residents on the ground floor. This in turn attracted other leading lights of the time: painter Ben Nicholson and sculptors Henry Moore and Barbara Hepworth frequented the Isobar in the 1930s.

It is not difficult to see why so many people were drawn there. Externally, the four-storey, concrete-framed building appears sleek and linear – Christie likened it to an ocean liner without funnels. It contained 32 minimalist flats, a staff flat and had two penthouses. Twenty-four of the flats were studios and contained a small bedsitting room, bathroom with dressing room and a tiny kitchen. Residents could eat in the Isobar or have food sent up. The remaining flats were one-bedroom, but the kitchens and bathrooms were barely any bigger than those in the studios.

Remaining faithful to the original layout style has exercised Avanti Architects' grey matter to the full. The main problem was the tiny kitchens; both Lamb and Lankarani felt it important that modern lifestyles should not be compromised by lack of space. This meant standard-sized appliances. "We wanted to use ordinary equipment; the trouble with bespoke goods is if they break then you're stuck," says Lankarani. The answer was to take 300 mm from the dressing room next door and add it to the kitchen.

The flats will be fitted with as many original fixtures and fittings as possible. "A surprising amount of stuff has survived, although it has been somewhat tinkered with," says Lamb. "For example, somebody has put flock wallpaper over some original curved plywood panelling." Fittings such as internal doors and built-in wardrobes will be restored and reinstated.

The other main challenge for Avanti was providing modern services without compromising the exterior of the building. Originally, the building had a communal heating system that had to be replaced in 1974, and the new pipework was not sensitively done. "There were 100 mm wide pipes on the external elevation that went up over the roof – absolutely hideous," says Lamb. "This demonstrated the difficulty of heating the building communally."

The project team first considered electric heating but that would require large water cylinders to store the water. Eventually they decided on individual combination boilers housed in the bathrooms. The snag with this solution was that it required rows of boiler flues sticking out of the rear elevation, which would spoil the building's sleek lines. Avanti has made the best of this by concealing them behind original cast-iron vents.

The most demanding job for Makers was cleaning off the old paint on the concrete exterior. This took a gruelling two months of grit blasting. However, Isokonophiles have a lot to thank the coatings for. "The structure is an extremely thin 100 mm, and it was in very good condition," says regional construction manager Duncan MacLeod. "We put this down to the thickness of the coating – this really saved the building." A lot of effort has been spent establishing the original colour of the building, which, surprisingly, was a pastel rose-petal pink.

New steel-framed windows have been installed throughout the building. Unusually for the restoration of a grade I-listed building, these are double-glazed, whereas the original would have been single-glazed. "We put our case to English Heritage saying people have to live comfortably in the future, so they agreed," says Avanti's Lamb. "A significant factor that helped our case was the fact that we didn't inherit the original windows." Camden council had already replaced those with double-glazed units in 1979.

Fortunately, the original steel door frames only needed cleaning up as, unusually, they were cast in with the concrete frame. "The original developers were innovation-heavy; they wanted everything to be new and modern," comments Lankarani. One innovation that has largely disappeared is the Bricanion partitioning, a kind of steel mesh lathing system suspended on wires. Fixtures such as the fitted kitchens were attached to this mesh and then plaster was applied around them. When the contractor removed the fittings, the brittle plaster just fell apart. Some of the Bricanion partitioning has been preserved, but conventional drylining is used elsewhere. Originally external walls were insulated with a 25 mm thick layer of cork then plastered over – insufficient for modern building regulations. Avanti's solution was to specify thermally lined plasterboard.

The building is due to be finished in June with 25 flats to be occupied by local teachers on a shared ownership basis. Eleven of the flats will be for sale on the open market to help fund the restoration costs. Those who cannot afford what will be a stiff price, or aren't eligible for a key-worker flat, at least get to see inside. Flat 15, once the home of Walter Gropius will be open to the public, and the original garages are being turned into an exhibition space by The Isokon Trust, a body formed to save the buildings. They are hoping to buy flat 15 and equip it with its original fittings. Sadly, the Isobar was converted into flats by Camden council in the 1970s and won't be reinstated.

Anyone with the cash should really try to buy the biggest of the penthouses, originally the home of Jack Pritchard. The owner of this flat will be safe in the knowledge that theirs is the plum property in what will once again be north London's trendiest address – just as long as they can stand the tourists peering through the window …