For years, every day on his way home from work, Christian Keesing passed a vast empty unit within the foot of the Grade II-listed Barbican tower block he lived in. “I’d probably gone past it at least a thousand times, until I had the idea,” Keesing said. The idea was to buy the four-storey space – which had lain empty for almost 50 years – and transform it into a vast and unique home at the bottom of the 43-storey Cromwell Tower in the 1970s brutalist complex. After years of protracted negotiations with the corporation of London, which owns and runs the Barbican estate, Keesing bought the leasehold to the space – which he has nicknamed The Void – for £200,000 in 2006. It took several more years to win planning permission, but now he is selling the space with approval for a 3,097 sq ft two-bedroom apartment for £3.5m. “When I finally got the keys and walked in, I really couldn’t quite believe it,” he said. “It’s a really huge space – four storeys high – it feels like walking into a cathedral. The first time people see it, they almost trip over themselves.” Keesing hired architects to reimagine the space – which still has reinforcing steel bars (rebars) sticking out of the walls – as a home. He added that it’s actually a house within a 43-storey roof on it. “It doesn’t have the proportion of an ordinary apartment – it is tall and thin, and I wanted a design that gently asserts itself but fits in with the surroundings.” “The incredible volume of a disused space at the foot of Barbican’s Cromwell Tower offers the rarest of opportunities to create one of London’s finest architectural homes,” the estate agent listing says. “The proposed design takes the form of three ‘shelves’ stacked vertically along the length of the Void to create long, elegant living spaces.” The spaces are to be connected by a modern steel staircase “simple in concept and form but spanning an extraordinary distance and scale”. They say the design “embodies a vertical hierarchy strongly akin to the formal arrangement of a London townhouse; the more public spaces of the home will be arranged on the lower levels, and the more intimate spaces will be set out across the upper floors”. The space has been empty since the tower was built in 1973. In the original plans, it was set to be a creche, but the children never moved in. Whoever does move in is likely to be rich, and could well be famous. “We have a stream of people come to look at it,” Keesing said. “They have been from various walks of life but the one thing that unites them is they recognise it is a unique opportunity to put their own stamp on a space within such a famous location as the Barbican.” Keesing said the space had already attracted two offers from prospective buyers, but he had rejected the approaches as too low. Asked to describe the prospective buyers, he said: “There was a couple from the hotel world who wanted to make their own apartment here, there have been some international developers, and a British creative person from the world of music.” When asked if they were famous, he replied. “Well, yes, it is a famous person. They’re someone you would know of their artistic endeavours.” Keesing, a retired solicitor, had planned to live in the space himself but quickly realised it would cost too much to do the conversion work. “But if it doesn’t sell for near-asking, I will fund it myself and then see what happens.” Nick Verdi, of estate agent Savills, said: “The Void Space is truly one of the most exciting development opportunities in the capital. I haven’t seen another space like it in nearly two decades of being an estate agent in London, and the chance to build a home of this size and scale within the Barbican estate is a one-off opportunity. “The landmark cultural enclave is home to an incredible array of interesting residents and has a very tight-knit community owing to its ‘city within a city’ feel. Some are drawn by the architecture, some by the culture and some by the city location, but most become long-term residents. “The Void is a cavernous space with ceilings more than 10-metres high, which is ready to be transformed by a visionary owner into a unique four-storey, light-filled home.”