But the result is exemplary. Some architects might have felt their hands were tied by the draconian requirement to faithfully follow the shapes of the old outbuildings, but Charlie Sutherland has exploited it magnificently for his clients, Carol and Philip Thomas. Each of the three parts is linked by glass. The steep slope of the site is dealt with by means of ramps inside - especially, a bravura ramp in the form of a bridge of oak trusses, leading from the entrance up to the first-floor living room. The engineer for this is bridges expert Matthew Wells of the consultancy Techniker. Based on the design of American railroad bridges, it’s not the kind of object you ever expect to find inside a house.

With its deliberately chaotic mix of materials and slightly rough-edged feel, Barnhouse retains an agricultural language that reminds you that this was once not London, but a rural hilltop village outside London. Which is still how the locals see it, and it is certainly how the Thomases, who hail from Clerkenwell, see it. But if they love the site with its amazing views - though feeling a bit bruised by the local reaction to their project - they have nothing but praise for their architects Sutherland Hussey. “They listened to what we said, and they brought things out in us as clients,” says Philip Thomas. “They have a generosity of spirit - no motives other than passion and skill.”

To which Carol Thomas adds: “They understood that what we really wanted was a tree house. It’s a wonderful place for dreaming. We went through many options, but in the end Charlie’s original back-of-envelope sketch was pretty much what we built.”

There are some smaller projects to be found in this year’s awards shortlist - a £79,000 extension to a 1960 house near Chichester Harbour by Foggo Associates is the littlest one on offer , and there is a £180,000 little two-bed new house in London by architects Scampton and Barnett for their own use - but the main range is from £300,000 to nearly £600,000. The house called “Jacob’s Ladder” by architect Niall McLaughlin in the Chilterns, or the Brooke Coombes house in Ealing by Burd Haward Marston - both featured recently in these pages - are £412,000 and £300,000 respectively. Both would be more, were it not for the hands-on involvement of their clients.