Jonathan Broom, one of the founders of Ecoigo, and his wife Deborah have put everything on hold while he pursues his dream of building a miniature Hollywood Hills-style mansion. They stumble across a scrap of land in north London, but it's fraught with problems and the only way they can complete the £1million home is by sinking half of it six metres below ground. Surrounded on all sides by their neighbours' back gardens, the couple have to dismantle walls and fences and promise to put them back together again, and that's just the start of their difficulties. Kevin McCloud follows their progress.