Marianne Davys, a London-based Irish architect working in Muswell Hill, is about to build a basement in her home. She got her next-door neighbours onside by designing and digging out their basement first to create the space above. “It’s not often you can say to a prospective builder ‘Here’s one I did earlier,’” she laughs. In her mirror-image semi, the floor-to- ceiling height at basement level will be 2.3 metres. The project will add 18sq m to the ground floor and 28sq m at basement level. The house was originally 78sq m, to which she has already added a 30sq m attic space from where she runs her business. The plan is that in future years, when she retires she can, by closing a fire door to the basement, turn the space into a self-contained holiday let, offering panoramic views over London.