Kevin McCloud meets Paul and Penny Denby, a couple who have lived on the same exclusive south London estate for nearly 20 years. They are surrounded by pioneering houses built in the Arts and Crafts style in the 1920s, but they're stuck in a dated 1940s mock Georgian home. That's about to change, as they are knocking down their million pound house to build a much bigger contemporary mansion. With its stark white render and huge expanses of glass, it aspires to be an exciting and classic piece of modern architecture, as pioneering in its own way as the original houses built on the estate. But Paul and Penny have hardly even changed the carpets in their old home, let alone built a house before. Soon the challenge of building this complex home starts to bite. Had enough of house porn envy? Robin Gibb off the Bee Gees is tracing his family history over on BBC1 in Who Do You Think You Are, also at 9pm.