Plot: Kevin McCloud, Damion Burrows and Michelle Ogundehin visit the final five properties, one of which is added to the shortlist, before the winner is announced. This week they focus on houses that reinvent beloved types of building, including a 21st-century reboot of the classic Kentish oast, a low-key eco-home in Devon which turns the idea of the country house on its head and a cool contemporary reimagining of the suburban family house in Surrey