We were commissioned to transform a large run-down flat occupying the ground floor and basement of a grand house in Hampstead into a spectacular contemporary apartment. The property was originally built for a gentleman artist in the 1870s who installed various features including the gothic panelling and stained glass in the living room, acquired from a French church. Since its conversion into a boarding house soon after the First World War, and then flats in the 1960s, hardly any remedial work had been undertaken and the property was in a parlous state. Photography: Bruce Heming