Castle Court, a penthouse apartment constructed in the 1990s, was in need of redevelopment to make the most of its panoramic views over London. At the heart of David Snell Architects’ renovation scheme is an inventive roof extension that maximises and reimagines the existing external space. While the original building form is retained, the reworked ‘cupola’ now employs large, curved sliding doors to maximise views out, and blur the distinction between inside and outside. The judges complimented the transformative qualities of the project, including the technically impressive, curved sliding panoramic doors, which have given the tired apartment a new lease of life.