The term “mansion flat” was coined in the 19th century to describe the nobly proportioned pieds-à-terre purchased by those whose primary residence was a grand country house. In recent years, London’s multihome purchasers have shown a similar appetite for apartments with stately dimensions, and two recent developments have been notably generous in this respect. At One Tower Bridge(www.knightfrank.com), three dazzling duplex penthouses (one remaining, £14.5m) will not only provide a private view of the Tower of London, but 427sq m of palatial living space. Over on the other side of town, in Chelsea’s The King’s Library (www.knightfrank.com and www.savills.co.uk), developer John Hunter (whose previous contributions to metropolitan splendour include Observatory Gardens, Earls Terrace and The Lancasters) has gone back to school – or rather to an Edwardian schoolhouse – to transform classrooms and corridors into 18 apartments. The double-height show apartment (£13.9m) has soaring ceilings, dramatic entertaining space and a party-perfect west-facing roof terrace.