Adaptive reuse of significant older buildings is not only a more sustainable alternative to demolition and redevelopment, it also enables cities to retain their inherent character. In the luxury residential sector, visionary developers are crafting homes that are brand new inside, with a restored heritage facade – giving buyers the best of both worlds, often in a prime inner-city location. In London, such properties may also come with a rich and storied past – as is the case with Twenty Grosvenor Square, a historic landmark in affluent Mayfair, now home to the world's first stand-alone Four Seasons Private Residences. Once the headquarters of US Naval forces in Europe, where General Dwight D. Eisenhower orchestrated the D-Day landings during the second world war, the property has been transformed into 37 exclusive residences with the high-touch service and amenities of the Four Seasons hospitality brand. Will the worldwide luxury property market slump end in 2020? Behind the retained historical facade of Grosvenor Square – once also home to royalty, prime ministers, actors, musicians and artists – residents can indulge in a lifestyle that incorporates private wine cellars, a spa with a 25-metre swimming pool, a treatment suite with a steam room and sauna, state-of-the-art gym, games room, cinema, business suite, crèche, garden library and private landscaped courtyard. The flats, in three-, four- and five-bedroom configurations, are inspired by the building's historic past and include design features such as elegant classical drawing rooms, soaring ceilings, media rooms, family and professional kitchens, and separate access for staff as well as valet parking. Alex Michelin, co-founder of Finchatton, the property development and design firm which partnered with Four Seasons on the project, says it offers unique possibilities. “Our building has the enviable position of being located in one of the finest and most famous squares anywhere in the world – Grosvenor Square. As such, we felt it vital to honour that heritage and keep the facades of the building, which are so characterful and so in keeping with London and Mayfair,” he says. “Our vision was to keep these fabulous facades but rebuild everything behind them to give our buyers the best of both worlds – the beautiful historic architecture but with all of the latest modern technologies and conveniences inside. We were able to add a couple of floors onto the building to make the most exceptional penthouse flats as well.” Architect Henry Squire, partner at Squire & Partners, also acknowledges the history and character of the building. “Timeless architecture is understanding that a building is part of an evolutionary process that connects the past and future in the present,” he says. “For Twenty Grosvenor Square, the past and future are connected with the creation of a new facade intertwined with a retained facade, through contemporary use of traditional materials, reinterpreting classical proportions and details in a modern way and using current day technologies to reimagine ancient craft.” In another masterful restoration, the largest penthouse flat at Television Centre, the BBC's reimagined former home in West London, has come on the market. Sitting over the top two floors of the historic Grade II listed Helios building, the flagship four-bedroom penthouse provides an expansive 4,068 square feet (388 square metres) of living space, with four outdoor terraces, one providing far-reaching views of the London skyline. Why India’s heritage sites have become hot travel destinations For the interiors, British design firm Waldo Works researched the graphic architecture of broadcasting towers, the curves of sound wave transmissions, and the pixelation of the early televisual imagery, applying them to layouts, patterns and textures throughout the flat. As entrance hall centrepiece, an imposing steel spiral staircase boasts a commissioned paper sculpture, designed by Deepa Panchamia, rising through it. The living room further complements the staircase and the building's shape with a bespoke circular sofa paired with an Anni Albers rug. Three reception rooms on the upper floor lead off the marble-lined Italian cook's kitchen, an internal garden/library space, study and media room. The lower floor sees a range of multiform bedrooms, each one individual. The first phase of this residential-led mixed-use development has delivered 432 new homes, a transformed forecourt with an eclectic line-up of restaurants, cinema, a new office building and White City House – an outpost of private members' club Soho House with a 45-bedroom hotel, state-of-the-art gym, and a public bar. Meanwhile, Banda Property, founded by Edoardo Mapelli Mozzi, fiancé of Princess Beatrice, is putting the final touches to 13-19 Leinster Square in Bayswater, London W2, a boutique luxury residential scheme within a row of Grade II-listed heritage town houses. How a room with a view can heap value on property prices Dating back to the 1850s, the building most recently operated as a hotel before falling into disrepair. Today it's been transformed into a 50,000 square foot development providing 15 flats comprising five maisonettes, eight expansive lateral flats, and two stunning penthouses with sunken roof terraces. And for those who fancy living within a piece of 19th century utility infrastructure, Gasholders London, in King's Cross, may be just the ticket. Built in the 1860s, the decommissioned triplet of Grade II listed cast-iron gasholders now house 145 distinctive flats and penthouses designed by two-time Stirling Prize winning architects WilkinsonEyre. Retaining its industrial aesthetic, the gasholders guide frames and columns have been painstakingly restored by hand. The imaginative design and circular nature of the gasholders results in flats laid out to take advantage of natural daylight and expansive views while central atriums give light to the heart of the structures. This scheme is the chance to live a landmark residence steeped in heritage, with the comfort and ease of a custom-designed modern space. There is a gym and spa, business lounge and entertainment suite with a screening room, dining room and rooftop gardens. Becky Fatemi, director at estate agency Rokstone, says rich mainland Chinese and Hong Kong buyers are keen on homes in restored heritage buildings. “These buyers are very sentimental and therefore love the idea that they are a buying a part of London and its history. They want to retain the character of a 'quintessentially English' property, but with the modern luxuries,” she explains. Modernised properties with the original facades command a premium over newer buildings. Prices have already been driven up, Fatemi says, citing Hong Kong property tycoon Cheung Chung-kiu's £200 million (US$250 million)-plus purchase of 20-bedroom Knightsbridge mansion 2-8a Rutland Gate. Looking for a new home? You’ll have to wait in line Buying guide What you can buy for £7.6 million: The penthouse at Television Centre – excluding furnishings and artworks – the reimagined former home of the BBC in West London. The pinnacle of luxury living, the penthouse is the largest flat to complete in this landmark residential-led mixed-use development, and one of the largest residences in West London. What you can buy for £17.5 million: The starting price for a four-bedroom flat at Twenty Grosvenor Square, a restored historic landmark in affluent Mayfair. Behind the original architecturally important facade, homes are equipped with the latest modern technologies and conveniences, and benefit from the legendary service and amenities of the Four Seasons brand. Want more stories like this? Sign up here . Follow STYLE on Facebook , Instagram , YouTube and Twitter .