Rooftop and basement extensions add value to your home

There is hidden value in your home. Sell to a builder who can spot it
Extending up and down has become pretty routine as Londoners seek to carve more living space out of their homes. But now developers are going a step further by buying roof space and installing prefabricated penthouses on the top of mansion blocks.

In less than two weeks, there can be a fully furnished flat on your roof selling for £5million. Leaseholders who initially object are soon won over with tempting upgrades such as a redecorated entrance hall, new lifts, improved landscaping and more parking — all likely to increase the value of their homes.

Penthouse extension on top of a St John's Wood mansion block
Developer Dekra lifted this 3,500sq ft penthouse into place by crane on top of a St john's Wood mansion block

Christian Lock-Necrews, head of Knight Frank's Marylebone office, says: "Rooftop extensions, and extended basements too, are hugely popular in central London, and in most cases the cost should be 50 per cent of the value added. Usually, building on top is most cost effective, because it realises more value per square foot as roof extensions can have views and benefit from natural daylight."

In St John's Wood, four neighbouring mansion blocks are getting taller. Penthouses have been erected on the roofs of three blocks and two more are being added to a fourth.

In Chaldon Road, Fulham, a homeowner has added £50,000 to the value of his £1.25 million house by building a fourth bedroom above an extension, says Barclay Macfarlane of the sales agents, Strutt & Parker. The 10ft by 9ft bedroom cost £25,000-£30,000 to build.

Height of good design

The biggest rooftop extensions are appearing on mansion blocks. Developer Dekra buys roof space and has built 90 penthouses on blocks in London. It erected a huge 3,500sq ft penthouse with a 2,300sq ft roof terrace in St John's Wood. Dekra's penthouses are mostly prefabricated units lifted on to the roof by crane.

This approach is less disruptive than building brick by brick, says Derek Cunnington, managing director of Dekra. Cunnington says leaseholders welcome it when they hear the deal that will benefit them and add to the value of their property.

To maintain aesthetic continuity, planners usually won't allow extra floors to be added to a building unless there are precedents or they fill spaces in a "gap-toothed" roof-line where some buildings are lower than others.

Planners are more willing to permit new basements, because they don't disrupt the look of a building, even though digging them can be disruptive for neighbours and there are concerns about rising water tables.

A £150,000 basement extension in Drayton Gardens, Chelsea, added £275,000 in value
A £150,000 basement extension in Drayton Gardens, Chelsea, added £275,000 in value

Digging down for gold

Another trend is for clever homeowners to tempt developers into a high-priced sale by first gaining planning permission for generous extensions.

A builder in Chelsea has spent £150,000 creating a basement under the front garden of a five-storey semi-detached house in Drayton Gardens. It provides space for a utility room, cloakroom, storage, reception room and bedroom with en suite bathroom and has added £275,000 to the value of the five-bedroom home, which is on the market through Beaney Pearce and Strutt & Parker for £5.95 million.

Hamptons International is offering an unmodernised four-storey house with planning permission for a double basement, at Douro Place, Kensington, for £8.25 million. The agency says the upper basement could become a media room and the lower basement a spa and swimming pool area.

A terrace house Doneraile Street, Fulham, for sale through Savills has a study added to the roof of the first floor, while a new basement has provided a family room, cinema and a fifth bedroom
£3.5 million: A terrace house Doneraile Street, has a study added to the roof of the first floor, while a new basement has provided a family room, cinema (above right) and a fifth bedroom

Go higher and lower

Some houses are having both basements and roof extensions added. Savills is marketing a £3.5 million terrace house in Doneraile Street, Fulham, where a basement containing a family room, cinema room and fifth bedroom has been created. Upstairs, a study has been built on the roof of the first floor, leading directly from converted loft space.

Basement and roof extensions are being added at large redevelopments. As part of its plans to turn the former Port of London Authority Building at Tower Hill into 10 Trinity Square — a hotel, members club and apartments complex — developer Bullet Investments will add two floors to the top of the building for penthouses, and extend two floors downward at basement level to accommodate a spa and car park. The scheme is scheduled to complete in 2015. Prices for apartments start at £3.25 million.

While central London property prices stay at record levels, approximately £1,600 a square foot, expanding up and down has quickly become part of everyday life in the capital.

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