Interiors: How to develop a roof of your own

THERE are three basic ways to build upwards: a loft conversion, a "mansard" - a building design named after a French architect which uses the house's original shape but incorporates another layer - or build an extra storey. And there are numerous points to consider before dreaming about sunning yourself on your fifth-floor patio.

First of all, who owns the air rights, or "rights of light"? Usually these belong to the freeholder, and to buy them can cost anything from zero up, depending on how the addition will affect the property's value and how keen the owner of the rights is on the extension.

Next, planning regulations. Submitting an application complete with drawings, then waiting for full planning consent, takes around eight weeks, and may cost up to pounds 1,000 (pounds 1,500 if the building concerned is listed). This includes, if it is in central London, several visits by the district surveyor to check that the work satisfies regulations. The more complex the extension, the greater the expense. Noise and nuisance will be taken into account by planners; you will need to allow builders about 15 weeks to add an extension costing from pounds 30,000 to pounds 70,000, and 10 weeks for something smaller.

Developing roofspace will obviously put more weight on the rest of the building. Which walls will support the weight - and who do they belong to? How strong are the foundations? Adding 10 or 15 per cent to the weight of the building may not affect a two-storey Victorian house, but for those wanting a sixth storey for a period building on the Pimlico mudflats, it may be more tricky. Costs for underpinning foundations start at around pounds 30,000. Any building above three storeys needs a secondary fire escape, which may be built either at the back of the building or through the roof. This could add another pounds 5,000 to pounds 10,000 to the bill, and will probably need planning consent.

Mansards, which utilise the space of a steeply pitched roof and are constructed with a steel frame, are particularly popular in areas such as Chelsea, Kensington and Belgravia; costs start at around pounds 50,000, according to Keith Broadbent of surveyors Bryan Packman Marcel. A loft conversion typically costs less at between pounds 25,000 and pounds 30,000.

Nick Davies, chair of the Royal Town Planning Institute's policy committee, says that no planning permission is needed for converting existing roofspace, although listed buildings or those in conservation areas may need special consideration, and you can't add a dormer window to a house facing a highway.

He adds: "The Government is encouraging people to live in towns to reduce the pressure on greenfield sites, and it's likely there will be a move towards increasing density and encouraging more flat conversions." Car parking facilities and amenities such as gardens are two additional factors to take into account.

When you start the building, you may need permits for access, scaffolding and materials; neighbours need to be consulted, as do any lessees in the house below. Any tenants occupying the basement will be less than thrilled, for example, to find the foundations of the house being dug from under them.

Young architect of the year Niall McLaughlin, who recently developed a roofspace into an additional bedroom for a client in Notting Hill Gate, believes that while a development may not increase the value of a property unless it includes extra rooms, it gives the edge in terms of light and space: "The advantage is that you can bring in light from above. Particularly in London, the best space is at the very top of a house. Most people have one eye looking towards the attic wondering whether something would be possible. People who want to do something for pounds 20,000 or pounds 30,000 are a bit afraid to contact an architect, but there are hundreds of young architects out there who would love that opportunity."

For information on buying a roofspace, contact John Weatherall at auctioneers Andrews and Robertson on 0171 703 2662. For advice on surveying, telephone Bryan Packman Marcel on 0171 834 7899. Niall McLaughlin is on 0171 792 0973. RACHELLE THACKRAY