Make do and extend: Space-savvy homeowners are changing the face of the urban extension

It's time to build up, not out, says Oliver Bennett

Onwards and upwards? The space above the nation's homes could be the final frontier for the its homemakers. New research from the Royal Institution of Chartered Surveyors (RICS) shows that the "don't move, improve" equation is alive and well, due to a combination of a stagnant market, high prices in some areas and the financial and emotional aggravation that comes with changing mortgages and moving. As a result, the nation's householders are building: downwards, sideways and upwards.

We've had the "banker bunker" (basement extensions)on the fashionable streets of west London and now, from Bradford to Basingstoke, we are seeing the rise of the people's penthouse. And why not? Building up is a way to create new rooms, especially bedrooms, (the most sought-after addition, according to the survey by RICS), and few can deny that there's something uplifting and even transcendent about the rooftop view; an eyrie from where the world can be safely observed. "Whether it's to accommodate expanding families or change lifestyle, homeowners have become aware of the potential for expanding upwards," says Tony Mulhall, director of RICS. "When you need more space, that neglected loft or roof begins to look very promising. In some locations I've even seen the entire pitched roof structure replaced."

In US cities the idea of building upwards is commonplace, and the notion of "air rights" – that is, the right to develop in the empty space above a property – is often part of a property deal. Elsewhere, in vertiginous cities from New York to Hong Kong, rooftop extensions, although often of dubious legality, attest to the need for urban space.

Now British towns and cities are slowly starting to think upwards. "Roofs are probably the most common type of extension in central London, where gardens are small and ground-floor extensions are limited," says Blaze Stojanovski of loft developer Blaze and Co. They may be the best way of gaining the space you need. "Developing a roof is generally a lot cheaper than moving, as there's no stamp duty or estate agent's fees to gain the same amount of space," says Hakan Olsson, director of rooftop extensions developer First Penthouse. Plus, you get an interesting and often light-filled feature.

Still, we don't think upwards enough, and are often hamstrung when we do. In the 1960s and 1970s there was a flurry of extension building on top of Victorian and Georgian terraces, but that has largely been curtailed by local authority planning departments, who wish to maintain the continuity of the street elevation. "The planners are looking for precedent," says Stojanovski. "So if your neighbours have built up in the past, you'll stand a better chance. If not, they'll want to keep the roofline as it is." To illustrate this, Stojanovski knows of two nearby and similar streets in Fulham, one of which has been built on and allows rooftop extensions, while the other hasn't and doesn't.

"Planning has become a lot more difficult," says Olsson, whose company created a rooftop extension for Carol Thatcher in her Southwark loft. "It's about issues such as daylight, views and having to work with the 'story' of the neighbourhood. Generally speaking, you should have a better chance on a 20th-century structure than a more historic building."

Nevertheless, if you crack these criteria and present a good case, you might be able to build up. Architect Jonathan Darke of the firm TP Bennett managed to put a third storey on his Victorian house in London's Stoke Newington.

"Hackney [the local authority] was open-minded and perhaps we were lucky," he says. "But I do feel that, at the moment, if you have a convincing case and a discreet design, then you might be able to build as planners are looking for density gain."

One of the chief contentions is the 'ridge' – that darned roofline – and it will help your planning application if the new storey isn't visible from the street.

Darke recently designed a rooftop extension in Bethnal Green that hid behind a parapet, a feature of many Georgian and Victorian properties that is actually rather a good base for a rooftop extension.

"For obvious reasons the most suitable roof is flat," says Stojanovski. The "London roof" – a shallow valley behind a parapet – will work, but with a pitched roof it would be better to build your space within, like an attic conversion.

Even in less historically sensitive suburban areas, there are issues to consider. "Gone are the days when a bulky box extension could suddenly appear on the front of a modest semi-detached house," says Mulhall. So if you're planning, think discreet but also bear in mind that Government policy aims to achieve sustainability by getting more people to live in the city. "Many local authorities want increasing density and also emphasise the importance of providing of family-type accommodation," adds Mulhall. "In many developed areas the only option may be a loft conversion or roof level expansion." Put that into your argument to the planners.

So, how much does it cost? "The higher up your extension, the more expensive it is because of the need for cranes, scaffolding and health and safety compliance," says Stojanovski, who estimates that a rooftop extension will cost about £170 to £200 per sq ft, with an extra 10 per cent more for each extra storey. Meanwhile, Ollson of First Penthouse estimates costs upwards of £250 per sq ft, but adds that building might be easier than one assumes. "There will probably be some demolition, and you might need to move services," he says. "Plus, you'll need to check the existing load-bearing structure and you'll normally need a means of escape, too."

On the plus side, the work will often be of negligible impact on neighbours and, depending on the scale, you might not have to move out yourself. There is also good news for those living in smaller towns and rural areas. "Planning for rooftop extensions is a lot more relaxed outside London," says Stojanovski. "But it's also less commonplace. After all, there's more space and it's easier and cheaper to build on the ground."

Therefore rooftop extensions tend to be more common in high-value urban areas, where space is at a premium. But costly as they may be, the dug-out basement is still the most expensive way to extend.

And there's no reason why building up shouldn't be done in rural or regional locations. Apropos Conservatories created a roof extension in a period property in Lancashire across an unused balcony: a contemporary addition to the traditional stone. "The original building looked disjointed with an odd balcony," says Paul Schofield of Apropos. And architects Kraus + Schoenberg gave a Victorian warehouse in Bradford a rooftop that levitates the building from the brick below.

Sometimes, a rooftop extension gives the whole neighbourhood a lift.

Roof extensions: A builder's guide

* Look at your local authority's policy on expansion through loft conversions or other alterations at roof level. Policy can change either way, but with the trend towards districts becoming designated as conservation areas, it could impact on the permissibility of these extensions.

* Think usability and building regulations. Is there adequate head room in the loft space and can the access and fire-safety requirements of the local authority be achieved?

* Make sure your house can take it. Loft and roof spaces were not designed to take the increased load associated with occupying the space, so additional strengthening will be required.

* There may be disruption. Make sure you contact your neighbours to discuss your proposals well in advance.

* In a flat or warehouse conversion, make sure you have the right to extend upwards as permission from the freeholder will be required.

* While planning regulations may have loosened under the Coalition, compliance with building regulations is required, including increasing standards of thermal insulation.

Useful contacts

* First Penthouese (ww.firstpenthouse.co.uk)

* Apropos Conservatories(www.apropos-tectonic.com)

* Kraus + Schoenberg (www.kraus-schoenberg.com)

* www.planningportal.gov.uk