Drive down any street - especially in London - at this time of year and you can see scaffolding bristling from the roof of many a Victorian or Edwardian terrace. Like the advent of Wimbledon, loft conversions are a sure sign that summer is here.

Loftspace Specialists, based in Chiswick, west London, says orders have doubled year-on-year since 2002, while Econoloft, which works across the country, says demand is growing at an annual 5%-10%. With many people unwilling - or unable - to move in the current market, the only way is up.

So, why the attic? In most cases, because it’s an empty space filled with the detritus of everyday life. It may be an ideal place to store suitcases, skis and defunct pieces of furniture, but think how much more useful it would be as a bedroom, home office or children’s play-room. At an average of £100 a square foot, a loft conversion is far cheaper than buying more space in the shape of a larger house - and about a third of the cost of digging out the basement. Unlike a back extension, it won’t eat into your precious garden, either.

Nor is it just a London phenomenon.

While the high price of property in the capital means moving upwards makes even more sense there, people all over the country are looking to their roofs. The online property portal Globrix (partly owned by News International, publisher of The Sunday Times), which allows you to search homes by feature, lists 2,670 for sale with loft extensions - 2,308 of them outside London.

So, what can you do with all that space above you? Although most people go for an old favourite - the extra bedroom and ensuite bathroom - this need not be boring. Take the loft extension that Mark Newman, director of Newman Zieglmeier, an architecture firm, and his girlfriend, Natasha Bulstrode, a make-up artist, have just finished above their two-bedroom flat in Kensal Rise, northwest London. They have created a boutique hotel-style bedroom, with a roof terrace, a freestanding resin bath at one end of the room and an enormous plate-glass window with views over the city. “It was quite an effort to get planning,” says Newman, 39, who carried out the work himself, with a skeleton team of builders. “They wanted something a bit more traditional at the front, but we managed to persuade them otherwise.”

Even if you’re not quite ready for all that glass, there are other ways to be innovative. Two years ago, Marcus and Anna Porter-Wright converted the loft of their Victorian terraced house in Wandsworth, southwest London, into a bedroom with ensuite - but, rather than go for a traditional bathroom, they built a wet room with a double shower and heated floors. There is also an enormous storage cupboard. “If you have a family house and no loft, you need somewhere to store the suitcases and children’s shoes,” says Anna, 36, a designer at Anya Hindmarch, the accessories brand.

Or why not put something completely different up there? Kevin Walker, who runs Loftspace Specialists, won the Master Builder of the Year award in 2002 for his conversion of a 660 sq ft roof space in a Victorian family home in Chiswick into an open-plan bedroom, bathroom, sitting room and gym area. It included a neon-lit glass-surround bath, a walk-in wardrobe, a box-in-the-wall fireplace and floor-to-ceiling windows. “Doing a loft extension is an opportunity to create something different,” he says.

So, you have decided to go up. What happens next? First, decide what you’re going to do: are you installing a couple of Velux windows and using the space as it is, or do you want, or need, to expand outwards, with a mansard extension or dormer window?

Before doing anything, establish how much head height you have. If the roof is only 5ft high in the middle, you’re in trouble - unless you lower the ceiling of the rooms below, which will add substantially to the cost. You should also look at the pitch of the roof: the steeper it is, the less room you will have.

Recent changes to planning law mean you no longer need permission for conversions that don’t exceed 50 cubic metres (1,766 cubic feet) for a detached or semidetached property, or 40 cubic metres (1,413 cubic feet) for a terraced house, and don’t go within 20cm (8in) of the eaves – which normally means you can add Velux windows. If you live in a listed building or in a conservation area, or want to build an extension higher than the original roof or one that will change the external appearance of your property, planning permission will be required. Work that affects any wall, floor or ceiling of an adjoining property will need a party-wall agreement. If your scaffolding or materials invade the space above a neighbouring property, you may have to pay for its use. Loft conversions must also meet building regulations, including high levels of insulation, fire precautions and headroom requirements - 2.3 metres (7½ft) in the centre of the room and 2 metres (6½ ft) over the stairs.

So, what else can you do while you’re up there? You could go greener. Building regulations stipulate generous amounts of roof insulation, but go one better by opting for an environmentally friendly material such as Thermafleece, made from British wool, or Warmcel, which uses recycled fire-retardant newspaper. The Energy Saving Trust estimates that the average family could save £110 a year on heating with proper loft insulation. You could also install solar panels: government grants of £2,500 are available for those installing solar panels to generate electricity; if the panels are for heating water, you can get £400.

What about a roof terrace? Councils don’t look on them favourably, largely because neighbours are unlikely to take kindly to you peering down at them while they are sunbathing in the garden. And think carefully before creating one without permission. “Because it’s on your roof, it’s rather obvious,” says Hugo Pace, an architect at Snell David, a London-based firm. “It’s not hard for nosy neighbours to learn you don ‘t have planning permission and make you apply for retrospective planning or take it down.”

Is converting the loft always a good idea? “It’s cheaper to build space than to buy it, so, if you can create the extra space, chances are you’ll be quids in,” says Phil Spencer, the Sunday Times columnist, Channel 4 presenter and director of Garrington, a property-search agency. “Square footage is money - the more you have, the better.”

Recent statistics from the Nationwide show that converting the loft of an average three-bed, one-bathroom home can add 19% to the property’s potential price. It certainly seems to have worked for Newman. The conversion cost him about £120,000, but according to a recent valuation by Foxtons, it has added £200,000 to the value of his home. Crucially, Newman retained the property’s balance, keeping it as two bedrooms and creating a living/dining space where the second bedroom once was.

This is a smart move for people converting houses, too, Spencer says. “If you have a four-bedroom house, you’ve got the living space to support those bedrooms,” he says. “You could probably get two bedrooms out of the loft - but then you’d have a six-bedroom house that would be top-heavy. So, unless you were going to extend the living space, too, you wouldn’t necessarily benefit in value.”

Spencer also warns against spending too much on a loft conversion: the average cost for the work tends to come to £30,000-£40,000, but make sure you don’t end up with the most expensive property on the street, especially in the current market. “Yours might be wonderful, but for an extra £50,000, someone might choose to live somewhere better,” he says.

Be a convert

Check your attic is suitable Not every attic is worth converting. If it is too low in the middle or the pitch too steep, you will be unable to create a decent-sized room without lowering the ceilings of the rooms below - which will cost a lot. Building regulations stipulate minimum headroom of 2.3 metres under the apex and 2 metres over the stairs.

Decide if you want to convert the existing space or extend You can convert existing space of up to 50 cubic metres for a detached or semi or 40 for a terraced property, but if you alter the roof line or change the external appearance of your home (by adding a mansard or dormer window, for example), you need planning permission. Building control will also be involved.

Talk to your neighbours Work affecting any wall, floor or ceiling of an adjoining property will require a party-wall agreement

Calculate the costs A basic fit-out, including a bedroom and shower room, should cost £30,000-£40,000. Set aside about 15% more to cover unforeseen expenditure.

Find the right people for the job An architect or loft company will manage the whole process, from planning and design to selecting a builder. The Federation of Master Builders advises obtaining at least three quotes before starting work. And don’t rely on the cheapest - get a potential builder to provide a schedule of work. A basic conversion should take 4-6 weeks.

Think about going green Converting your loft is the perfect opportunity to add solar panels and improve your insulation.

Don’t try a covert conversion Nosy neighbours are sure to notice - and you’ll need a completion certificate for the work from a building-control officer when you sell the house.

Who else do you need to tell? Keep your mortgage lender and insurer informed, as a conversion could affect your premium.

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