The “don’t move, improve” trend has meant that the British have already spent £13 billion in the past year extending, repairing and making over their homes. This race for space has been prompted by the downturn. Homeowners unwilling to accept low prices for their properties have opted instead to increase their size in the hope that the extra square footage will boost value when the recovery arrives.

London has been the site of the most extreme extensions and excavations. Jon Hunt, the former owner of Foxtons, the estate agency, secured permission to dig down beneath his Kensington garden to house his collection of Ferraris. He has, however, been thwarted in his plan to add floors beneath the house.

But homeowners elsewhere also have grand ambitions, such as Harry Wolton and his wife Julie, who turned a poky two-bedroom farmhouse in Herefordshire into a six-bedroom Elizabethan-style mansion on a 46-acre estate. The property, which the couple bought at auction for £290,000, is now 200 per cent larger than its original size and on the market for £2.35 million (via Strutt & Parker: 01584 873711).

Harry, an eminent property QC with an extensive knowledge of planning law, transformed the house incrementally over 20 years. “There was no masterplan,” he says. It now has six reception rooms, the highlight of which is the bright, timber-framed kitchen — part of the original cottage — a two-bedroom “staff” cottage, a one-bedroom guest apartment and several other outbuildings, as well as lakes, herb gardens and a cider press. The project has cost more than £1 million and the couple — who are downsizing now that their children are grown-up — say that, though they love the house, they are not sure they would have started had they understood “quite what we were taking on”.

The Woltons did not, however, seek planning permission, which can be lengthy and expensive. Instead, they relied on “lawful development certificates”, which homeowners can apply for if an extension (on certain properties) has been completed without interference for four years.

Richard Brindley, a director at the Royal Institute of British Architects, says that homeowners should be wary of taking on a project without proper advice. Not many of us have the necessary expertise to avoid planning permission on a project such as this. “The UK has some of the strictest planning regulations in the world,” he says. “Additions that make a property 10 per cent larger are normally OK, but anything more will need permission.”

You will also need planning permission if the house is listed or in a conservation area, and it is more likely to attract attention from the authorities if the area is built-up.

However, Jeff Atkins, of Strachan, the building design consultancy, says that it is becoming easier to make home improvements: “We have had a huge rise in inquiries over the past year, not only because of the credit crunch, but also because a change in planning laws in October made it easier for homeowners to organise simple extensions without permission.”

Creating en suite bathrooms or adding a bedroom remain the most popular — and cheapest — improvements. Prices vary wildly, depending on the people and materials you use. A basic ground-floor extension could cost as little as £20,000, but items such as expensive tiling or rare timber will raise the price straight away. Loft extensions are likely to cost at least £40,000 and basements even more.

If the project is not a simple “knock-through” job, the easiest option is to consult a company that specialises in the alterations you desire. It will sort out the design, build-works and permissions for a fixed fee. If you don’t mind doing a bit more paperwork yourself, hiring an independent architect to draw up your plans could help you to negotiate a more competitive fee with the builders.

Mr Brindley warns against assuming that a particular home improvement will add a certain value to the property: “In London, which has expensive properties and the market for them, a £50,000 loft conversion could add £100,000. But if local buyers have a lower budget, or are more interested in flats, an extra bedroom is not going to be quite as valuable.”

Expansion plans

• Try to check whether an extension will nudge you into a higher council tax band by looking at similar extended houses in your area.

• Complicated extensions can mean living with builders for months, especially if there are lengthy dealings with the council. Most applications to the council take at least five weeks.

• Whatever your budget, allow for another £10,000. Not only do many owners overspend, but there are costs such as building regulations (£500) and planning permission (at least £150).

• Any changes must now comply with energy and environmental regulations, which means that they will be more expensive. However, measures such as improved insulation could bring lower energy bills in the long run.

• Remember your neighbours. They can complain about noise and other disturbances, regardless of whether you have planning permission.

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