Research conducted for The Sunday Times, however, shows for the first time what precisely will boost an asking price, and in what area of the country. An extra bedroom, for example, could add up to 40% to the value of your home, according to an analysis of house prices in England and Wales.

In London, you can add up to £131,293 to the sale price if you create a fourth bedroom in a three-bed house, while in southwest England an extension that takes a four-bed property up to five adds an average of £63,831.

A typical two-bed home in East Anglia increases in value by £24,642 if a third bedroom is put in. In Yorkshire, expect an extra £52,609 on the value of a home that is extended from four beds to five.

“The big gainers are those who extend from three bedrooms to four, or from four bedrooms to five,” says Richard Donnell of Hometrack, the property consultancy that conducted the research. “That’s because 46% of all homes are three-bed units, so moving above that means you’re automatically entering a sector that has fewer homes but often proportionately higher demand.”

He used data from the Land Registry, which monitors all property sales, plus information from local authorities and about 4,000 estate agents across England and Wales to calculate, on a per-bedroom basis, the extra added to a property’s sale price and the percentage increase in its value.

In northern England and Yorkshire and Humberside, you add an average of more than 40% to a property’s value if you extend from three bedrooms to four. In the northwest of England, southwest, West Midlands and Wales, you can add 30% or more by extending from four beds to five.

Within each region, there will be significant variations. Towns with a high number of larger houses will produce smaller premiums for extended properties, and vice versa.

“Maximum value is added if the extra bedroom is completed professionally, without damaging the integrity of the original bedrooms. But this will not always work in the owner’s favour. It is possible, especially with a smaller two-bed place, to spend too much on an extra bedroom compared to the value it will add to the sale price,” says Donnell. “But when three-bed properties in particular have been enlarged appropriately, they sell for the same price as a home originally built to that larger size.”

There is good news, too, for those hoping to let an extended house. “Providing an extra bedroom is adequate for a double bed, and ideally has an en-suite or an additional bathroom nearby, you’ll get the same rental income as if the property had been built to that size to start with,” says Lisa Hulyer, lettings manager at Savills’ Chiswick branch, in west London.

The Royal Institution of Chartered Surveyors (Rics) says building an extra bedroom, usually accompanied by a small separate or en-suite bathroom, will cost between £15,000 and £35,000 outside London. This will include costs of planning applications and abiding by council building regulations as well as materials, labour and fittings. Adding an extra reception room could take the price up to £50,000, it says. In London and parts of southeast England, the costs are likely to be up to 30% more.

Hometrack’s figures suggest it would not pay to add a third bedroom to a two-bedroom house if you plan to sell soon afterwards. Owners adding an extra bedroom to a three- or four-bedroom home are much more likely to turn a quick profit.

“In all cases, the best plan is to extend into a loft or some other unused space, or to physically extend the property to create extra living areas. Critically, we assume you won’t lose some of the current usable square footage to squeeze in a new bedroom — that may lead to much less value being added,” says Rics.

There are risks if you overdevelop: experts say the full added value can only be achieved if the rest of the home supports the extra bedroom. For example, estate agents say the number and size of reception rooms are critical.

“If the ground-floor living area is designed for a three-bed house, for example, and two bedrooms are added through a loft conversion, it will not necessarily bring it up to the value of the neighbouring five-bedroom property unless the living space is equal in size,” warns Matthew Wender of Kinleigh Folkard & Hayward’s West Hampstead branch in London.

Garden size is also important. “Four or five bedrooms are optimum, whereas properties with more than five bedrooms attract a very niche buyer,” says Nick Evans of Humberts, an estate agency specialising in country properties.

“Properties built as four bedrooms will have larger gardens and would usually command a higher premium than an extended three-bedroom property with a smaller garden,” suggests Evans. “In the context of houses built since the second world war in provincial towns, it would be typical for an additional fourth bedroom to add 10%-15%.”

Alexandra and Simon Young could not afford to move on from their first home, a two-bed Victorian house in Fulham, southwest London. Instead, they extended it by adding a third bedroom with an en-suite bathroom, an expenditure of £35,000 that has added about £66,000 to the property’s value, according to Hometrack.

“In theory, we could have put a fourth bedroom in, but that would have meant sacrificing a big bathroom and may have made the house a little overdeveloped. Although we’re thinking of having children here, we know that in the long-term, when it comes to selling the house, most people wouldn’t look in our street for something with four bedrooms, because the gardens are so small,” says Alexandra, who is studying theatre design.

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