If you want to build an extension to your house, now could be the time to take the plunge.

It has become worth the expense in swathes of the country as the value added to properties in 30 areas now exceeds the building costs.

So, not only do you get more room, but your house will be worth more and you should make a profit on the costs if you sell.

Those in the richer parts of London and the South East have long taken advantage of this as the big price difference between, say, a three and a four-bedroom house made it cost effective.

But in the past year, as growth in property prices slowed in the home counties but accelerated elsewhere, the phenomenon has spread – much to the delight of families in the rest of England who are desperate for more space.

Analysis by the estate agent Hamptons International of Land Registry, Office for National Statistics and Energy Performance Certificate figures identified the 30 districts, the most profitable of which is central Bedfordshire.

A typical extension there will add £57,025 to the value of the average home costing £300,130 – £5,459 more than it cost to build.

In second place, extending the average house in Canterbury, Kent, will add £56,830 to its value of £308,110 – £5,264 more than it cost to build.

Others include Castle Point, Maldon and Southend in Essex, Worcester, Wiltshire, Derbyshire, Craven and Hambleton in North Yorkshire and Manchester. But in some areas, the extra value created by building an extension is only marginal.

Those thinking of extending in Swindon in Wiltshire and Thanet in Kent will be just £66 better off after building costs.

The overall gains in the newly cost-effective areas pale into insignificance compared with the value homeowners in more expensive postcodes can generate.

For example in Kensington and Chelsea, where houses cost almost £2million on average, extending will add value of £212,706 after £45,120 of construction fees.

However, the figures compiled for The Sunday Times show that those living in districts where no value is added by building on to a house can be left with big losses.

In Burnley, Lancashire, where a typical house costs £92,925, residents will pay more than £29,000 for an extension, but will only gain an added value of £17,180.

David Fell, of Hamptons International, said: ‘When the cost of an extension is outweighed by the value it adds, an area tends to reach a tipping point. Change starts to ripple up and down the street, with investors extending as a matter of course, or getting planning permission prior to selling.’

But he added: ‘People extend their home regardless of how much it adds to the value.’