A well-designed extension with a classy finish could add 10-20 per cent to the value of a property

COSTLY and problematic, adding an extension should not be undertaken lightly, but estate agents agree that a well-executed extension will increase your home's value. Aside from the inconvenience, extensions come with a health warning - a badly laid-out or below standard extension can make a house more difficult to sell.

A well-designed extension with a classy finish could add 10-20 per cent to the value of a property or pull an all-important two or three extra bidders into an auction, says Pat Mullery of Douglas Newman Good.

There is a "strong appetite" in the market for period homes with good extensions which add bright breakfastroom/living areas extending into the garden, says Simon Ensor of Sherry FitzGerald. Nine times out of 10 such an extension will "significantly enhance the value of the property", he says.

He recalls a period residence on Palmerston Road which had a stunning garden-level extension. When sold, the house set new standards for values on the road.

Felicity Fox advises homeowners to be careful not to interfere with the integrity or proportions of a property when extending. Keeping a target market in mind is also important. "You can bring a house up to 186sq m (2,000sq ft) but what's the point if you are aiming at the family market and have sacrificed the garden. You may have four or five bedrooms but where will the children play?"

Getting the experts in from the beginning and paying for a high-spec finish is the only way to go, says Geralyn Byrne of Sherry FitzGerald. "I don't think the public responds to a cheap and cheerful makeover and it certainly doesn't do much to improve the value of a house. People's expectations have been raised to exceptional standards and they are not prepared to take something half done."

People are willing to pay a premium for houses at two ends of the market, she says - either a lovely period house with its features intact with potential or else a home in walk-in condition.

Some sound advice is to think like a buyer and figure out what your house is missing. An open-plan livingroom/kitchen? A fourth bedroom?

A big extension is not always the best way forward, says architect Peter Roberts.

"Very often a smaller extension and re-organising the existing space - rather than banging on a big extension - is best because the remaining rooms will become internal rooms and become alienated from the garden and light."

People make lots of mistakes with extensions, and often huge extensions can detract from the value of the house, he says. Other pitfalls for extensions are dark unlit corridors, which should be avoided at all cost. Extensions that are totally out of character from the original design are risky. The cost of an extension is not cheap and Roberts advises clients to budget for around €3,500 per sq m for an extension.

Allowances must also be made for the cost of work ensuring the extension is properly integrated to the main body of the house, he adds.

In addition, he warns that there is "nothing like an extension to bring out the best in a neighbour". He advises his clients to tell their neighbours about building plans as soon as possible. Most of the time this strategy works well and if it "blows up in your face" at least you know early.