Expand to fit

More floor space

Last year, the size of house extension a person can build without planning permission rose to 430 sq ft (40 sq m), a sizeable addition when you consider that a small three-bedroom house measures around 840 sq feet. You have to comply with building regulations which will require, for instance, certain standards of insulation.

Above and beyond

Because of the cost of housing, people are building bigger extensions than they did previously rather than trading up. Instead of a quick, flat-roofed kitchen or diningroom extension, they are opting for two-storey extensions with an extra bedroom as well. As bigger extensions are more visible, people are tending to take more care over the design to help it blend in with the house.

No trespassing

One of the biggest problems with extensions is disputes with neighbours. There have been cases where overhanging roofs have had to be cut back after the extension is built. Even gutters can't overhang a neighbour's property - it's trespassing. When you go for planning permission they will check the plan but not the site boundaries - that is up to you. You need to establish who owns what before touching a boundary wall.

Average costs

Extensions have become more sophisticated and are better designed. They are also more expensive. The average cost has risen from £20,000-£30,000 (€25,400-€38,100) 10 years ago to around £60,000 (€76,200).

Go to the pros

Professional advice, whether from an architect or a quantity surveyor, is essential. You can build an extension without professional help but you could run into problems, not least when you come to sell the property. The purchasers' solicitor will look for a Compliance Certificate to show that the extension complies with building regulations and planning requirements.

Add all the costs

People usually budget for between £750 (€952) and £2,000 (€2,540) a sq m but it is difficult to talk about averages. The mistake people make when considering how much an extension will cost is to just think of the structure itself but you need to look at other costs, such as changes to the walls linking the house to the extension, relocation of toilets in the house, perhaps, and redecorating. Generally, a single-storey extension will cost around £1,100 a sq m.

The budget

You need to get a proper budget and consider everything from new kitchen fittings and flooring to drainage alterations. It is not always best to go for the cheapest quote until you're sure you know exactly what's been quoted for. Quotes will differ for various reasons. One is that it is difficult to get contractors at the moment - you might find someone who is finishing a job and has time before starting the next one, so he might be keen to get yours. Another builder might be able to fit the work in but is not pushed about getting the job so will quote a price that will earn him a sizeable profit; someone else may not want the job at all and give a huge quote.

Eye on detail

To help save money you have to consider in detail what you're going to do. If you change your mind during building work, costs will rise. You need to think about built-in wardrobes, fitted kitchens, redecorating the rest of the house, new furniture, carpets and curtains. You need to position all the sockets, light fittings and radiators before work begins. If a builder has to move them it will cost you extra.

Specify all specs

Another reason sums vary is that builders may be quoting for slightly different things. If they're quoting on an architect's drawing, or on specifications that aren't thorough, the builder could add extras on at the end of the job, for everything he put in that wasn't specified. So you have to make sure each builder is quoting for exactly the same work. You need to go through everything with the builder and architect and then confirm it all in the contract. You should also get a building programme.

Architectural gems

While architects will charge you around 10 to 15 per cent of the overall building cost, they may actually save you money. They are used to working to a budget so you can tell them what you have to spend. Architects can also spot any problems as the extension is being built and have them rectified there and then instead of you finding out that something is wrong after the building is complete.

Do some research

People often use builders who have done work on a neighbour's house but check just what kind of job he did. Talk to previous clients and ask whether the builder returned to fix things and whether he added extra costs on during building.

Extended usability

People often don't get the extension they want even when they spend a lot of money. At the outset you must tell your architect how you use your home so they can create a practical design that doesn't create complicated routes through the house. People buying a house with an extension that is 20 to 30 years old often have to knock it down as it has been badly planned, cutting out light into the existing house and perhaps difficult to get to. It is very important to have any extension as part of the living space. The worst thing people do is plan the extension as a diningroom and put it at the opposite end of the house to the kitchen, which means you have to carry food through the house.

Let in the light

Glass walls and a solid roof will mean that you will get a lot of light and be able to insulate the roof to prevent heat loss. You should also invest in double glazing. If you're using lots of glass, make sure the structure is bright enough. There's no point in et the extension they want even when they spend a lot of money. At the outset you must tell your architect how you use your home so they can create a practical design that doesn't create complicated routes through the house. People buying a house with an extension that is 20 to 30 years old often have to knock it down as it has been badly planned, cutting out light into the existing house and perhaps difficult to get to. It is very important to have any extension as part of the living space. The worst thing people do is plan the extension as a diningroom and put it at the opposite end of the house to the kitchen, which means you have to carry food through the house.

Let in the light

Glass walls and a solid roof will mean that you will get a lot of light and be able to insulate the roof to prevent heat loss. You should also invest in double glazing. If you're using lots of glass, make sure the structure is bright enough. There's no point in extending the north side of a high building as it will lose heat and get very little light.

East or west

If you put the extension on the south side you may have to protect it from the sun - east or west is best. Plan the garden at the same time - you don't want to step out of it on to a muddy lawn, so put a patio there.

Emma Cullinan is the editor of Select magazine