More often than not these days, people buy houses that feel too small or not quite right even as they move in. As a nation, we are getting used to less space.

As an architect working on many domestic projects over the last few years, I have found this to be a very positive thing. It is positive because it has forced people to examine a little bit more carefully how they live in their houses. When we look at how we live in our own house, we often find the unexpected. We never use our house in the same way as our neighbours do, because we have a different set of needs, wishes and habits to them.

So many clients show me around their houses when I visit them first and tell me a pack of lies - "here is the diningroom", they say, showing me a room that has seen many more children doing homework in recent years than families dining. They bring me in to show me the garage which is so full of lawnmowers, old furniture and unused exercise machines that it would take two days of hard labour to clear the space to fit a car in to it, and why is the computer always in the spare bedroom? - because visitors like to surf the web while staying in the house?

How we choose to live in houses is not only individual, it also constantly changes, as our situation and society alters. Technology is one driving force - what car needs to be kept in a garage these days? Who would have dreamt that by the year 2000, lots of people would want to, or could afford to have a computer in the house. When we add to this that many people buy houses that were built one hundred years or more ago - when a maid did the cooking, and the children were seen and not heard - we realise that we have to carefully re-examine how the house should be used.

Being comfortable in your house must be the most important thing, but what is comfort? When you sit down, if there is a cushion that you can move to exactly the right place, then you are comfortable. When you get pins and needles in your arm 15 minutes later you can move the cushion.

This is what really drives people to build extensions. The extension is the cushion that they can put in the right place on the chair. It makes the house right for them. But how do you go about effecting this change? Building any thing is a complex, expensive and messy business, having builders in your own home is one of the most stressful things that we have to live through. The first thing to do is to talk to an architect. People are never quite sure what an architect does. I think of architects as being more like doctors for houses than image consultants. You will need to tell them what you don't like about your house - the symptoms - and they should find remedies for the problem that don't cause allergic reactions, and that you can afford.

This stage of planning things with an architect can and should take a while. You must learn to trust each other and get a sense for each other's tastes and ideas. You know that your architect is doing a good job if they are asking lots of questions. Designing extensions is not traditionally one of the more glamorous or lucrative kind of projects that an architect can be involved in, however, by employing a young architect who is starting out in business you can be sure of working with someone who is both enthusiastic and has time to dedicate to your project.

In my practice, I have started to use the analogy of acupuncture to describe my approach to altering and extending peoples houses. In the practice of acupuncture, the traditional Chinese form of medicine, the human body can be healed by the insertion of small needles at particular pressure points. This often replaces the need for surgery. Similarly, we can find pressure points in houses. At these strategic points, through the careful insertion of new structures or by the opening of new connections, we can redefine how a house works or feels. This is always much more economical than wholesale demolition and extensive rebuilding.

IT is an exciting thing to alter or extend your house. You can tailor-make it to suit your habits and lifestyle and if you approach it in the right way, the process of designing it will really inform you not only about your house, but also about your tastes, your interests and your dreams. This equips you to treat your house as an ongoing project that will evolve with you as your needs as a person or as a family changes. As with all things these days, it may be re-invented many more times in your lifetime.

Dominic Stevens is a practicing architect and lecturer in architecture at UCD. His new book, Domestic, is available from bookshops, or directly from the publishers, Mermaid Turbulence (tel and fax: 01-6768570) from December 8th.