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Converting a garage may seem an ideal solution if you don╣t have enough space in your home because the structure is already there - but garages are not built to be habitable spaces. They are not usually constructed to high specifications so are often poorly insulated, have no damp-proofing and low ceilings (although in many modern houses the garage does have the same headroom as the rooms in the house). Having a garage converted into a living space will probably mean addressing all these problems as no-one wants to live in a cold, damp and dark room (and building regulations won't allow it, anyway).

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All garages are not the same so the first step is to see what you are working with. In some cases it will be cheaper to knock down the garage and build from scratch. An architect or engineer will be able to assess the condition of the building. "For a start, you'll need to make sure that the garage has adequate foundations," says architect Kevin Mellon, "Garages are supposed to have foundations but I have seen many without them because the builders have taken short-cuts. Sadly, I've also seen extensions without them." If the foundations are poor the solution is to underpin the walls, which can be expensive.

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Because many garages have low ceilings you may need to lower the floor or raise the ceiling, if you want comfortable headroom. Building regulations suggest a room height of at least 2.4m (around 8ft). Lowering the floor is the cheapest option but it will make the floor lower than that in the existing house so you will have to step down into the room. Raising the ceiling will entail raising the level of the walls as well as the roof so this option will be expensive.

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If your garage is attached to rear or side of your house and you are not greatly extending the size of the structure then you won╣t need to apply for planning permission. This applies to domestic dwellings only - you will have to apply for permission if you want to extend an office into the garage. You will also have to apply for permission if the garage is detached from the house (the planning department would be concerned that you may be creating an extra dwelling altogether) and if your property is a protected building. Technically, you are not allowed to build out beyond the existing building line so if you put a large bay window on to your garage conversion you may breach this. While a slight over-run is unlikely to unduly upset your local planning department, it is best to notify them in advance. "While there are no design guidelines in the planning regulations we do encourage a conversion that is in character with the house," says David Dunne, senior planner with Dublin Corporation.

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You won't always have to insulate the room, by law, but it is worth doing if you want a comfortable living space. Many garages will only have four-inch brick walls, plastered each side, which will be freezing without insulation. One way of insulating the wall is with internal insulation held in by foil-backed plasterboard (taking into account that this will reduce the size of the room slightly).

If the roof is to be raised (rather than lowering the floor) then insulation can be incorporated while this is being done, otherwise insulation can be put in between the existing joists above a plasterboard ceiling. Insulation in the walls will also help avoid condensation.

Condensation isn't a problem in a garage because it is unheated and so the wall will be at the same temperature inside as it is outside. If the room becomes a living area, and is heated, the warm air will hold moisture which causes condensation when this hits a cold wall. Cavity walls, used to build houses, usually prevent this problem but, as the existing garage wall will probably be a single, solid one it will need insulation to help prevent condensation.

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You'll need to put a damp-proof membrane on the floor (which costs around £20 (25.39) a metre). If you're lowering the floor then it can go in beneath the new concrete floor. If you are keeping the existing floor then the membrane goes on top of the concrete floor slab and can be covered by a floating floor (such as wooden floor boards on timber battens). A damp-proof course should also be put into the walls to stop any rising damp. In brick or rubble walls this can be done with a chemical injection or the Lectros system (this comprises thin electrical wires nailed along the base of a wall - behind the plaster - which creates a small magnetic field that prevents damp rising). In hollow-block walls damp-proofing is painted on (in what is known as a tanking system). All these methods cost around £20 (25.39) a metre. Companies such as the Damp Store (01 874 5055) can advise on this.

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Building regulations stipulate that rooms should have an opening to the outside of at least one 20th of the floor area to allow natural ventilation (there are some exceptions, such as tiny kitchens, where the opening needs to be larger). In a garage conversion this could be achieved by having a large window where the garage door used to be or by installing a number of smaller windows.

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You need to think about how the new structure will be linked to the existing house. The best place to have the link between the two is into a general area such as a hallway rather than having to access the new room through a living room or kitchen. If the garage is converted into a bedroom, entering it through a utility room won't be very pleasant, but if it is converted into a diningroom, it makes sense to situate it next to the kitchen. If the garage isn't attached to the house you will need to build a covered walkway between the buildings, unless you are prepared to face the elements between the two.

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Pipes from the house's existing central heating system can be extended into the conversion only if the boiler has sufficient capacity - ask a plumber about this. If your garage was used to store lots of family clutter, you will have to consider where this will go. It may mean building a lean-to, shed or utility room as well.

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At all stages you must make sure that your new room complies with building regulations. If you employ an architect or engineer, they will do this for you.

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Depending on the quality of the existing building, the conversion will cost roughly £80 to £120 (101.60-152.37) a sq ft, although if the garage is well built and needs little modification you may get away with £40 (50.79).