Under the new regulations that came into effect from 1 October 2008 the permitted development regime covering garage conversions changed and planning permission is not usually required, providing the work is internal and does not involve enlarging the building.
As a result it has become an increasingly more popular and cost effective way to increase the living space of your property.
Depending on the garage you have, various options may be available to you. A single garage will add around 150 square feet of floor space to your home. With a double garage you may either convert the whole area, or just the nearest side to your house. If you convert it all, you can expect to add around 300 square feet of floor space to your home.
A tandem garage might be a possibility. As before, converting only the back will leave you with the best of both worlds.