If you don't use the garage for a car it can make a great space for kids to call their own leaving the grown-up rooms for adults. As long as it's warm, well-lit and all harmful tools, liquids and other possible hazards have been removed, it's a blank canvas. To make it child-friendly, lay down rugs and beanbags and put up shelving stacked with easy-to-reach toys and games. A desk and chair are handy additions too.
String system in white with white floor panels, from £72, skandium.com