My parents did this in two houses. The took the garage and built a wall so that the garage was now only about 8 feet long. The space on the other side of the wall was turned into a game room for us kids by adding carpet and painting the walls. Worked great when we were teens and had friends over.
Check local Building codes first. Not all municipalities allow it. Consider resale value. Sometimes it can lower the property value. If not a concern, William provided an excellent link.