To build a garage, add an extended driveway to your already existing one and
then add a garage door between the two parts. Next, surround it with walls to
form a room and there you go, you now officially have a garage! Of course,
you can style your garages anyway you want, like in the case you want to make
your garage an underground racing hangout for the slow and sedated! Oh yeah,
the garage door can be entered by sims on foot as well as by car, so you
don't have to worry about building another exterior door to the garage if you
don't want to. If you want your sim to drive their car into the garage, you
must manually place the car within the driveway inside the garage, the next
time your sim drives out, they'll drive back into the garage. Basically, the
car remembers where it is parked, and it'll always be parked there while at
home.