Remember Maxis states there should be only ONE entrance to an apt. there may be private yard space but it must be completely enclosed by some type of fencing, as you have to use a door to get to it. The point being that if you have any other doors besides the apt front door other sims will attempt to use it and it will mess up the lot. Same for garages. I have built mine attached to side of apt with a door from kitchen into garage and it works fine. Remember, visting sims will not use a garage door as an entry, so only the sim who lives in that apt will use the garage door to access the car. If my sim is in the common yard area and I click on the car to tell him to go to a community lot, he will sometimes enter the garage through the garage door instead of going through the apt first-- no problem. AL sees the garage as another "room" of the apt. I can purchase a car after sim has moved in to the apt with no problem. Just remember to place all the appropriate driveway pieces while still building the apts as a residential zone. A bonus! I build the garage wing with a flat roof accessable via a door on the second floor of apt and voila-- my sim has a nice private rooftop patio! By making a little private yard in the back of apt (with completely enclosed fencing) your sim can buy gardening plots to use there. Or if there is enough room on the lot you can attach a greenhouse to the apt for additional plants also.
Just remember that the main key is to never have but one entry door (the apt door). Any other doors that lead outside or to garages must open to completely enclosed areas. Since visiting sims do not regard a garage door as an entry door to apt or home it sort of is a barrier the same as fences and walls.