Kathy,

We are currently considering the same thing! We are wanting to keep our overhead low the first couple of years on our acreage.

We have come up with a floor plan for a home where we could build the garage with a second story now and attach the rest of our floor plan later on when we are needing more space. (and maybe have more money?!) :)

I am curious to see what kind of replies you get to your post!

You would first need to check your local building codes to see if it is permitted.

There can be issues with fire and vehicle exhaust safety.

I built the space over the garage for future needs when constructing the garage. The additional cost is well worth the added space.

I built it detached purposely, aliened it for the breezeway (future kitchen extension) and interior entrance to the space.

Since you're talking Apartment, I'm assuming Income space.
Some states insist on two points of egress.
For my own piece of mind, I will have two!

The biggest concern I had was locating the separate entrance (or second point of egress).
I personally do not like exterior stair entrance. I believe it takes away from the over-all look of the home. MHO..
Keep in mind as you design, not only the entrance, but also an escape route.

Second is Plumbing.
I've also always been opposed to running water lines into the room above the garage.
I have found, in most cases, if you're willing to give up one room in the main house (typically the back/spare bedroom) you can get a bath and kitchen in.
In my case, giving up the upstairs bath would not be an inconvenience and turning the spare bedroom into a kitchen (extended over the breezeway) would not only be large, but it's right over my existing kitchen (making the plumbing very easy).
Now, if the space is empty during the winter, I don't have to heat it!

Everyone's situation is different, so what will work for me, may not for you.

Just my sticking points..