If you are just using the residential calcs, I doubt there is any requirement that you include the utility buildings if there is no fixed in place equipment. (HVAC etc) The general lighting load by square foot exempt them.
That is still not a good "design".
If you look at 220.103 there is some guidance for "farm loads" and if the garage is going to be a shop, you should try to determine what kind of loads are likely to be there.
The trick is trying to take into account "diversity".
It might be sufficient to just put in any HVAC load and add in the 3va per sq/ft to give them enough, assuming no extraordinary loads.

Greg Fretwell