It depends.
It probably is- until you try to sell it!
There are very few buyers (or banks) that will place much value on a building like that if it is the only living space.
Because of that, you may not be able to finance the construction.
It would also depend on whether you were going to heat and cool the garage, and how you insulate.
Where I live, many houses are built on slabs. The ground temperature remains stable enough that it is not necessary to insulate the floor.
Therefore, it is cheaper to build a slab floor than to buy the wood and insulation to build the 2nd floor floor. It would be better for me to build additional square footage on 1 level, and I'd get it back at resale.
This is probably not the case where you live.
Scenario #1 (2 story) vs Scenario #2 (1 story):
Assumption: No heat in garage, concrete slab floors on 1st floor
- Roof area will double with #2
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1 will have 1500 SF slab floor + 1500 SF framed floor, #2 will have 3000 SF slab floor
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1 will add 1500 SF fire rated ceiling
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1 will add 1500 SF flooring insulation
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1 will add stair case and exterior entrance
- They will have equal exterior wall areas
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1 A/C load will increase because of floor over unconditioned garage
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1 will likely have more plumbing and less electric wiring
The only thing more expensive in Scenario #2 is the roof area and the land to put it on.
If you pay 10% more to build Scenario #2 (I don't think you would), but take a 20% hit in resale value when it comes time to sell, its a bad idea.
UNLESS you site the building with the intent of your living space becoming a future apartment for a buyer who is going to build a bigger house in front of it.
Sorry if it sounds like I am not answering your question. There is a lot more to it than most people think about.
And anybody who tells you "Houses cost $xxx per square foot to build" is lying to you.