It looks as if you would have to put a dormer on one side to make it useable at all. It's just way too small unless you are less than 4 ft tall.

Do you mean to convert the auto area in the garage to a habitable room, or do you mean convert the overhead truss space to a habitable room. Your description is confusing.

First step: check with your local jurisdiction to see if your zoning requires two enclosed or covered parking spaces. Don't do anything which would make your house nonconforming to local zoning.

Second: any "habitable room" in the existing truss space is going to be very small and not really usable. More importantly, it will have to be insulated, heated, cooled with electrical power added. Will you need water supply and waste lines as well?

How will you access the overhead space? Will there be interior or exterior stairs?

The expense of all of this makes it very questionable as a cost-effective project, IMO.

You will certainly need an architect or structural engineer to inspect the existing conditions to see if your project is feasible or even possible.

Good luck!

Tear off the trusses and replace with floor joists and roof rafters, and stud walls if you require that much space.

And write a check for $75-$100 grand.