Hi @edpmay,
I do not know the exact conditions of the parking garage. If it is well ventilated and you could use natural ventilation (you will need some operable windows) and set the room to unconditioned. If the floor between the apartments and the garage is well insulated, the difference between the two options wil not be that big. When a floor is regarded as a groundfloor the outside temperature will be set to 18 degrees. If you want to mimic outside temperures in the garage you can move the building a bit upward.