If the garage structure is in visually good repair, then you can probably convert. However, if the existing concrete slab is cracked, uneven, heaving, spalled, or otherwise severely damaged, this could add cost to the project. This goes for perimeter foundation as well; it needs to be in condition. Another item is the elevation of the garage floor relative to the main house floor. In some older houses, if the concrete floor is close to the same elevation or even level with the main house floor, you would need to step up into the converted area due to the new floor framing. This may not be desirable for your situation or may not be allowed in your jurisdiction. You will also want to note the ceiling height of the garage relative to the main house floor. Ideally you wan the new floor to be level with the existing main house floor but the garage ceiling may be too low per code to accomplish this. You will need at least 7'-0" clear for the ceiling; there are some exceptions but this is the typical rule the building department will follow.