Wow, that's a tough one to thumbnail-sketch wins. A stick built home and a stick built gar apartment are virtually the same structure, only varying based on architectural detail. As far as cost, I can only offer that our project (1200 sq ft gar, 1100 sq ft live space) was spec'd by the insurance company as $140 K to replace. Personally, I think that's a bit high and will shop the policy a bit when I have time. Regarding safety, whenever a garage area is attached to a living space, there are specific precautions during construction that must be implemented to protect from fire hazard (ie; fuel tanks in the vehicles). Fire rated sheet rock and taping must be used inside the garage, fire rated doors must be used between the gar and living space. There are other code issues; do your research. The potential for hazard is all based on the reliability of containing potential fire and fumes residing within the garage area.
