The door between the garage and the living area of the home is generally required to be a proper fire-rated door. Since many fires start in the garage area, having a pet door in the door to the house may allow a fire in the garage, to spread more rapidly.
Inspectors will often refer to one hour fire rated doors. Most building departs consider a solid core wood door (not a hollow core wooden door) 1 and 3/8 inch thick, or a 1 and 3/8 inch thick honeycomb metal door, or a 20 minute fire rated door; as sufficient to meet this requirement. Once a doggy door or a glass window is put into the door, it loses its fire rating, unless the doggy door or glass is fire rated for this use. Most are not.
Fire rated doors will sometimes have a metal plate on the hing edge, indicating the door is fire rated. However, many doors conform to the fire rating requirements, but do not have a plate, or tag, on the edge of the door. Another requirement of a fire door is that it be self closing. (READ ABOUT SELF-CLOSING DOOR REQUIREMENTS )
Leave or replace
Many buyers are content with leaving the existing door, especially if they have a pet that would use the door. This is risky because many fires start in the garage. The safest thing to do is to replace a non-conforming door with a properly rated fire door.
The problem for many people is that they loose the pet door doing this. Should a buyer decide not to correct the problem, then at a minimum they should make sure that the garage has a heat detector and properly installed smoke and carbon monoxide detectors in the house.
Who to consult? Door hangers, finish carpenters and qualified handymen.
DIY home owners may install a fire door on their own. Door hangers, finish carpenters and qualified handymen are the most proficient at installing and working on doors.