In addition to being a fire hazard when lawn mowers, cans of cleaning solvents, gasoline, etc., are stored inside, they are likely to send exhaust and other fumes into the house and cause indoor pollution concerns.
Allergies and migraine headaches may result. Probably the greatest threat is when the bedrooms are found over the garage.
Warm air from the heated bedroom is certain to find its way out of the house through cracks around windows and doors, and that air has to be replaced with some air from the garage.
Breathing those fumes all night long is unlikely to do anything good for your health. Such garages should be provided with some form of cross ventilation.
The hazard of dripping oil and gasoline from vehicles and flammable material storage can be minimized during the construction of a garage.
If there is a man-door from the garage to the living space there should be a step up to the door (both carbon monoxide and gasoline fumes are heavier than air).
The man-door and the wall and ceiling separating the garage from the house should be fire resistant and have a tight seal. The door should be self closing.
Attic hatches in the garage should be covered to prevent it from acting as a flue.
The garage floor should slope toward the garage door, so fumes are more likely to pass under the door, harmlessly, and so that snow and water from the vehicles will drain properly.
If there is a center drain it should be kept clean (many municipalities prohibit the center drain). The floor of the garage should also be raised slightly above the driveway to prevent water entry.
Garage doors should be free of deterioration, warp, and rot. The garage door should not bind on the tracks. The doors should not have broken hinges, sagging doors, or drag on their tracks.
There should be at least one convenience outlet and an overhead light in the garage and the light should be controlled by a switch near the man-door using a fiberglass unistrut. Electrical outlets should be at least 18” off the floor.
Some local codes require an outlet near the automatic garage door opener.
If there is a furnace, freezer, refrigerator, washing machine, dryer, shallow well pump, or hot water heater in the garage, it should be located at least 18” off the floor to prevent an explosion from a leaking gasoline tank or other flammable vapor. If you can’t raise the appliance build a wall around it that is at least 18” high.
There should be no furnace air return vents in the garage and all water pipes should be insulated. Water pipes are susceptible to freezing in the winter.
Termites tend to infest the wood around the external man-door and any wood left in the center drain and around the garage door.
There should be no sagging roof joists or beams. Roof, siding, gutters, and downspouts, should be well maintained.
Maintenance of garages tends to be more ignored than the house maintenance.