A good majority of homes have attached garages these days. People use them for everything! Modern garages have evolved to the point that a recent straw poll indicates a mere one quarter of homeowners are able to park a vehicle in their garages. About half say they can squeeze one car in, and the other quarter is somehow able to park multiple vehicles there. Most folks use their garage space to store personal items. Other uses run the gamut, from hobby shops to rehearsal studios to a budding dot-com headquarters.
My rule of thumb has always been, if you want to park one car in the garage, you need a two car garage, and two cars needs a three car garage... etc. Junk, errr... I mean "stuff" is almost always prevalent in most garages. Regardless of how you use your garage, parking vehicles, or storing items (stuff), Stuff like paint, gas for the lawn equipment, small propane tanks for BBQ, cleaning solutions, paper or clothes... if a fire starts in your garage, you do not want it to spread to the attached house. Additionally, if you start up your vehicle in the garage, you do not want carbon monoxide entering your home.
Fire separations are required between residences and attached garages and their attics, but are not well defined by the prevalent building codes. These are often not constructed correctly, do not perform as intended, and constitute health and fire safety hazards for homeowners, especially where the homeowner modifies the space. Curiously, there is no mention in the prevalent building codes regarding gases or vapors other than CO. Even when properly configured, given the lack of clarity and comprehensiveness of the pertinent building codes (such as the International Residential Code or IRC), safety is often not maximized. So some common sense must prevail!
One example and in my experience, the biggest problem area is the entry door between the garage and the residence. The IRC (International Residential Code) 302.5.1, states that a fire-rated door or its equivalent is required.
R302.5.1 Opening protection. Openings from a private garage directly into a room used for sleeping purposes shall not be permitted. Other openings between the garage and residence shall be equipped with solid wood doors not less than 13/8 inches in thickness, solid or honeycomb-core steel doors not less than 13/8 inches thick, or 20-minute fire-rated doors, equipped with a self-closing device.
I have seen people cut holes in the door to install a pet door. Yes, send your pet out into the garage to breathe in some Carbon Monoxide, or gas vapors! Cutting or modifying the door like this destroys the integrity of the door and is dangerous! Letting vapors into the home, or a fire to easily spread throughout the home quickly. In the case of residences that have natural gas water heaters in the garage, there is already a flame in the garage in the pilot light.... waiting for fuel to ignite. That is why it is required to be installed 18" above floor height. If there are any vapors present, vapors are heavier than air and will run across the floor in the lowest area and sit, unless there is enough ventilation to let it escape to the outdoors. Garage ventilation is imperative!
So, before you close in half your garage space, think about the consequences. All holes in the drywall, both ceiling or walls should be sealed. Any opening to the attic must be sealed, even if it's pull down stairs, the door should be sealed when it's closed. If you're not sure, get some professional advice from a qualified contractor or your local handsome Home Inspector before the hammer and saw comes out. It could mean yours and your families lives!