Why it matters
Older homes, additions, and DIY garage conversions are where this gets missed. A door from the garage into the house without a self closer, unsealed penetrations where wires or pipes pass through the shared wall, or drywall that was never installed on the garage side can all let exhaust and fumes drift into living space, usually into a mudroom or the room directly above the garage. It's rarely dramatic day to day, but it's a real health and safety gap that's inexpensive to fix once you know it's there.
How to spot it
Check the door between the garage and the house. It should be solid wood or steel, not hollow core, and should close and latch on its own rather than needing to be pulled shut. Look at the garage side of the shared wall and ceiling for exposed drywall with taped and sealed joints, not bare studs, insulation, or exposed framing. Sniff for exhaust or gasoline smell in the mudroom, adjacent closets, or the room above the garage, especially right after a car has been parked. Ask whether the garage was ever converted from a different use or added on, since additions are the most common place this gets skipped.
What it costs
Adding or sealing the drywall barrier and swapping in a proper self closing solid door typically runs $100 to $500 for the self closing hardware and weatherstripping alone, and $500 to $2,000 where drywall and sealing repairs are needed, depending on how much of the wall needs to be opened up and finished. It's a modest cost relative to most of the items on this list, and one worth fixing quickly once identified.
What to do
If you notice smell, a missing self closer, or exposed framing on the garage side, ask about it before your conditions come off and budget the fix. It's inexpensive work for a licensed contractor and worth doing before move in day rather than after.
Education and triage, not a home inspection. Casaroo flags missing separation, an unsealed wall, or a non closing door from your photos. A licensed contractor confirms what your local code requires and completes the fix. We flag; we don't inspect.