I was reminded this project in my city. For more ideas, I’d google “garage ADU.”

zenbox portland ADU design/build · More Info

Garage has concrete floor?

If you're ever going to park a vehicle in this building, then you need to be able to clear a space for it quickly.

Instead of a standard bed and bedroom furnishings, consider making the wall against which you have the bed's headboard floor to ceiling built ins with a Murphy bed in the center.

You should also make the wall between the great room and the bedroom a solid wall, perhaps with French door. That way, with a draft dodger to help block smells, the living area might not reek of exhaust fumes and/or gasoline or other car smells when you return from vacation to swap the use back from garage to bedroom.

You'll also want to paint the concrete floor of the bedroom so that cleaning it can be done with bleach and a microfiber mop, perhaps a dark gray, rather than try to put down any kind of flooring which would be ruined by automotive fluids.

One idea worth considering, budget, space and building codes permitting, of course: ADD 6'-8' to the back of the building for your kitchen and bathroom plumbing rather than damage the concrete floor of the garage.

Where is the rest of the yard in relation to this garage? If it's on the kitchen/bath side, consider adding a garage door on that side too. That will turn it into a covered patio for parties. Friends of mine did that in a city area of detached homes on very small lots. And they put a deck on the roof and ran a walkway from the second floor of the house to the deck. Very cool.

My father-in-law built exactly such a space nearly fifty years ago. We fondly refer to it as our garage/guest house. The concrete floor has never been sealed and cleaning it is impossible. We plan to get it polished after they replace the roof Irma blew off. That is the other caveat, make sure any ceiling fixtures are high enough to clear the car roof, depending on your vehicle. You will also need to come up with a screen solution for when the doors are open.

I actually love this idea.

The garage door issue can be solved (I can get an R9 door that includes the sealing areas - not just one measurement through the middle) It is not cheap and not easy the more glass you want.

I'd try to eek out a little more room for a sink in the bath. And agree with the Murphy bed idea.

Seal the floor - epoxy it - immediately. Either use a commercial chip or quartz aggregate floor. I've had both in my garage and basement. We've parked a 7700 pound truck on the garage floor for 13 years and it still looks new. Mop it down and it still has a sheen. The chip floor (real kind not "sprinkled" but covered up to an 1/8th inch thick and then clear'd over several time. Take care on making it grippy - especially if you are entertaining (spilled drinks - slipping / falling)

Can you make the entry door a nano wall? (can you use that as just a single door when necessary?)

Also agree on making the doors tall enough and managing lighting - especially if you do a solid garage door.

Make sure the vehicles you plan to have in the structure fit.

Sometimes things planned for dual-purposes do not work well for either purpose.

if it was me: i'd find a different place to park your car while away. it could even be cheaper than what you'd have to do with this to make it work.

I would build up. Use the second story for an apartment and the ground floor garage. I have this arrangement. I rent it out. It makes my house payment. What's not to love?

Build up!

You can also build down! We have a detached "carriage house" ... a 3 car garage with a guest house attached, but we built the guest house with a full basement below. There is a large storage closet and the rest of the space is wired for a home theater, which we don't really need. We have a pool table, Foosball table and a drum set in the space. It has 10 ft ceilings and a large light well that lets in natural light. With lots in the LA area being on the smaller side and the lot prices being very high, it makes sense to maximize your living areas.

I find that garages tend to get quite dusty/sooty and wouldn't want to share living space with my cars.

Pulling a car into a garage to leave for a few weeks while one travels - and not in and out every day should make this a moot point assuming your car is in decent repair. The OP states the the garage use will be rare. Expect that from between the occasional car storage, and the guest use, there will be plenty of open air uses to remove any car smell. A damp mop can remove tire tracks.

Invest in a garage door with windows in it, at least a full row of windows along the top, preferably windows that open.

You might also invest in a screen that opens and closes like a curtain.

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i didnt explain my reasoning for not using it for parking... its not the smell... its every design decision that gets compromised just to store a car once in a while.

Will water come in on the tires? I personally would build up, even if only partially like bedroom and living area.

I have observed in many of my finished garages that spiders and bugs inevitably make their way into the garage space. I'm always having to clean the spider webs from around the corners of the doors. An I mean a lot of spider webs. Garage doors are not constructed or finished to the same degree as regular doors and windows. Add that to the ick factor.