You need a local contractor on site to review and price.

Somehow the existing foundation has to be evaluated to see if it is sized to support a second floor.

Adding a habitable second floor to an existing garage involves many considerations:

--As Mark points out, are the foundations sufficient for a second floor live and dead load;

--How will the new supporting floor joists and any needed columns (for second floor point loads) be designed and installed;

--How will the interior of the garage cavity be made airtight and fire resistant to meet code for habitable spaces above the garage;

--Where will the access stair be located--internal or external;

--What are the exterior, interior materials and finishes;

--How will the necessary utilities be routed to the garage and installed in the second floor habitable space--HVAC, plumbing waste and supply, electrical and lighting systema, sub panels, detectors, and signal/communications systems;

--What will the second floor space actually contain--bed(s), sitting, study, studio, kitchen, toilet, closets, special built-ins;

--Will there be special roofing conditiions--dormers, skylights, clearstories, etc.

--Is the roofing asphalt shingles, metal or clay tile?

--Are there any special requirements required to be met by local codes, ordinaces or HOA requirements?

Answer these, add a fee for architectural design and engineering, add a sizable construction contingency percentage, add fees for permits and approvals...and you'll be getting close to a reasonable budget.

Check your zoning to see if they allow plumbing in a detached garage. My town doesn't; they don't want people to add rental or other multi-dwelling units. We couldn't even put a spigot for the hose in our garage wall. Your area may allow it, but you will need to check.

What does an entry level new build house cost in your area? Because that is basically what you are proposing. With a lot more complexity about getting services to it.

I have friends who did this when they did a gut remodel of their house. It was not cheap. And yes, they cannot have any plumbing for that upstairs room. They would have to have had an enclosed staircase to do that. I know - what's the difference. Well, the town building permit dept sees a difference.

Same friend and her parents decided to build an attached addition to the house in which her parents could live in the summer. They could have a bathroom, but no kitchen. It was a single-family zoning area and a kitchen meant another family could live there. Her parents were simply "house guests" with no kitchen in that wing - LR and BR and bath only.

there currently is NO zoning in Houston, TX

We are in the planning stages of adding a second story to an existing similar-sized addition to our home (20x25 on slab). So far based on convos with an architect and several design-build firms, this is not a complicated project once you get beyond the foundation issues (and even that I'm told can be addressed pretty simply .) We're in a very high-cost area and the project would involve renovating 2 existing baths, adding a 3rd, and doing some reconfiguration to the space below the pop-up. The very very preliminary quotes I've gotten so far range from 175k to 400+k, with the addition (including new master bath) representing 120k to 220+k of those totals. We are still talking to other design/build firms, and weighing whether we're likely to do better in terms of time, money, and outcome by finding the right architect and bidding the project out to contractors. Having lived in Houston, I'm sure the costs there would be considerably lower, but I figured I'd share these just for some perspective.

Houston has zoning. Otherwise a bed and breakfast warehouse hostel, or a dog boarding kennel, or nudie bar, could pop up right next door to you in any residentially zoned area. I don't think you understand what zoning means if you think there is no zoning. There are permits and inspections too.

Houston didn't have zoning when I lived there. There may be deed restrictions and there are certainly building permit issues.

If you believe the interwebs, you're both right.

https://kinder.rice.edu/2015/09/08/forget-what-youve-heard-houston-really-does-have-zoning-sort-of

I'll ask: What is your budget?

I'll answer, given that the OP has disappeared. They probably thought that it was going to be a 10-15K "simple" job. Or 10% of what would be a realistic number for a pretty complex job.

I think the first question to ask is: "What is your budget?" I don't waste my time with any advice until I know how much money they want to spend. If they say: "I don't know" then it's a waste of time with a long answer.

Thanks for the insight/questions.

Just curious what the general range would be. Are we talking $25K, $50k, $75k, $100k ,etc. I think somewhere around $85k is most I would pay before reconsidering ways to add on.

I don't know a lot of answers to the questions asked. Staircase would be interior connecting to walkway to the house.

No one can tell you how much it would cost to remodel your garage as you describe until you answer the questions above. If you need a number, figure between $10,000 and $100,000. Could be more, depending...but it won't be less!

Let's assume the foundation is sufficient, standard finished interior/exterior (hardiboard, drywall). The space will just be an open playroom of sorts with one 3/4 bathroom (small with standard sink, toilet, small shower). Roofing will be standard shingle. Nothing special here. Obviously it will have to meet standards to be interior space and HVAC/electrical will have to be run from house to upstairs.

Thanks, good to know. I'm in Houston so building is very very cheap. About 1/4th of the houses in my neighborhood have 2nd story add-ons, so it's certainly possible. Guess I will just need to go through the hassle of having someone out and running through the steps.

You have to do your homework and due diligence. There's no short cuts. Unless money is no object.

I can tell you to do this in canada is between 100-150k for a full suite as we added one onto out garage in our build. Keep in mind that we have 650sq ft living space, 50 sq ft unfinished storage, and a small deck. The bottom triple garage will house the mechanical for the suite and the indoor stairs. The garage itself is 36ftx24ft.

Hi! We are adding a similar garage currently. I would love to see the exterior elevations and know the total height of the building. Can you share?