Costs vary by region of course, but our detached 2+ car garage with interior stairs and upstairs unfinished space was about $35k, about 4 years ago. (That doesn't include the cost of the driveway or any plumbing or HVAC, but we did put a separate electric panel out there in case we want to add more at some point.) So $25k sounds reasonable.

However... be sure to check your local zoning. Ours doesn't allow for plumbing in a detached garage (they want to avoid things like rental units); even a spigot for a hose outside is not allowed from the garage.

There are also considerations like egress (think you'll need more than one exit for the space to be considered safe) and also building an envelope between garage space and living space so that fumes don't carry.

Also, not sure if your garage is currently existing or to be built. Interior stairs will take up some space inside, so space for your car will be shortened by several feet, even if the staircase turns. (We handled this by bumping forward one half of the garage to be a few feet deeper than the other half. I wanted this anyway so that it wasn't one big flat expanse across the front, and it looks nice.)

To be honest, I think you'd be happier with an addition that's part of the house, especially if you'll use it frequently.

I doubt that 25k number is too far off. Unless you want to use it as more of a guest suite, I would add it to your main house. Eliminating the bathroom would cut the bulk of your costs out however. Depending on the size of the garage, you could do it with bonus room trusses. You would not need an elaborate heating/cooling system. A simple single head mini split hp would do the trick. They only run around $2k online.

Detached garages are already much more expensive than attached. Adding in living quarters (IF it's allowed0 will about double the cost. I think the quote is actually too cheap for all of the engineering that this will need. If you want to lessen the costs, attach the garage to the house and save at least 50K over a detached residence garage.

Yeah - we figured that those #'s were very rough estimates.

The living quarters ARE allowed in this development as some other houses have over detached quarters.

However, they ALL have no bathroom & essentially serves as a bonus room.