Is there a hallway on the second floor that dead ends into the exterior wall where this room would go? Meaning is there going to be an easy way to access the new bedroom or will you have to do considerable reworking of the existing rooms? This would add to the expense. Normally I tell people to look at moving before making such a major, expensive change, but if your current mortgage is two points lower than current rates that probably wouldn't be a good idea.

We received an estimate from a design build firm 6 years ago to do something similar except it was over a 3 car garage and the house was a rambler. There was plumbing and electrical in the garage for access and the structure could support it. Bid was $125000 and that included a detailed floor plan and this is for a large metro area in the midwest. Again 6 years ago. We ended up buying a different house 3 miles away with the number of bedrooms we needed for our growing family.

Seek cost information locally.

Just to talk -- does the second story give a nice view - and have a balcony?

is the backyard at all a walkout suite option, with semi-private patio etc.? is the backyard a sun exposure that you'd like?

A gable facing front would instantly give the house much more curb appeal. Cost estimates are totally dependent on your location and the level of finishes you desire. The biggest challenge will be living through the construction.

My strong suggestion would be to hire a local architect to both give you the most efficient layout and also an idea of what the cost will be.

Thank you all for the feedback. We will consult with a local firm to get an idea of what the cost and understand potential ROI to determine if it’s worth it.

"...understand potential ROI..." Additions like this typically do not provide 100% ROI. You should do it because you want to do it.

It Can get very costly. I am working on something similar and is very expensive. YMMV.
1st. Have to get an engineer to se IF modification of foundation is needed.
2- tear 1st floor to add the second story. I stick framed everything, it was much cheaper for me.
3- Get several quotes and see if it makes sense.
4- you usually don’t recoup the money.
The reason it made sense for me was because I don’t have any plans of moving and have over $200k in equity and know a lot of trades, I’ve been managing the job and keeping cost down, But it gets very expensive.

I am attaching pictures of the work I am doing. Hope this helps.

depending where you live make sure to insulate well, garages tend to be cold many in the northeast have to heat the garage to make the rooms above the garage livable

You are unlikely to see substantial if any ROI, so only do this if you really, really want it. Also, bedrooms over garages are notoriously noisy and cold, particularly in northern climates. Insulate for heat and sound!

If you insulate properly, there will be no problem. We have two bedrooms over the garage and neither one is noisy or cold. As for ROI, I agree there are no guarantees, but you should proceed if you can afford it and it will enhance your lifestyle/enjoyment of the house.

We attached a garage and master suite above it. I’m in a Midwest city where our cost of living is probably about 20% lower than say a larger city like Chicago. We also did a little bit of work in the old master because we had to divide it up to create a hallway between the new and old areas (our old master was very large but the space was used strangely). Used higher end materials: marble, Zellige, wainscoting, f and B wallpaper, all unlacquered brass etc. it was 330,000. We were given estimates from 200k - 250k (and we went over budget). We used the contractor who gave us the highest estimate (not because they were the highest, but we felt they were going to do the best job, had experience with older homes, larger company so could probably work faster, etc) you can also ask your bank to appraise what the value of the house will be with the addition if you are concerned about ROI — we actually made 75% based on the appraisal which isn’t bad :) We kind of had to add on if we were going to keep the house because our house had a crazy basement driveway that are bad for all sorts of reasons. I will post a before and after pic. We are eventually going to add a front porch and landscaping so not the final version of our house :) but to us it was a huge improvement! Our first step was getting an architect.