You need a local architect.

Firstly most garages are not made to support another storey and in many places building over a garage is not even allowed so better check all of that and then hire an architect.

Got a couple of hundred grand? Then go buy a different house. Wasting it on that project won't be give you much return.

Actually you will find most of us do not have expertise doing what you want to do . I would never want a bedroom above a garage if I had a choice so I would be looking at a different idea

Look into adding the master bedroom in the back behind what is probably the m/b now. Making an L shape, you will also gain a room under and keep the same roof height. You can turn the existing room into the walkway, closets and master bath. Then you will maintain the clean line look you have now.

If you are spending this much $$ on an addition, I think the existing garage should then be connected to the house. I personally wouldn't buy a house with a detached garage. That said, I'm not sure a master on top of the garage would be the best option. If you could extend it out the back like woodteam suggested above and connect your house to the garage I think that would be the best use of remodeling money.

Looking at the picture, I think the garage looks small and out of place. I think a second floor on the garage might make the home look more balanced.

I imagine it would be easy to connect the two. I'd love to see a floorplan of your house to give more design input.

We added a master bedroom and bath once. The best part was being able to design my own closet! A good closet is a huge luxury.

I think you should do a combo, either add the master bedroom on the left of the house or on the back to make it a L shape like woodteam5 suggested, then also add an addition to connect the garage to the house, in that addition you could even have a nice mudroom if you have the space. Having the garage attached could add value to you home even if you don't want to move.

I like the idea of the room above the garage and then making a breezeway/mudroom attaching the two. It will make the house appear much more grand and give you the opportunity to give your whole exterior a once-over towards a more striking aesthetic.

As someone else has pointed out, the garage's foundation may not in fact support the second storey, so you might have to build a new garage in order to make this happen.

Posting a floor plan would help us better advise you. I agree with auntthelma about visual balance. Contrary to some comments above, building over a garage is not a problem so long as all code requirements are met. As mentioned, you will need to confirm that the structure of the garage is adequate for a second floor, and upgrade as necessary. You will also need to make sure the new bedroom floor is well insulated since the garage isn't heated.

You would be well advised to hire a local architect to help you look at all the options.

I lived in a tri-level many years ago and 2 of the bedrooms were over the garage. The garage was an attached garage though and not a detached.

Rapp brings up a good point about insulation above a non insulated garage.

When we built our master bedroom, we ended up with the bathroom about a screened porch. We live in coastal New England. Winters are cold. The plumber told us we were crazy.

The carpenter insulated the heck out of the floor and around the plumbing and also added an easy to open section in the ceiling of the porch so that IF the plumbing did freeze, we could easily get to it without damaging the floor or ceiling.

We lived there 10 years and never had an issue with the plumbing. As long as you know your house, you can manage 'quirks' or unusual situations.