Is it advisable to build a bathroom above the garage? What kind of obstacles might we face if there is a bathroom above the garage in terms of piping, insulation, etc.?
List of issues:
Getting the space conditioned. This may be really easy or insanely difficult. Depends on what is on the other walls of bathroom and where your ducts are in your house.
Getting electric. Should be no big deal.
Getting water. Might be a bigger deal than electric but probably not a huge thing.
Getting exit plumbing. Given that the bathroom is on a second level you have a lot more options but you will probably have to open a few walls to get plumbing to your main stack. Your shower and faucet aren't a big deal. It is your toilet that would be concern. You don't want to run a long solid waste line with low slope.
Venting. Should be easy since you are on second level.
So biggest issues are #1 and #4 for a bathroom above garage. If you get these easily figured out the rest shouldn't be a huge deal.
Generally speaking building anything on top of garage isn't be idea in the world, but I'm guessing that you are in need and it's not because you have nothing better to do, it can be done, and if you ask me it is not more complicated than doing it anywhere else. First and the most important thing: WATER PROOFING and THERMAL INSULATION. You should have two layers of water proofing; one that is protecting the bathroom from "outside" water (humidity etc.) and the other which is protecting everything else from bathroom water. First layer should be placed on top of garage slab, and second one right under ceramic tiles. I have skipped one step, well actually I'm taking it for granted and that is the question whether your slab can support extra weight. After water proofing put some thermal insulation, and cover it with cement screed. Also keep in mind that your walls and ceiling should also be thermally insulated. Then you put some simple roof on top of it cover it with metal sheets or something similar, if possible put some gutters and that's it. When it comes to plumbing (water and sewerage) that could be the biggest problem, put I cannot tell anything about it since I’m not fully familiar with your situation.