I can also recommend a builder, however you might not need one. There is probably a "backyard builder" in your area that can do the job. Just have your significant other ask around. Usually works our when you point out what you want, then you go buy the material and his crew does the work. A bathroom is a very low level job and we actually renovated one of ours just how I described.
One thing about Thai building is that they don't look ahead, they think nothing of tearing up the wall they rendered 2 days ago to put in a water pipe.