Having built 2 houses here - my main piece of advice is choose your builder wisely!
Our first house, 5 years ago, 1.7Mill Baht for a 4 bedroom 190m2 house. Not premium, but no problems with this one.
Our current house - 5+ Mill Baht for 3 bedroom 2 living area 2 storey house - about 230m2 living area. With this build, we had to watch our builder (who came highly recommended!) like a hawk. He was constantly looking for ways to shaft us. Cheaper materials, poor building practices etc - but we called him out on most things, and made him rectify issues. Less than 3 months after moving into our house - he was ghosting us! Not a great issue - as we didn't want him working for us anyway! - and in the next 18 months we have spent another 1.5mill repairing/replacing/rectifying his faults! When we built our separate gym/pool room/motorbike house - we contracted our builder from our first house - have had no problems with this.
Don't expect to 'make' money from a house in LOS. Our first house is still sitting empty after 2 years. Thai are not keen on buying second hand houses. Too many Ghosts!! LOL