Finding builders in Thailand does seem to be quite a daunting task, from my experience when they say 'easy no problem' comes with the unsaid 'if we go i can just walk off site and the problem goes away'.
Far various reasons we now have a shit job to deal with. we need to put a shit tank in the gararge and replumb the toilets to it, which does involve installing pupping under the ground floor concrete slab. Personally I would describe it as major renovation... rather than the 'easy, no problem' we have been getting out of builders so far.
Does anyone have any builders or crews in bangkok who they thing might be up to the job?