I am just having a house built and I cannot believe that i fell for the bathroom small door frame trick it is only about 2 inches so no real problem, the builder (bless him) tried to say something about what we are having on the floor was the reason, so i told him whats that got to do with the height of the frame, anyway the wife agreed with him so the argument hasn't finished yet

Most likely the reason: Many Thai houses have a step or threshold of 5cm (about 2 inches) to prevent the water from escaping the bathroom. Most likely a breakdown in communication. They started to do the same on my house, but I caught it early enough to easily rectify.

Saves you the trouble of changing the rotten frame in a year or so when it's not soaking on the water on the floor.

One member post this problem with pictures just few days ago. Take a look to see what might happen.

I have seen many Bathroom Door frames rotten from the floor several inches up!

The frames that are cut short,do not rot,because they are not submerged in water,from semi blocked bathroom drains.

The rot problem is also why many Bathrooms have Plastic Doors.

Saves you the trouble of changing the rotten frame in a year or so when it's not soaking on the water on the floor.

One member post this problem with pictures just few days ago. Take a look to see what might happen.

The frame sits on the bedroom floor not the bathroom floor

Of course this thread made me go and look at our condo bathroom.

The door is indeed about 2" lower than the adjacent bedroom door, about the same difference as the bathroom floor is lower.

In this part of the world, a bathroom or toilet door frame should be mounted on heel stones.