I have just moved into a brand new flat, I realized that the bathroom doors are almost 1 inch above the surface and the way the flat is designed, anybody even uses a tap in the master bedroom bath, a person sitting in the hall can listen to it. Anybody talking in the main hall can be clearly heard in the bathroom. I feel that the sound waves passes through the surface without any obstruction so this could be the reason. If anyone has any solution to this then suggest

If anyone has any solution to this then suggest Earplugs come to mind. That's about your only solution unless you can talk the landlord into soundproofing the bathroom. Good luck, Tom

You could also use a throw rug at the thresh hold of the door to block some of the open space. 1" is not standard, bring it to the attention of the owner, could be a bad install

Where do you live? This could be some type of city code...

The gap is supposed to be smaller then that.. we had the same thing in our bathroom and we just installed a door flap it attaches to the bottom of the door and is a little longer then 1". Made of soft rubber it seals it against the floor better and makes less noise come through