After a time of holiday in Copenhagen [B-)] i tried again to select my surface and there it was!!! [:p] The upload via mediafire didn't work but i managed to create a picture to show what's happening:
i tried to select the surface of 1,24m² by clicking in it, but it was always the big room (walls) that was selected. Now i tried to select with a right mouseclick but that didn't worked when i pointed the mouse in the room somewhere. Finally i pointed the mouse in the surface of 1,24m² and now i could select between different surfaces!. See my picture to know what i mean because i am dutch and my english is not super... [:(]
thanks to everyone who tried to help!


[Note from moderator: updated link to image]