Thank You for your quick answer. I will change the model in the way you suggested. But I did not understand your point concerning "inside out". I can change the faces from blue to white using reverse but I do not understand the use of it.

Hi
I'm new on this forum and I have following problem. I'm modelling a house using different layers (1 for each floor) and components.
Here the snapshot "before"

At one moment the component has a strange reaction: adding or removing lines has the result as seen on the following snapshot. I am removing the line where the mouse cursor is:

Can anyone help me?
Thanks