I usually use the command 'ShrinkTrimmedSurface' which shrinks the underlying surface to the boundaries of the new shape.
You can find the explanation here....... http://moi3d.com/wiki/Hidden_Secrets
12th Secret from the top.

Cheers

I also struggled a bit with NURBS at the start. I had only just started to get the hang of polygons :-)

I try to think of NURBS more as a block that I am going to cut bits away from to make my final shape. I use other shapes to cut the block into whatever you want. I use booleans and extrudes more than planes.

For your building I would have started with a block then used another block to cut out the corner. This would have left me with a solid model.