Chikopee,
I am at a loss as to why you want to do all this in model space. There is no way of doing it other than what you know as far as copying and circling a section of the main drawing and deleting and trimming. Remember, every time you do this, your drawing file is growing in size.
Getting back to viewports in paper space. If your main drawing is in a large viewport and you want to make an enlarged detail of some feature in your drawing, create another viewport and zoom in on the area you want to detail.
There is no need to copy any part of the house so in effect your drawing is not going to get huge.
Lets say you want to put dimensions and notes on the enlarged detail. You will notice that they will show up on the main drawing in the large viewport as well because there is only one model and anything you do in one viewport, shows up in the other,which is undesirable. However, if you create a separate layer for the enlarged detail dimensions, when you make the "whole house" viewport active,go to your Layer Properties Manager,pick the layer for those dimensions and checkmark the box "Freeze in current layer". The enlarged detail dimensions will disapear from the "whole house" viewport but will remain on your enlarged detail.
This is also the fastest and less laborious way to do it. it is also the safest as you don't have to edit and fudge dimensions.
Don't know if I've helped any. Let me know.