Selecting perimeter of whole floor plan

Hi, first time here so not sure if this is the right place to ask but here goes.
I Have drawn a set of 3d floor plans, I incorporated cavity walls and have since seen I needed to build a raised base of the outline so that push pull and other bits work correctly! I cannot now make a base that connects to existing plan to make it work. any ideas? I was thinking of selecting the parameter and making it a solid that way some how. hope this makes sense as I am extremely illiterate soz.

It might be as simple as drawing a large horizontal rectangle at the base and the deleting unwanted sub-faces. But without seeing the specific model that’s just a guess. Is it possible for you to share the model?

Hi thanks for reply! I have managed to sort the problem I was able to remove all internal walls leaving just the parameter and made it 150mm thick then attached everything back on. thanks

I think you mean ‘perimeter’ - is that right?

yes that is right!