Make the common room shell
Copy it twice
Make one into the existing
Make one into the proposed
Group/Component each one
Put each one on a separate layer
COPY each so that they are on top of each other.
Set up camera views/anims of/within the overlapped models and save the scene(s) as CAMERA ONLY
SWITCH OFF the Proposed layer
Export the scene(s)
SWITCH ON the Proposed and OFF the Existing
Export the scene(s)

By making a copy of the existing and proposed component'd rooms you can continue to build/modify them and the overlapping ones will update accordingly.

I do this all the time.
Another way of using this method is to make all the individual component'd floors of a building and its roof, have them layed out NEXT to each other for ease of working on them but to ALSO have a copy of each stacked on top of each other as the building will appear.

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As you can see I have Ground, First, Second, Third and roof all neatly layed out 40m apart co i can easily copy stuff between floors (Kitchens, furniture etc) but I also have them stacked for generating sections, testing service runs etc.