Interesting thread.
I usually model the whole building in 3D because as was stated earlier, by the time you've finished that exercise you understand the project. To model a small building shouldn't take more than a couple of days and the basic information can be used for the renovation.
My routine has been to model the building then copy the model to a dedicated Demolition layer set. I try to keep the Demolition layer set to the bare minimum of stuff needed to provide a credible demolition set (they can, after all, go and look at the building!!). After all, the demo set really only needs a 2D representation.
I've tried keeping the existing elements et al current with the new design but it really is a logistical nightmare. You get walls with old window openings to be filled but they somehow just kind of intrude on new openings...
In the final analysis, it probably really depends on the extent of the demolition and the proposed renovations.
Cameron Hestler, Architect
Archicad 27 / Mac Studio M1 Max - 32 GB / LG24" Monitors / 14.5 Sonoma
