Well, mine wont be a popular opinion here, but:

Rebuilt it, in your template. From the ground up.

If you arent contractually responsible for verifying the existing conditions, then MAYBE link theirs in, and trace it. But having done a LOT of renovation work in my Architectural career, you COULDNT pay me enough to start a job with someone elses existing model.

Thanks for the response, but at least there is not that much to change since they were obligated to start from our template and to follow our standard.

Having said that, my problem is more related to how you would work on the model if you had re-model the existing, for exemple.
Do you keep everything from the existing in a linked file and when you need to demo stuff, you change it inside that link.
Or do you have everything inside the same model (if it fit for size) and you just play with the phases ?

I did found a reply to a post you did a while back Aaron, and you had these guideline a while back from a document called Modeling Existing Buildings- Requirements-v1 :
"7. We separate models in to Buildings, not Phases. If the entire project happens (demo and new work) in one building, the Existing Model will become the Project Model. We do not need to have an “Existing Model” and a “New Model” unless there is another reason to divide the two files"

Is it still what you are doing ?

Thanks,

If you know the existing model is good, you can just use it and model the new phase in that revit file, no need to link stuff.

Yep. I wouldn't separate existing in to its own file just because its existing.

I prefer the one model, but how do you deal with the wall join between existing and new so that new wall do create weird modification to existing wall?

I use disallow join on the walls. Works fine, and its simple.