This is more of a brain-picking exercise, looking for options, advantages, disadvantages, on how to best address this situation. I'm guessing it's a common occurence, just not something I've dealt with much.

The owner is a manufacturing plant, that's been around for decades on the same piece of property. Over the years they's added, demolished, renovated, replaced facilities
on an as-needed basis. All horizontal control on the plant property is tied to a local coordinate grid, with a BM at a known ( and readily relocatable) point in the public RoW, and an assumed bearing on the CL of that public road. Any communication with the owner has to be tied to that grid, ranging from 800 North to 800 South, and 1000 West to 800 East.

Since the property has a real world location, any point on it can be identified by lat-long or state plane coordinates. In my typical day-to-day work, everything is based on state plane. That makes is simpler to bring in aerials, GIS data, adjacent properties, flood zones, etc. Normally the WCS in my drawings is the state plane for that region of the State.

What's the 'best' way to deal with the different schemes, and why?