The answer to this question will take into account your mother's goals on how the money needs to work, transition plans in the future, risk tolerance, asset segment preferences (retail, industrial, MF, self-storage, etc.), and many other factors.

-Sauna-Hot Tub-Patio/courtyard (large cement tiles on crushed bluestone with lighting, natural gas line to bbq)-Secondary patio off primary suite-Losing sq feet in the primary bedroom to enlarge the bathroom-High grade cabinets-Appliances with cabinet fronts-High end appliances (Viking/Miele instead of GE Cafe)-Outdoor shower (teak box walls with a dry well beneath the footprint)-Wiring for Cat6 to every room-Wiring for surround speakers (or including the speakers that are built into the walls)-Sonos whole-home audio system-Hardwired cameras (with on-site storage) to the exterior of the house-Smart features: thermostats are a given, but how about light fixtures, switches, aromatherapy, fans ... anything else?

I had the same intention in 2019, but available duplexes weren’t feasible.

We do Self storage.  

I recently just sold a 47-unit and 36-unit self-storage deal. 

DSTs function as investment vehicles that hold ownership of income-generating properties, granting investors fractional ownership in various types of commercial real estate such as apartments, self-storage facilities, build-for-rent properties, and NNN (triple net lease) properties.

I'd tell them to stay at a hotel/motel or STR for 3 weeks and put their stuff in storage.

The zoning will define the permitted uses (this will tell you if you can have an RV/Boat storage there) and it will define the density (this will tell you how many tiny homes you can add).You'll want to make sure you don't conflict the existing duplex to a new use or mixed use where you might put the existing use out of compliance. 

If you have the storage room, having one of these might be handy for future SC summers, as I'm sure this will happen again at some point.

That said, I do want to respectfully offer some counterpoints to Dan's assessment that may provide a broader perspective on the value and feasibility of ADUs.1-2.