San Antonio Rental Zoning- Asking For A Friend

[Trampas Tanklage]

I'm selling a property in the City of San Antonio that's zoned R-4 (single-family residential). The property has 2 structures, a 2 bed/1 bath original house and a 1/1 back casita (fully permitted). Would a buy and hold investor (not living in the property) legally be able to rent out the units to 2 unrelated tenants (which would increase the rent substantially), or would they need to re-zone the property to a multi-family designation? If re-zoning is necessary, how hard would that be for a property like this?