This depends on several items, but mainly is predicated by two things: your type of financing and the area you want to live. If you're going to be using a conforming loan (FHA, Conventional, VA, etc) then you will be limited to 1-4 family structures. Most of the true small 2-4 unit buildings are located within the urban core, however there are pockets throughout the inner-ring and suburbs that have small multi-family developments.
Next you will want to look at the base zoning for the area: RM-4, RM-6, & MF-33 are the most common zoning classes that allow for multi-family buildings (including ADUs). There are several IDZ zoning classes that can be applied to properties to allow for increased density, relaxed setbacks, and multiple units. Assuming you're looking for an existing building and not planning to build new, zoning is extremely important. Purchasing a multi-family building or a property with multiple detached units without the proper zoning may keep you from obtaining your loan due to underwriters taking issue with "non-conforming use." I have seen this happen both on new purchases and refinances. Aside from re-zoning the property (which is expensive), the city does allow for a conditional use permit, but you have to apply for it annually and there are other requirements. Usually a conditional use permit is enough to satiate the loan requirements.
R-4, R-6 and other zoning will allow for an ADU as long as it's owner occupied. Should you move from the property or move your homestead exemption to another property any ADU automatically reverts to an "Accessory Structure;" which does not allow for living quarters. It would seem this would be difficult for the City to enforce this, but I have definitely seen it happen- and keep in mind, jamming up residents is just another way for them to generate revenue.
My suggestion would be to find a good agent that understands the nuances of zoning relative to your desired property/strategy. It's been my experience that most realtors in San Antonio have no clue about zoning, and/or zoning requirements as they apply to property/ land use; this includes understanding how the Short Term Rental Ordinance applies.
Best of luck with your search.
