These areas, designated by a new “z” Constrained Sites overlay zone, will retain existing regulations for corner duplexes or one accessory dwelling unit with a house.Lots that do not meet minimum lot sizes for three or four units (R2.5 = 3,200 sq ft; R5 = 4,500 sq ft; R7 = 5,000 sq ft)Infrastructure constraints, including lack of paved streets, sewer or water service.Increase the allowed size (FAR) for duplexes and triplexes (no increase for a fourplex).

The lot has a small accessory dwelling unit, and we plan to build a larger main home. 

Personally, I would consider alternately looking for a well priced SFH and building an accessory dwelling unit (ADU), also known as an in-law unit. 

You might consider doing the renovation,  but also subdivide the lot and build an Accessory Dwelling Unit (ADU). 

If your title document says it is 4 units, you have 4 meters on the property, and 4 addresses, yet according to the tax accessory you have 5 units, you may be overpaying taxes for your units.

Would it be possible to originate notes on accessory dwelling units for homeowners who cannot qualify for a HELOC or refi to build one on their own?

, and some other accessories/fix up stuff for it. 

https://www.arlnow.com/2019/03/07/county-readies-zoning-changes-to-encourage-construction-of-new-detached-accessory-dwelling-units/

Has anyone in the Austin area added an Accessory Dwelling Unit (ADU) to their primary residence or investment property to increase cash flow?

Arlington County recently relaxed the regulations on accessory dwelling units, and there is a process in place where you could request to build an in-law suite.