I'm seeking advice on how difficult it may be to convert a SFH with an ADU (accessory dwelling unit) into a legal Duplex.

"(3) (A) A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit less than 750 square feet.

Not sure what you'd call this, but when we bought our sfh back in 2016 California had just passed the Accessory Dwelling Unit ADU law.

Then California passed the law legalizing Accessory Dwelling Unit's (ADU's) in January 2016.

We want to add a garage to a 4plex but the city has re-zoned the lot to SFR use and says our property is legal non conforming so we are not able to expand.i think a garage is an accessory that is consistent with accessory's for a house,  so it should be OK to my thinking, Is there a tact full way to approach the city with this?

@Nathan ClaireThanks for the substance and silliness ;-)A modification/addition to the BRRR method I’m considering are ADU’s/Accessory Dwelling Units.

https://www.sanjoseca.gov/business/development-services-permit-center/accessory-dwelling-units-adus/adu-amnesty-programAlso, you could try speaking with a local architect like @Adam Mayberry. 

A SFH zoned for an accessory dwelling unit in Denver at that price?

Investment Info:

Small multi-family (2-4 units) buy & hold investment.

Purchase price: $350,000

Cash invested: $70,000

SFH with tons of deferred updates and zoned for an additional dwelling unit ...

:)An ADU is accessory dwelling unit, think a granny flat, in-law suite, guest house, etc.