My current servicer has the property as a single-family in their system, might I qualify for single-family terms by describing the detached garage duplex as an accessory dwelling unit?

Many basements are converted into ADUs (Accessory Dwelling Units) and rented out as studios.

If the lot is really small (and with that density limit) you may be looking at a single family home only but some jurisdictions have begun to allow for ADU's or DADU's (accessory dwelling unit, detached accessory dwelling unit) which could allow for someone to have a small apartment on a single family lot spreading out their mortgage costs and / or allowing you or them to condo it out and sell separately.

That being said, I do know that small hacked multi-families like that are not uncommon in that area.With its layout above the garage, it is technically classified as an Accessory Dwelling Unit (ADU) and requires the main home to be occupied by the owner.

While we currently rent out a legal accessory apartment in our basement, we are looking to grow our portfolio of SFH and small multiplexes through the BRRRR method.

What's the best way to protect yourself as an owner if you're renting out an accessory dwelling attached to your own primary residence?  

Unless Chicago ends up legalizing accessory dwelling units which would be huge boost for 2-4 unit owners.

If you're in Los Angeles and want to keep your investments there I'd seriously look at some foreclosures to rehab and add an ADU - Accessory Dwelling Unit.

I was working under the assumption that the ADU would be an "Accessory Dwelling Unit" and not making the home a duplex.  

There is a two car detached garage that has potential for an accessory dwelling unit to be built on top of it.