You could definitely run 1 outdoor unit, but you will need a few indoor units, depending on the apartment's configuration.

The configuration is all 2BR units and it's at 80% occupancy with an appraised value of $310,000.

You don't know for sure what you will be allowed to build there.What is the size configuration of your lot you are looking at?

Perhaps they are trying to re-configure something to their needs and wants.

The current configuration of two units that are roughly the same size, and one smaller unit, is apparently OK. • One of the ADU units must either be built to a high environmental standard OR must be reserved for low-income housing.

Generally this would be if sold to a parent or child, or a spouse, or possibly some configurations with entities. 

(My job requires a lot of creative floor plan configuring) Also, if you decide to walk from the deal, I may be interested in buying the house from your seller  (if the numbers work out) and giving you a finder’s fee of course. 

My properties are configured such that a deck would go over a driveway. 

I do more like what @Julie McCoy does.When we purchased our vacation rental, I pulled as much data as I could from surrounding rentals that came even close to ours in configuration.

I picked one out which fit my criteria (location, price, property configuration) and pitched a price to the contact.