You'll have to truly opposing forces.

You might want to consider that "extra money" an incentive for the buyer to purchase YOUR house as opposed to SOMEONE ELSE's house. 

@John Woodrich Good call out John, our structure is not a series LLC (at least not the way that people usually use the term) as each LLC is a separate LLC with its own EIM and operating agreement as opposed to what most people mean when they say Series LLC . 

@Kim LeJeuneI have to assume that it was a rental house, as opposed to your personal residence or a flip.Each monthly payment has 3 components: interest - taxable as ordinary incomereturn of your principal - not taxablecapital gain - taxable at various ratesYour down payment has components #2 and #3.Interest should come from an amortization table. 

The ADU would offset our mortgage enough to where we would be paying on average around $1,100/month as opposed to the $1,600/ month we are paying currently for rent.

FSBO's almost always think their property is worth more than it is, are difficult to schedule showings with, haven't prepared the property to sell, are unwilling to negotiate, and they almost never know the sales process or have the software or the contracts to make the transaction go smoothly and keep the deal from falling apart over little things (very common when there are no agents to act as mediators, buyers and sellers have inherently opposing interests and tend to argue then cancel the deal).

Perhaps when you call, you can tell the department that you, and other investors, would not be opposed to THAT solution.

Completely different metrics/analysis as opposed to a traditional rental property because you get an additional, massive benefit: shelter. 

If you want an "easy" hire, paying more through a vetted system as opposed to winging it on Upwork.com will cost more.What's a good way to setup a system with expenses and getting the bookkeeper this data?   

I'd suggest you do your own calculations as opposed to trusting figures from the developers, who unsurprisingly can be wildly exaggerated.Now, a reasonable $/SF is subjective and might be different for everyone, especially if the purchase is partly or fully for lifestyle.