Depending on the style/approach for your str business.

There are so many other factors that impact the final price that the square footage of the property is dwarfed by them - beds, baths, age, style, condition, acreage, garage, basement, HVAC, etc, etc, etc.

The "narrowed search" are things like specific styles of design, types of flooring, open concept, etc.

I know it will be just as much a re-sale consideration in a decade as it is for me currently, but since I see this as a long-term buy-and-hold, the more immediate concern is determining ARV and what it would appraise for when the time comes to back into a conforming mortgage after seasoning (BRRRR style). 

What style of house is it?

Unfortunately, that 800k is chump change to too many people these days.

The house currently has more or less a Cape Style 2nd floor with dormers.

It's a popular loan style overseas that allows all normal banking deposits/idle funds to sit on your remaining balance and thus save interest cost.

Imagine a ranch style home going from a 2 bed/1 bath to a 4 bed/2 bath which is what would happen if you finished the basement. 

I'm taking family style lenders.