Zillow doesn’t just the square footage in the listing you provided. 

My new thought is to make the basement a master bedroom, with another "bonus" room to add some square footage and be more attractive to investors.

@Clay Tolbert It depends on the sq footage and are you changing the footprint of the property?

Would an appraiser adjust for the square footage difference this way?

Pay attention to bedroom count, bathroom count, square footage, lot size, etc.

Basement is basically exactly 50% of the total sq footage of the home, so I'm good on that regulation. 

Filter last 6 months, filter for similar BR/BTH with +-5-10% of the square footage, sort high to low, and then you have a list of the top-tier closes for that housing product (hopefully they are renovated if that's your end goal), with the list decreasing in value as you scroll down.

The square footage figure mentioned in the post is a target figure but is certainly flexible. 

It doesn't make much difference in price, you can have a 2100 square foot two bedroom and the square footage sells, this is because it is common. 

@Bryan Place an appraiser might have a hard time getting comps that have an ADU, so they will most likely be conservative and only give you something like $100/sqft or maybe less, but if it's attached to the home and you make it accessible from the interior then the square footage can be counted as part of the home and it will likely be worth significantly more, something like $200+/sqft.