How will finishing a basement affect ARV?

[Thomas Enright]

I'm currently under contract for $300k to purchase a house with 2,100 sqft above ground that's finished. It's in pretty rough condition and based on comps in the area it should end up being worth $380k-$410k once I have it all shined up and this doesn't include the basement. The basement is 1,000 sqft, unfinished, but I plan on finishing it as an inlaw suite because it's got its own entrance. In the county I'm doing this in, all the tax cards read "above grade living area" for square footage. As finishing the basement would obviously not add anything to the "above grade living area" but I would add about 50% more living area, how would an appraisal, CMA, or ARV be calculated with a finished basement vs without a finished basement? Would an appraisal use all of the newly created sqft in his/her calculations? Or stick to the above grade sqft and the basement would just be an amenity similar to a fenced-in yard? I'm very familiar with the appraisal process but I haven't ever seen the "above grade living area" before and instead just seen the "heated living area." Any help would be greatly appreciated, thanks!