Adding value by finishing an attic

[Charles Campbell]

We currently live in a 3300 sq ft, 2 level, single family home with an unfinished attic. I want to get the attic finished and turned into a media/home theatre room. I'm curious how to calculate how much more value that would add to the house? The attic is a pretty big space, runs the whole length of the house, and if it were finished it would turn our house into 3 levels. Does that mean if I finish the attic and turn it into a media room, I can simply add that additional floor into the total square footage calculation of the house? For example, lets say, currently, each floor is approx 1650 sq ft, thus a total of 3300 sq ft for the 2 floors. And if I have the attic finished, that would be another 1650 in sq ft...is it as easy as looking up other homes in the area that have approx 4950 sq ft to figure out how much value the extra room would give our house? Thanks in advance. Hopefully this is the right place to post this question.