Reconfiguring existing rooms

Not sure if this belongs here or another sub. We recently bought a very nice 3 bedroom, 2.5 bath, 3500 sq ft house and was the worst, oldest house on the best block in town. There are two additional unfinished, partially framed out rooms on the second floor with space for another full bathroom. It’s a 35 year old house and the square footage is off by about 300 sq ft. It’s currently roughly 3200 sq ft of finished space but the square footage is calculated as if the two additional rooms are finished, which they aren’t. We intend to finish the rooms, thus making it a 5 bedroom, 3.5 bath house. My question is, since this technically isn’t changing the square footage of the house as we aren’t altering the exterior, would our taxes go up? We’re merely reconfiguring and finishing existing space. Ideally, we’d like to do this the proper way and notify the town since they are the ones who have records listing it as 3500 sq ft, but in theory, I’m not sure if we should since there’s no exterior alteration being done to the building. I’m most concerned they would increase our taxes as it’s a very affluent area and all the other homes around us are 5 bedroom McMansions while ours isn’t and is a cape. This is our forever home and we see ourselves here for decades, but my big concern is if and when we go to sell, we’d be stuck listing it as a 3 bed, 2.5 bath.

Appraiser here. Sorry trying to follow your conversation. Taxes, I think you’re saying you’re already taxed at 3500 sq ft, although those rooms aren’t finished. Of course, the assessor is always privy to new permits that are drawn on your house, usually staff fills in the blanks. Different departments across the country act differently with exterior areas and what is viewed as finished. I mentioned permits because it sounds like you’re gonna need electrical permits to implement your finish. If these rooms are finished, I would suggest viewing ANSI standards. For those rooms to be bedrooms, they’re gonna have to meet certain standards. Some people use rooms with short ceilings as bedrooms, that doesn’t mean it’s marketable that way. You need an exit window, most buyers expect closets in their bedrooms. Etc. I posted a link to the most common standards of living space.