First the whole attic needs to be brought up to code very often attics were never made to support live occupation only for storage. So a lot more involved than just adding windows. The bonus is you already have a stircase going to the attic .So you just need to make sure about the code. As for the rest I see no point in the den space so utilize that for entertaining space maybe even 2 seating areas.. If you are adding bedrooms upstairs then maybe lose one on the main floor to get a larger kitchen and a larger main bedroom .

Thanks, Patricia! We've started speaking to contractors to bring the 2nd floor up to code. agreed that the den area is not the best use of space. I'm thinking about partially closing it off with french doors and turning it into a TV room / guest room