I was just reading your question and recalled a similar situation when I sold a house a few years ago up at Pepperbush in Newington. I would recommend that you speak with your Realtor and then do some research at the Newington Town Hall to see exactly what was done to the unit. If the lower level had a permit pulled and inspected than you could surely use the square footage as additional space but I would be very careful in describing it as a bedroom.
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Using it as a bedroom is up to you...assuming you understand the danger of limited egress for the family member using the psuedo bedroom. If you have the window and door, it counts, and the family member has the two potential exits in case of emergency!
Is it COUNTED as a bedroom? Probably not. Bedrooms must be above-grade.
Lower levels/basements are just that - lower levels. They are not to be included in square footage measurements, and their rooms (beds, baths, family rooms, etc.) are not to be included in room counts.
Also, I have seen condo associations limit not only the number of bedrooms per unit, but also the number of people that may reside in a unit. If you are needing to utilize that basement room as a bedroom, you will want to review the condo documents to make sure you do not go over any such limits.