A couple of quick points that come to mind that I didn't read on previous answers is the ability to heat the room. Also a bedroom can not be adjacent to a room that uses combustion air. (This is not a complete list)
Feel free to contact us for more non-official information on room requirements.
Similar to what the person below mentioned- you should find out about what the septic system / design is to backtrack to what is a legal bedroom in the house.
If the septic was approved and built as a 3 bedroom and that's on record, it doesn't matter if this room is perfect, it cannot be used a bedroom without septic alterations/ enlargements. So it would not officially be a bedroom.
If I can help in any way further, please let me know. Thanks,
The room needs to have a closet to be considered a true bedroom. Living in Bridgewater myself, one bigger concern is to find out the size of the septic system to determine the amount of bedrooms actually existing in the house.