It may depend on many factors, such as:
1) what is the norm for that area. If there are 90% 3 bedroom homes, and someone expanded to an atypical 5 bedrooms, then the value offset may be more limited
2) The sq. ft. should be reasonable for the number of bedrooms. For example if there is a 1,400 sq. ft. 3 or even 4 bedroom home, some consumers would not like 1,400 sq. ft. living area for a 5 bedroom home.
3) A local appraiser could let you know the offset more accurately, especially if you are considering adding one.
Please let me know if you would like the contact info of a local appraiser.