Should I add an extra BR during renovation

[Mar Now]

I am renovating an old split level in a Chicago suburb which has a large addition on the main level. Because of the  extra space I have an opportunity to turn the basement into a 4th BR for only a couple thousand more. There is already a full bath on the basement level. 

Any downside to this? It's a last minute decision so I want to try to think through it quickly.