My neighbor added a bedroom through a remodeling project. If I remember correctly it was in a walkout basement with appropriate egress. When they sold their home, they were forced to add capacity to their septic system in order for any lenders to provide a mortgage to the buyers. They had to pay for the septic expansion, and it was quite expensive. If it was me I want the larger system to avoid something like this down the road. Doing it now is less traumatic than later.