I am currently in the process of selling my home. A radon test
revealed that my radon is at a level of 6.9 pCi/L. The buyers
interested in my home are requesting that a mitigation system be
installed.

I currently have a passive mitigation system. It consists of a pipe
that starts below the basement floor, runs through an interior wall of
the home, and vents above the roofline. My thoughts were that a fan
could be installed in the basement on this existing pipe. (Installing
the fan in the attic is not an option, as the home has vaulted ceilings
and no suitable location for an attic fan.) I got an estimate from
someone who stated that law and/or regulation now states that the fan
cannot be installed in a basement. This means that I would need to
reroute my existing pipe and have the fan installed on the exterior of
the home. Is it true that law and/or regulation now prevents the fan
from being installed in the basement on an existing passive system? (I
live in the state of Pennsylvania, if that has any bearing on your
response.)

Thanks in advance for any responses!