Multifamily retest via Alpha Track Hi Jeff,
My dad always said, "the only stupid question is the one you don't ask." We deal with these types of situations on a daily basis. In my professional opinion I would recommend testing all ground floor units in the four buildings,especially if this is being requested after prior readings were elevated.
In the perfect world if you had copies of the prior reports, that would be helpful in determining what protocol was followed initially and what units were actually tested in all the buildings. Not having this information, one would have to consider this as initial testing. If this is not a HUD project, then the 100% plus 10% of the upper units in all the buildings would apply per the ANSI/AARST standards. If you are protecting your clients interest in this property and covering your assets :-) performing a full survey should be the way to go in order to determine the need for mitigation at the entire property. I do realize however that budgets are always an issue and you have to deal with that as well. If the testing is only performed on the four buildings and without the full knowledge of the prior results your testing would have to be considered a limited screening and not a full radon survey. This also can be the push your client may need to gain access to copies of the original testing reports as well. I hope this helps.