Thanks to everybody for your reply.
The state requires disclosue of mold so I got "mold disclosure" of potential mold on every foreclosure I bid. If I don't know the house has mold then I don't have to disclose. But if I have to hire mold remediation contractor, I will have to disclose.
The cost I got from three contractors are 4K, 7K, and 20K. So roughly in the range of beachlver said. I would require to get a mold free certificate if I decide to keep the house.
Mold is the biggest concern after a*****esto and lead paint have been banned.
