First, did you buy the house "As Is" If there is anything in your purchase agreements, disclosures, contracts, etc that you signed saying you purchased the house "as is", you're out of luck.
Second, it sounds like the work was performed by the homeowner, if a general contractor had performed the work in MN, he'd be on the hook for 10 years and have a bond you could go after
Third, check out your states contracting laws, in certain cases in MN, I believe homeowner performed work can be held to the warrantee period a general contractor would of been required to warrantee their work, (I think 10 years in alot of cases) unless you sign off that you are buying the house as is.
You could run it by your realtor and they should be able to tell you right off the bat if you have legal recourse based on the contract and disclosures you signed