Rent Back to Seller for Extended Period

[Richard NA]

I'm looking to buy a property as my primary residence. The sellers are about to start building their new home. We have negotiated a deal where, in exchange for a discount on selling price, they get to stay in the property rent-free until their build is complete. This obviously might take a while. Our agreement will be for 1 year. How can I do this without causing the lender to classify it as investment property (which is a different loan with higher rates AFAIK) ?