A legal agreement extending a duty of care from a designer or contractor to a third party (like a funder, tenant, or future purchaser) who wasn't part of the original contract. Without a collateral warranty, a building's future owner or lender would have no direct contractual right to sue the original architect or contractor if a defect appears - this is the legal document that gives them standing, separate from the main construction contract and separate from a product Warranty covering physical materials.