Yes, the co-borrower's credit matters a lot, and it can drag your pricing down in a way most people do not expect. For pricing, lenders take each borrower's representative score (the middle of three scores, or the lower of two) and price the loan off the lowest representative score among all borrowers. Pricing follows the weakest link. One wrinkle worth knowing: for qualifying eligibility, some conventional automated and manual underwrites now use the average of the borrowers' median scores, so a file can be eligible on the average while still being priced off the lower score. Confirm current guidelines. We saw this directly with a client who had an 800 score refinancing on her own, then watched her terms get noticeably worse the moment a co-borrower with a 680 was added. The fix in that case was keeping the co-borrower on title as an owner but off the loan, so she qualified on her own income and credit and kept the better pricing. That points to the general rule: - Need the co-borrower's income to qualify? They go on the loan, and you accept the pricing hit. - Qualify without their income? Keep them off the loan (they can still be an owner on title) so their score does not set your rate. A few non-QM and HELOC lenders will price off just the primary wage earner's score, but that is uncommon. Whether you actually need the extra income is exactly the kind of thing we sort out on the free Roadmap conversation.