Mostly the math just combines: both incomes, both debt loads, one shared DTI. The wrinkle to know about is credit scores. When two co-borrowers will live in the home together, whether spouses, a couple, friends, or family, the lender combines all income, debts, and assets and qualifies the loan off the totals. Credit works differently: - Pricing follows the weaker score. The loan is priced off the lowest representative score among the borrowers, meaning each person's middle score of three (or lower of two). An 800 paired with a 621 gets priced like a 621, with no averaging for the rate. - FHA uses the lowest borrower's minimum decision score the same way. - One newer conventional nuance: for eligibility on certain files, Fannie Mae now uses the average of the borrowers' median scores rather than the lowest. So a weaker co-borrower score does not always sink the approval itself, even though it still drives the price. That lower-score pricing rule can matter a lot, so weigh it before deciding who goes on the loan. A non-occupying co-borrower, say a parent co-signing for a recent graduate, follows somewhat different rules depending on the program. If you are deciding whether adding a co-buyer helps or hurts, we can model the loan both ways in the free Roadmap conversation, about 20 minutes, where we run your real numbers.