My self-employed tax deductions make my DTI look nonexistent — can a friend (a doctor) co-sign on a conventional loan to help?

Yes. What you are describing is a non-occupant co-borrower, and conventional financing allows one, so your doctor friend can go on the loan without living in the home. How the programs handle it: - Conventional (Fannie Mae and Freddie Mac): allowed, and the co-borrower can even be added for strength without their income being used at all. When their income is used to qualify, expect roughly a 95% max loan-to-value with an automated approval, or 90% on a manual underwrite, where the occupying borrower's own ratios are also held to a tighter standard. - FHA: also allows non-occupant co-borrowers. Full 3.5%-down financing generally requires the co-borrower to be a family member; with a non-family co-borrower the loan is limited to 75% loan-to-value, which makes FHA far less useful with a friend. - VA: joint loans with a non-spouse co-borrower do exist, but they need VA's prior approval, VA guarantees only the veteran's share (so the loan is no longer zero down), and many VA lenders simply will not do them. Only a spouse preserves the full zero-down treatment. - Jumbo and non-QM: some programs do not permit non-occupant co-borrowers at all. Two more thoughts. First, an individual lender can still have an overlay against this even though the agency guidelines allow it, so a first no is a reason to shop the file, and a lender should never be assumed to speak for Fannie or Freddie. Second, you may need less help than you think. You have real income; the write-offs just make it look thin on paper. A correct cash-flow analysis of your returns often finds more usable income than a quick glance suggests, so it is worth having your file analyzed properly before you add anyone to the loan. Both questions get answered in the free Roadmap conversation, where we run your real numbers.