- A co-signer adds their income to the combined ratio — and their debts. A parent with a big mortgage can make the combined number worse, not better.
- Fannie Mae’s Selling Guide (B2-2-04) caps the occupant’s own ratio at 43% on manually underwritten loans. DU casefiles are evaluated on the combined ratio.
- In practice, many lenders apply the 43% occupant test to DU files too. That’s an overlay, it isn’t published, and it’s where these files get declined.
- If the combined number clears and the occupant number doesn’t, the decision is the lender’s policy, not the guideline. Ask before you apply.
Two ratios, one file
The appeal of a non-occupant co-borrower is simple: the buyer’s income isn’t enough, a parent’s income is added, and the combined debt-to-income ratio comes down to something a lender will accept. What surprises people is that the buyer’s own ratio doesn’t stop mattering once the parent signs.
Underwriting looks at the household that will actually be making the payment. If the person living in the house couldn’t carry it on their own income, that shows up differently in default data than a household that could, and the guidelines and lender policies both reflect that. So there are two numbers, and the calculator above shows both, always — because a tool that only shows the combined ratio is showing you the one that’s easiest to pass.
Combined DTI = (housing + all debts) ÷ all income
What the published rules actually say
The thresholds in this calculator come from Fannie Mae’s Selling Guide. They differ by underwriting path, which is why the tool asks.
| Path | Combined DTI max | Occupant-only max | LTV max with non-occupant |
|---|---|---|---|
| Desktop Underwriter | 50% | No separate test | 95% |
| Manual underwriting | 36%, up to 45% with credit & reserves | 43% | 90% |
Fannie Mae Selling Guide B3-6-02 (DTI) and B2-2-04 (non-occupant borrowers). Conventional only; FHA has its own non-occupant rules.
Read the DU row carefully. The guideline says the occupant’s ratio isn’t separately tested on a DU casefile. That is true of the guideline. It is frequently not true of the lender, and that gap is the whole reason this calculator exists.
Where these files get declined
The pattern is consistent enough to describe in advance. The combined ratio looks comfortable — a parent earning well can pull it into the 20s or 30s. DU returns an Approve/Eligible. The file goes to underwriting, and the underwriter runs the occupant’s ratio on their own, because the lender’s credit policy says to. If it’s above 43%, the file is declined or conditioned on a larger down payment from the occupant, and the borrower is told the reason is “DTI” — which is confusing, because the DTI they were quoted was fine.
Nothing dishonest happened. The lender is applying a stricter standard than the agency, which lenders are allowed to do. The borrower just planned around the published rule and was judged by the unpublished one. The overlays article covers why that happens and what to ask.
The calculator’s default inputs show this exact case: occupant-only ratio near 50%, combined ratio in the mid-20s. Switch the path to manual and the guideline itself fails the file. Leave it on DU and the guideline passes it — and the note tells you the outcome depends on the lender.
The co-borrower’s debts are the part people forget
A co-signer is a co-borrower. They bring their income, and they bring their obligations — including their own mortgage payment, if they have one. That cuts both ways.
A parent earning $9,500 a month with a paid-off house adds $9,500 of income and almost nothing to the debt side; the combined ratio drops sharply. The same parent with a $2,800 mortgage and a car payment adds $9,500 of income and $3,300 of debt, and the combined ratio moves far less than expected. In extreme cases it moves the wrong way. Enter the co-borrower’s full monthly obligations, not just the ones they mention.
High income, low debt
A co-borrower with a paid-off home and no car payment is close to pure income. The combined ratio drops the most.
High income, high debt
A co-borrower with a large mortgage brings a large monthly obligation. Net benefit can be a few points, not twenty.
Occupant-only test
No co-borrower changes the occupant’s own ratio. If the lender applies the 43% test, only the occupant’s income and debts count.
What actually fixes an occupant-only problem
Since the co-borrower can’t move the occupant ratio, the levers are the occupant’s own: lower the housing payment (smaller loan, larger down payment, a less expensive property), pay down or pay off the occupant’s debts before applying, or document more occupant income. A lender that treats the 43% test as a hard overlay will usually accept any of those. What it will not accept is more co-borrower income, because that isn’t in the ratio it’s testing.
The other lever is the lender. Overlays vary by institution. A file that fails one shop’s occupant test can clear another’s where DU’s combined evaluation is honored as written. Ask the question with the number in it: do you apply a separate occupant-only DTI test on DU files with a non-occupant co-borrower, and what is the cap?
What this calculator doesn’t know
It applies Fannie Mae conventional thresholds. Freddie Mac has its own non-occupant rules, FHA has different ones (including relationship requirements), and VA doesn’t use DTI the same way at all. It doesn’t know your credit score or reserves, which decide whether the manual path allows 36% or 45%. And it can’t know your lender’s overlays — nothing published can. Use it to see which of the two numbers is your problem, then ask the lender the right question about that one.