How mortgage lenders actually set a housing budget

Fannie Mae’s Selling Guide, section B3-6-02, is blunt about the limit that matters. For a loan underwritten by hand, the maximum total debt-to-income ratio is 36% of stable monthly income. That can rise to 45% if the borrower meets the credit-score and reserve rules in Fannie Mae’s Eligibility Matrix. If the loan is run through Desktop Underwriter, the maximum DTI is 50%.

There is no matching sentence that says housing itself must stay at 28%. Section B3-6-03 tells the lender to add up the monthly housing expense on the home being financed and drop that sum into the DTI calculation. The pieces are principal and interest, property and flood insurance, mortgage insurance when it applies, real estate taxes, ground rent, special assessments, and owners’ association dues. Lenders call that stack PITIA.

The 28/36 rule survives because it is easy to remember and because some lenders still apply their own overlays. An overlay is a tighter rule than the investor’s minimum. If a loan officer quotes 28%, ask whether that is their company’s overlay or a Fannie Mae limit. Those are different things.

Say stable gross income is $6,450 a month, about $77,400 a year. Other debts on the application are a $385 car payment, a $210 student loan, and $75 in credit-card minimums. That is $670 before any new house payment.

Debt-to-income is (housing + those debts) ÷ gross income. Rearranged, the most the housing stack can be is the DTI ceiling minus $670.

The jump from $1,652 to $2,555 is not “a little more house.” On a 30-year fixed loan it can be tens of thousands of dollars of price, once taxes and insurance are in the payment. It is also the difference between a file a human underwriter can approve at the base limit and a file that needs either stronger reserves or an automated approval.

Taxes and insurance are inside PITIA, so they shrink the principal-and-interest piece. A $2,200 housing budget in a high-tax county is a smaller loan than the same $2,200 in a low-tax county. Association dues do the same thing. Our home affordability calculator and mortgage payment calculator are estimates you can stress with those extras. They are not a preapproval.

Installment loans and revolving minimums generally count. So does the new housing payment. Child care, groceries, retirement contributions, and the cost of commuting usually do not, unless a specific program says so. That is why two households with the same DTI can have completely different lives after closing.

Income has to be “stable” in the guide’s sense: salary, documented hourly work, or self-employment with a history the lender will accept. A new bonus that has been paid twice may not count at the full amount. Overtime often needs a two-year pattern. If the offer letter and the tax return disagree, the tax return wins more arguments than people expect.

FHA loans are a separate rulebook, HUD Handbook 4000.1. FHA has long used manual benchmark ratios around 31% for housing and 43% for total debt, with room to go higher when compensating factors or automated underwriting support it. Do not paste a conventional DTI onto an FHA quote, or the reverse.

The lender’s question is whether the loan meets an investor’s rules. Your question is what is left after the payment, the debts, and the costs the application ignored. If the only way the house works is the 50% automated maximum, price a cheaper house or a larger down payment before you fall in love with a kitchen.

Renters comparing the two decisions can use the rent vs buy calculator. For the public-housing history behind the separate 30% rent guideline, see where the 30% rent rule comes from. Those are different ratios, built for different decisions, and mixing them is how people talk past a loan officer.