Your $780 of Monthly Debt Costs You $120,000 of Mortgage

Mortgage underwriting is less mysterious than it looks. Almost all of it reduces to two ratios comparing your monthly obligations to your gross monthly income, and understanding them tells you both how much you can borrow and, more usefully, exactly which of your current commitments is limiting you.

The two ratios

The front-end ratio is your proposed housing payment divided by gross monthly income. Housing means principal, interest, property taxes, homeowners insurance, any mortgage insurance, and any homeowners association dues. The traditional guideline is 28%.

The back-end ratio is housing plus every other monthly debt obligation, divided by the same income. Car loans, student loans, personal loans, and minimum credit card payments all count. Conventional loans generally cap this at 43%, with automated underwriting stretching to 50% for strong applications, and it is the binding constraint for most buyers.

Where existing debt actually lands

Here is the part that surprises people. Existing debts do not reduce your borrowing capacity proportionally. They come off the top of the back-end allowance, dollar for dollar, and every remaining dollar has to cover taxes and insurance before any of it reaches the loan.

Suppose this buyer has $780 a month across a car payment, a student loan, and credit card minimums, and the property carries $520 a month of taxes and insurance.

What counts and what does not

• Counted: car loans and leases, student loans including deferred ones, personal loans, credit card minimum payments, court-ordered child support and alimony, and any other loan reported to credit bureaus.
• Not counted: utilities, phone bills, insurance premiums other than those escrowed with the property, groceries, childcare, medical expenses, and retirement contributions. Underwriting ignores your actual cost of living almost entirely.
• Handled specially: a car loan with ten or fewer payments left is often excluded. So is a debt demonstrably paid by someone else, if you can document twelve months of payments from their account.
• Deferred student loans still count. Lenders generally use either the documented payment or roughly 0.5% to 1% of the balance per month, so a $60,000 balance in deferment can be treated as a $300 to $600 obligation even though you are paying nothing.

That second bullet deserves emphasis, because it is the single largest gap between what a lender approves and what you can actually afford. Underwriting does not know you have three children in daycare, or a $600 monthly childcare bill, or that your income is commission-based. The 43% limit is a credit risk threshold, not a budget.

Improving the ratio, in order of effectiveness

1. Pay off a small loan entirely. Eliminating a $450 car payment does more for your capacity than reducing a $900 payment to $600, because underwriting counts the payment, not the balance. Retiring the smallest balances with the largest payments is the fastest lever.
2. Pay a card down below its minimum-payment threshold. Minimums are usually 1% to 2% of the balance, so cutting a $6,000 balance to $1,000 removes most of that monthly figure from the calculation.
3. Document all income. Bonus, overtime, and side income generally count if you can show a two-year history. Many applicants understate their qualifying income by omitting it.
4. Extend the term rather than the price. A 30-year loan qualifies you for materially more than a 15-year loan at the same price, because the ratio tests the payment.
5. Increase the down payment. This lowers the loan and the payment, and above 20% removes mortgage insurance from the housing figure as well.

The number that should actually decide it

Approval tells you the ceiling. It does not tell you the right answer. A useful discipline is to compute your own back-end ratio including the things underwriting ignores, such as childcare, retirement contributions at the rate you intend to keep, and a realistic maintenance allowance of roughly 1% of the house value each year.

If that fuller number lands near 43%, you have bought a house that works only while everything goes well. If it lands nearer 30%, you have bought one that survives a job change. The gap between those two houses on a $90,000 income is roughly $100,000 of purchase price, and it is the most consequential decision in the entire process.