A lender will tell you what you qualify for. That is a genuinely different question from what you can afford, and the gap between the two answers is where most housing stress originates.
Lenders approve on debt-to-income ratio using gross income and only the debts that appear on your credit report. That calculation cannot see childcare, commuting, groceries or retirement saving — so the approved figure is routinely more than a household can comfortably carry. The useful method runs backwards: decide what you can pay monthly after tax while still saving, then convert that into a price.
What the lender is actually calculating
Debt-to-income ratio — your monthly debt payments divided by your gross monthly income.
Two versions matter:
Front-end ratio covers housing alone: principal, interest, property taxes and insurance, together abbreviated PITI, plus any HOA dues and mortgage insurance.
Back-end ratio adds everything else on your credit report — car loans, student loans, credit card minimums, personal loans.
The old 28/36 rule said keep the front end under 28% and the back end under 36%. It remains a sensible target and it is not what most programmes enforce. Many approve considerably higher, and some stretch toward 50%.
That is why "how much did they approve you for" and "how much should you spend" produce different numbers.
What the ratio cannot see
This is the whole problem, and it is worth being concrete about.
The calculation uses gross income — before income tax, before payroll deductions, before health insurance premiums. You do not spend gross income.
And it counts only debts that report to credit bureaus. Entirely invisible to it:
- Childcare, which for many households rivals the mortgage
- Commuting costs, which often rise when you move further out for a cheaper house
- Groceries, utilities and phone
- Retirement contributions
- Any saving at all
A household can pass a debt-to-income test comfortably and still have nothing left at the end of the month. The ratio was never designed to measure that.
Property taxes and insurance. Both sit inside your monthly payment, both are set by parties other than your lender, and both rise over time while your principal and interest stay flat on a fixed-rate loan. A payment that fits today can drift upward for reasons unconnected to your mortgage. Look up the actual tax bill for the specific property — it is public record — rather than accepting an estimate.
Maintenance. A frequently quoted rule of thumb is around 1% of the home's value per year, averaged. It is a rule of thumb rather than a measurement, and the real pattern is lumpy: nothing for three years, then a roof. The articles on roof replacement and HVAC describe the sort of expense that arrives without warning.
The method that actually works
Run it backwards.
1. Start with take-home pay, not gross.
2. Subtract what you actually spend — everything, including the categories the lender ignores.
3. Subtract what you intend to save, including retirement. Treat this as a bill, not a leftover.
4. What remains is your true housing budget. All of it, not just the loan.
5. Take roughly 20–25% off that figure before converting to a price, to leave room for taxes, insurance, maintenance and HOA dues.
6. Convert to a price using a mortgage calculator, entering current rates and your actual down payment.
The number this produces is usually well below the approval letter. That difference is the margin that lets you absorb a broken furnace without borrowing.
The other inputs that move the answer
Down payment. Affects the loan size, the rate, and whether you pay mortgage insurance — see how much down payment you actually need.
Credit score. Moves your interest rate, which moves the payment on the same house. Worth improving before applying rather than after — see what actually moves your credit score.
Loan term. A longer term lowers the monthly payment and raises total interest substantially.
Existing debt. Paying off a car loan can raise your borrowing capacity more than saving the same amount toward the deposit, because it improves the ratio directly.
Get pre-approved, then ignore the number
Pre-approval is worth having. Sellers take it seriously, and it confirms there are no surprises in your file.
Then set your own limit below it and hold to it. Estate agents and lenders both work from the approved figure — it is the number in front of them, and neither is being dishonest by using it. But nobody in that transaction pays your bills afterwards.
What to do
- Work out your true monthly housing budget from take-home pay, after saving
- Look up the actual property tax bill for any home you seriously consider
- Get quotes for insurance on that specific property, not an average
- Add HOA dues if any, and check what they have done over five years
- Convert to a price, then treat it as a ceiling rather than a target
This is general information, not financial advice — see our disclaimer.
Frequently asked questions
What is the 28/36 rule?
A guideline that housing costs stay under 28% of gross monthly income and total debt payments under 36%. It is a starting point, not what most lenders actually enforce today.
What do lenders really use?
Debt-to-income ratio, comparing monthly debt payments to gross monthly income. Many loan programmes approve well above 36%, and some approach 50%.
Why is the approved amount usually too much?
Because the ratios use gross income, before tax, and count only debts appearing on your credit report. Childcare, commuting, groceries and retirement saving are invisible to the calculation.
What does PITI include?
Principal, interest, taxes and insurance. Taxes and insurance are the parts buyers underestimate, and both rise over time independently of the loan.
What number should I use instead?
Work backwards from what you can pay each month after tax while still saving, then convert that to a price. That gives a figure you can live with rather than one you merely qualify for.
Sources
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