Lenders calculate what you can borrow, not what you can afford

This looks at mortgage affordability from the practical end — what holds up once conditions stop being ideal.

What the lender is actually testing

An affordability assessment estimates whether you can meet the payments, using your income, your recorded commitments and standardised assumptions about household spending. Those assumptions come from statistical models of typical households, not from your actual budget. If your spending on childcare, travel, medical costs or supporting relatives is above typical, the model will not know.

The output is a maximum the lender is willing to advance, which is a statement about their risk appetite.

Stress testing and what it covers

Regulators in many countries require lenders to test affordability at a rate higher than the one being offered. The purpose is to reduce the chance of default when rates rise, which is a lender and system protection rather than a household comfort test. The stressed rate used has changed over time as regulation and rate environments have shifted.

In numbers, passing a stress test means the model thinks you would not default, not that the payment would be comfortable.

Run your own calculation

Take your actual budget, insert the proposed housing cost including all the running costs, and see what remains. Then repeat the exercise at a materially higher interest rate, and at a lower income if either earner's job is uncertain.

Practically, the number remaining after the stressed scenario is the one that tells you whether the purchase is sensible. This calculation frequently produces a lower figure than the lender's maximum, and that gap is the margin worth keeping.

What raises and lowers the maximum

Existing credit commitments, dependants, and in some systems student loan repayments all reduce the amount available. Clearing a small loan before applying can increase borrowing capacity by more than the balance cleared, because the monthly commitment is what counts. Income treated as reliable — basic salary — usually counts more heavily than bonuses, overtime or self-employed profits.

Self-employed applicants generally need several years of accounts, and the income figure used may be lower than they expect.

Borrowing to the maximum removes options

A household at the limit of what it can borrow has no capacity to absorb a rate rise, a job change or a period of reduced income. It also has no room to overpay, which is the main mechanism for reducing lifetime interest.

For most households, the extra amount borrowed buys a marginally better property and costs flexibility for decades. This trade is rarely presented explicitly, because the assessment produces a maximum and the market negotiates against it.

Agreements in principle are not offers

A decision or agreement in principle is a preliminary indication based on unverified information and can be withdrawn. The formal offer follows full underwriting and valuation, and can differ in amount or be declined.

In numbers, relying on the preliminary figure when making an offer on a property is a common source of collapsed transactions. Lending rules, stress-test conventions and consumer protections differ by country and change, so anything material warrants regulated advice.

The takeaway

Run the numbers at a higher rate and a lower income. The lender's maximum answers their question, not yours.

Costs compound as reliably as returns do, and in the same direction.