Ask a lender how much you can borrow and you'll get a number back fairly quickly. It feels precise, almost official. But that number is the output of a fairly mechanical process, and understanding how it's built tells you two useful things: what you can do to influence it, and why it isn't automatically the amount you should actually take on.
The starting point: income multiples
Most UK lenders begin with a simple ceiling based on income: commonly around four to four-and-a-half times a single salary, or the combined salaries on a joint application. A handful of lenders will stretch to five or more, usually reserved for higher earners or specific schemes. This multiple is a first filter, not the final word — it tells you roughly which ballpark you're in before anything else gets checked.
Then affordability testing takes over
The income multiple gets the headlines, but the number you're actually offered comes from an affordability assessment. Lenders look at your regular outgoings, existing credit commitments, dependants, and sometimes your general spending patterns, then work out what you could still repay comfortably — including if interest rates rose. This is why two people on identical salaries can be offered noticeably different amounts: the multiple is the same, the affordability picture underneath it isn't.
Your deposit changes more than the sum you need to find
Loan-to-value matters on both sides of the equation. A bigger deposit doesn't just shrink the amount you need to borrow; it often unlocks meaningfully better interest rates too, since lending at 75% of a property's value is less risky to a lender than lending at 95%. Our Mortgage Calculator is a quick way to see how deposit size feeds through to a rough monthly repayment.
What you're offered isn't the same as what you should borrow
Lenders test whether you could afford the maximum if things got tighter, but they can't see everything about your life: the car that's about to need replacing, the plans to have children, the job that might change. Borrowing right up to the ceiling leaves the least room to absorb any of that, and it means a rate rise at the end of a fixed period — most UK mortgage deals fix for two to five years, then move to a different rate — has the most bite.
A quick way to sanity-check it yourself
As a rough starting figure: take gross annual income, multiply by four to four-and-a-half, then add whatever deposit you have available. That gives a ballpark for the property price you might be looking at, before affordability checks narrow it further. For an actual repayment figure at a given rate and term, running it through a proper Mortgage Calculator gives a far more useful number than the multiple alone.
The actual point
The figure a lender is willing to offer is a ceiling, not a target. It's worth knowing how it's calculated, and worth checking against a calculator yourself, but the number that actually matters is the one you're comfortable repaying if a few things go wrong at once — not the biggest one anyone's prepared to lend you.
This article is general information, not personalised financial advice. For guidance specific to your situation, including how much you could actually borrow, a mortgage adviser or broker can help.