An affordability assessment is the detailed calculation a lender performs to decide how much they are willing to lend you. While the income multiple gives a rough starting point, the affordability assessment is what determines the actual figure. It takes into account not just your income, but your outgoings, your living costs, and what would happen if interest rates were to rise.
What expenditure lenders check
Lenders look at your committed monthly spending in detail. This includes existing credit commitments such as credit card minimum payments, personal loans, car finance, student loan repayments, and any other mortgages. They also factor in regular outgoings like childcare costs, school fees, maintenance payments, and ground rent or service charges if you are buying a leasehold property.
This is why two people on the same salary can be offered very different mortgage amounts. Someone with no debts and low outgoings will typically be offered more than someone with car finance and credit card balances, even if their gross income is identical.
ONS data and living costs
Beyond your declared expenditure, most lenders also apply a minimum living cost figure based on data from the Office for National Statistics (ONS). This covers essential spending like food, utilities, transport, and clothing. The figure varies by household size — a family with children will have a higher assumed living cost than a single applicant. Even if your actual spending is lower than the ONS benchmark, the lender will use the higher figure.
The stress test layer
On top of all this, the lender runs a stress test to check that you could still afford the mortgage if interest rates were significantly higher. The combination of income, expenditure deductions, living costs, and stress testing means the final affordability figure can be substantially different from a simple income multiple calculation. This is exactly why understanding how the calculation works and checking across multiple lenders matters.
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Check Your AffordabilityLast updated: April 2026
