How UK Mortgage Affordability Is Calculated

What is mortgage affordability?

Mortgage affordability is a UK lender's view of the largest loan it can responsibly give you, based on your income, your monthly outgoings, the size of your deposit, and the rules the regulator imposes on the lending market. It is not a single formula. Two lenders, looking at the same applicant on the same day, can land tens of thousands of pounds apart — because each one combines an income multiple, a stress-rate test, an expenditure assessment, and its own credit policy in slightly different ways.

The mortgage affordability calculator gives a quick upper-bound estimate using the salary-multiple method, the same model UK banks display on their public-facing tools. It is a sense-check, not a mortgage offer. A lender's full assessment goes deeper — and can come out higher or lower than the headline multiple suggests.

How UK lenders actually calculate affordability

There are two regulatory touch-points every UK affordability assessment has to satisfy. Together they shape what comes out of the calculator.

The FCA affordability rules (MCOB 11.6)

The Mortgages and Home Finance: Conduct of Business sourcebook (MCOB) chapter 11.6 sets the affordability framework. The headline rules: a lender must verify the borrower's income, must assess their committed expenditure and basic essential expenditure, and must take into account "the impact of likely future interest rate increases on affordability for a minimum of 5 years." That last requirement is the stress test.

MCOB 11.6 does not prescribe a single multiple, a single stress rate, or a single expenditure formula. Each lender designs its own model within the rules. The FCA confirmed in March 2025 that lenders have flexibility in choosing which rate to stress against — the reversion rate, the initial rate plus a margin, or a likely follow-on product transfer rate, depending on the lender's policy. The result is genuine variation between lenders rather than a one-size answer.

The Bank of England LTI flow limit

Sitting above MCOB is the Financial Policy Committee's loan-to-income flow limit, originally introduced in June 2014. The rule caps the share of each large lender's new residential mortgages that can be at or above 4.5× household income to roughly 15% of new lending. In July 2025 the rule was eased: individual lenders can now disapply their own LTI cap as long as the aggregate UK figure stays around 15%, and the de minimis threshold (the size below which the rule doesn't apply) was lifted from £100m to £150m of annual residential lending.

In practice this means 4.5× is still the de facto mainstream ceiling. Some lenders stretch to 5–5.5× for higher earners, professionals, or borrowers passing strict affordability tests, but the bulk of residential lending sits in the 4.0–4.5× range. The calculator's 4.5× default reflects this. If your lender of choice is known to use 4.0× or 5.0×, change the multiple to see the impact.

Worked example

Two applicants, joint application. Primary income £40,000. Joint applicant £20,000. Combined committed outgoings £200/month (a personal loan and a credit-card minimum). Deposit £30,000. Lender's multiple is the FPC mainstream 4.5×.

• Combined income = £40,000 + £20,000 = £60,000
• Headline borrowing = £60,000 × 4.5 = £270,000
• Outgoings reduction = £200 × 12 × 4.5 = £10,800 (£200/month removed from affordable income, then run through the same multiple)
• Maximum borrowing = £270,000 − £10,800 = £259,200
• Maximum property value = £259,200 + £30,000 = £289,200
• Loan-to-value at maximum = £259,200 ÷ £289,200 ≈ 89.6%

Two things to notice. First, even £200/month of committed outgoings knocks £10,800 off the headline number — this is why borrowers who clear a small loan or close an unused credit card sometimes see a meaningful uplift in what they can borrow. Second, the implied LTV is nearly 90%, which means the example couple would need a lender willing to lend at 90% LTV. Plug your own figures into the mortgage affordability calculator to see how the relationship between deposit, multiple, and outgoings plays out for you.

Factors that affect what you can borrow

Income type and consistency

Lenders treat different income types differently, even when the headline number is the same. Basic salary from a permanent contract is the easiest case — usually 100% of gross is used. Bonus and commission are typically counted at 50% of a two- or three-year average, sometimes less if irregular. Self-employed applicants are usually assessed on the lower of the last two years' filed accounts or the two-year average, with a few lenders accepting one year's accounts for established professions. Day-rate contractors are sometimes assessed on day-rate × 5 × 46–48 weeks. None of this changes the multiple — it changes what counts as "income" before the multiple is applied.

Committed monthly outgoings

Anything you pay every month that you cannot easily cancel reduces affordable income. Personal loans, car finance, credit-card minimum payments, student loan deductions, child maintenance, and nursery fees all fall in this bucket. Subscriptions, food shops, and energy bills are usually folded into a separate essential-expenditure assessment rather than this debt-style adjustment, but they still reduce the affordability number a lender's full model produces. The calculator applies a simple debt-style reduction; expect a real lender to be a little stricter.

The stress rate

MCOB 11.6 requires the lender to test that you could still afford the mortgage if rates rise — for at least five years' worth of expected increases. Different lenders pick different stress rates. Some use the product rate plus 1%; some use a floor around 8–8.5%; some use the reversion rate plus a margin. A higher stress rate cuts the maximum loan, sometimes meaningfully. Two-year fixes are usually stress-tested harder than five-year fixes, because the borrower is exposed to a rate change sooner. This is one reason the same applicant can borrow more on a 5-year fix than on a 2-year deal at the same headline rate.

The deposit and resulting LTV

The deposit doesn't change the income multiple, but it sets the loan-to-value ratio and therefore the lending market you have access to. Mainstream lenders cap residential mortgages at 90–95% LTV; the cheapest rates appear at 60% LTV and below. If the headline multiple gives you a property price the deposit can't support at the LTV thresholds your chosen lender will accept, the affordable property price is effectively the deposit-constrained one, not the income-constrained one. The mortgage affordability calculator flags the implied LTV alongside the headline borrowing for exactly this reason.

Credit history

Two applicants with identical income and outgoings can land in different lender tiers based on credit file alone. Recent missed payments, defaults, CCJs, or high credit-card utilisation can disqualify mainstream lenders entirely and push the applicant towards specialist lenders with smaller multiples and higher rates. A clean file gives access to the full mainstream market. None of this is captured in a salary-multiple calculator — it's a hard constraint that sits behind the calculation.

How to improve your affordability

• Clear small committed debts before applying. A £200/month payment removes around £10,800 of headline borrowing at a 4.5× multiple. Paying it down before the credit search runs has an outsized effect.
• Increase the deposit if you can. Crossing an LTV threshold (90% → 85%, 85% → 80%, 80% → 75%, 75% → 60%) usually drops the rate, which loosens the stress test, which raises the loan allowed.
• Apply jointly where it makes sense. Adding a partner on a permanent contract can take household income from £40k to £60k+ and lift the headline multiple by tens of thousands of pounds. Be aware joint applicants share liability for the full loan.
• Use a whole-of-market broker. Brokers know which lenders are using which multiple, which stress rates, and which income types they're flexible on. The best fit for an unusual income profile (contractor, bonus-heavy, recent self-employed) is rarely the lender with the best advertised headline rate.
• Choose a longer fixed term. Five-year fixes are often stress-tested less harshly than two-year fixes, which can unlock a slightly larger loan from the same lender. Whether it's the right product for you depends on rate expectations, not just the affordability number.
• Time the application around bonus or year-end. Lenders that count bonus income usually look at a multi-year average. Applying after a strong bonus year — with the payslips on file — can lift the assessable income compared to applying mid-cycle.

Common mistakes

Treating the headline number as the offer

A salary-multiple result is an upper-bound estimate. The lender's full affordability model — including stress test, expenditure breakdown, credit checks, and product-specific criteria — will almost always come out lower for at least some lenders. Borrowers sometimes pick a property based on the headline figure and then find that the mortgage-in-principle they actually receive is meaningfully smaller.

Ignoring the costs around the mortgage

The maximum borrowing figure is just the loan. On top of that you'll need budget for Stamp Duty Land Tax (SDLT — see the stamp duty calculator), conveyancing, mortgage product fees, valuation and survey, building insurance, and moving costs. SDLT alone can run into tens of thousands on higher bands or on additional-property purchases. Use the mortgage repayment calculator to sanity-check the monthly payment before committing.

Misreporting outgoings

Lenders cross-check declared outgoings against bank statements and credit files. Under-declaring committed payments — especially loans and credit cards — usually surfaces as a discrepancy and slows the application down or kills it. Over-declaring discretionary spending unnecessarily reduces affordability. Be specific: list the obligated debt-style payments, not the lifestyle ones the lender will assess separately.

Forgetting the stress test exists

At low headline rates it's tempting to assume the affordability number will be generous. The stress test floor — typically 8–8.5% for many lenders — keeps the maximum loan tied to a rate well above the rate you'd actually pay on day one. A property that "feels" affordable at a 4% pay rate may not pass the lender's 8% stress, even if you'd comfortably handle the day-one payment.

When to seek professional advice

The mortgage affordability calculator gives you the maths in 30 seconds. It cannot tell you which lender to approach, whether to fix for two or five years, whether your bonus income will be counted at 50% or 100%, or how a recent change in employment affects your application. Those answers come from a regulated mortgage adviser.

Look for a CeMAP-qualified, FCA-authorised mortgage broker — ideally whole-of-market rather than tied to one lender. Their advice is regulated; mortgage calculators are not. Calculators are planning tools. A broker can pull a decision-in-principle, which is the only thing estate agents and sellers will treat as evidence you can fund the purchase.

For tax and pension-related questions that touch on your purchase (using ISA savings, gifts from family, releasing equity), a financial planner authorised by the FCA is the right next stop. A mortgage broker advises on the loan; they don't advise on which assets to liquidate to fund the deposit.

Frequently asked questions

See the FAQ section on the mortgage affordability calculator page for answers to the most common questions about salary multiples, joint applications, deposit treatment, and how this calculator differs from a real lender's decision.