The 4.5× figure comes from a financial stability rule designed to keep mortgages at or above that multiple to no more than 15% of new lending in aggregate. Individual lenders and products vary. The rule protects the financial system; it does not say what an individual household can comfortably afford.
| Income | Borrowing | Payment | Of take-home | At 7.5% |
|---|---|---|---|---|
| £30,000 | £135,000 | £750/m | 36% | 48% |
| £50,000 | £225,000 | £1,251/m | 38% | 50% |
| £75,000 | £337,500 | £1,876/m | 42% | 55% |
| £100,000 | £450,000 | £2,501/m | 44% | 58% |
| £150,000 | £675,000 | £3,752/m | 49% | 66% |
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The pattern nobody mentions
The share rises with income. At £30,000 the payment is 36% of take-home; at £150,000 it is 49%. That is the opposite of the usual intuition that higher earners have more room.
The reason is that the multiple is applied to gross income while the payment comes out of net. As income rises, a larger share of it is lost to tax, so the same multiple of gross is a larger multiple of what actually arrives. A £150,000 earner keeps a smaller proportion of their salary than a £30,000 earner, but is offered exactly 4.5 times the whole thing.
A higher-rate illustration
Lenders assess income, debts and regular spending, and many must consider likely interest-rate rises during the first five years. The FCA does not prescribe one uplift for every application, and an initial fixed rate lasting at least five years is generally exempt from that particular future-rate test. The final column is therefore a household planning scenario at 7.5%, not a reproduction of lender underwriting. In that illustration, the payment reaches 66% of take-home at the top of the table.
A more useful number
If you want the payment under a third of your take-home, you are looking at roughly 3.5× income rather than 4.5×. That is not a rule, and plenty of households live comfortably above it — but it is the multiple that corresponds to the comfort level people think 4.5× represents.
Work out your own on the affordability calculator, which shows the payment against your estimated take-home rather than your gross.
Sources
Reviewed 27 September 2026 against the Bank of England's July 2026 Financial Stability Report, the FCA interest-rate stress-test rule and MoneyHelper mortgage affordability guidance.
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