In some areas, repayments swallow up a quarter of income, data from trade body UK Finance shows.
Affordability pressures are the highest in East Anglia, with borrowers in North Norfolk spending 25.7% of their income on bills, followed by those in Hillingdon, Greater London, who spend 25.1%.
Spots in the London commuter belt make up the rest of the top 10 least affordable areas.
Londoners also have the highest average mortgage debt of £280,000, whereas those in Northern Ireland have the lowest with £99,500.
Seven out of 10 of the most affordable areas were found to be in Scotland.
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Borrowers in East Ayrshire and Inverclyde see 17% of income go towards mortgage repayments.
Overall, some 723,000 purchase mortgages were advanced in 2025, up 17% year-on-year.
London and Northern Ireland were found to have the highest percentage of borrowers on variable rate mortgages, at 16% and 18% respectively.
Borrowers in the capital also had the highest percentage of interest-only deals, at 12%.
All regions of the UK saw an increase in buy to let (BTL) in 2025.
However, the data also showed the highest BTL yields – of more than 9% – are in Scotland.
The lowest yields are in South Hams, Devon, offering 5%, followed by Kensington and Chelsea, offering 5.1%.
James Tatch, head of analytics at UK Finance, said: “It’s been challenging times for those trying to buy a property in recent years, with affordability pressures weighing heavy.
“But the pain is not felt equally across the country.
“Property prices, wages and demographics vary greatly across and within regions. All of these have an impact on affordability and, if you’re a landlord, how profitable your investment property is.”
