New research suggests many borrowers are being overambitious when buying a new home. But a rise in interest rates means they could get a nasty shock.
Thousands of homeowners are likely to face a sharp and unaffordable rise in their monthly mortgage payments.
The Bank of England base rate has stood at a record low of 0.5% since early 2009.
But an imminent increase will force repayments up for millions of borrowers.
Mortgage affordability warning
Now credit reference agency Experian is warning that many borrowers are overestimating their ability to afford the extra costs.
The problem is particularly serious for people who have recently bought a home or who are planning to in the coming months.
Experian said that a significant proportion of these borrowers were already overstretching themselves when applying for a home loan
Its research showed that the typical potential buyer was planning to buy a property worth £235,000 and had a total household income of just over £50,000.
'Get your finances in shape'
The likely monthly mortgage repayments required to finance a purchase of this size would be around £1,300 a month based on a 10% deposit.
Yet, on average, those surveyed said they would only be able to afford £780 a month.
Spokesman Peter Turner said: "These findings show just how important it is to get your finances in the best shape possible in advance of a mortgage application.
"It's not just a case of making sure you're accepted: it's a case of using your finances to land the best rate possible.
"Ensuring your mortgage application gets the highest credit score possible can make a difference of hundreds of pounds a month, and thousands over the course of the mortgage."
Tough new mortgage lending rules
Recent regulatory changes mean that conditions are now tougher for all mortgage applicants.
The Mortgage Market Review, which came into force at the end of April, forces banks and building societies to be more rigorous when scrutinising potential borrowers.
This means that everyday spending and other financial commitments such as loans and childcare costs are more likely to be taken into account when deciding if an applicant can afford repayments.
Experian said that it had been "widely reported" that the Bank of England was to raise the base rate in 2015.
Mid-2015 interest rate rise predicted
This would have an immediate effect on the cost of variable and tracker mortgages.
And borrowers on fixed-rate loans would face higher repayments when their current deals came to an end.
Mark Harris, chief executive of mortgage broker SPF Private Clients, said he expected the base rate to be raised in mid-2015.
"Nobody has a crystal ball, or at least not a reliable one," he said.
"We are not expecting interest rates to rise any time soon as the economic recovery is still too tentative.
"Indications are that when interest rates do start to rise they will do so slowly before settling at 2 to 3 per cent, lower than the 5 per cent we have been used to in the past."
'Fixed-rate mortgages offer insurance'
But Harris added that even if rates were not to return to previous highs, borrowers should still take action to limit the potential effects of a rising base rate.
"Fixed-rate mortgages provide insurance and protection from interest-rate rises," he said.
"Only in five years when you look back will you know what was best.
"However, if you cannot afford to be wrong – that is, if interest rates were to rise you would struggle to pay your mortgage – then you should always fix."
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