The social researcher, 43, and her husband Paul, 42, live with their children Ines, six, and Louis, eight, in Faversham, Kent.
They currently pay £1,718 a month for a 28-year mortgage on a four-bedroom new-build bought for £485,000 in 2020.
Her three-year 2.08 per cent fixed deal will expire next year – and, with cheaper deals already vanishing from the mortgage market, the Josephs know they are facing higher monthly payments.
Mrs Joseph said: “I can’t sleep at night for worrying about how I’ll pay the mortgage.
“The repayments are likely to rise to more than half my take-home pay – or even more if interest rates go through the roof.
“I could leave my current deal and lock into a new seven-year one now that I can afford, but my current lenders are demanding £12,000 for me to switch.