To remortgage is to pay off your existing mortgage with a new one secured on the same home. Americans and Australians call it refinancing. In Britain it is routine, because mortgages are commonly sold as fixed deals of a few years that fall back to the lender's standard variable rate when they end, so many borrowers take a new deal every few years, either from a new lender or as a product transfer with the one they have.
The test for whether it pays is the total cost over the new deal, not the rate alone. Add the arrangement fee, any valuation and legal costs, and any early repayment charge if you leave before your current deal ends, then compare that with the interest you would save. Moving £200,000 from 6% to 4.5% saves about £3,000 of interest in the first year, which comfortably covers a £999 fee; moving from 4.8% to 4.5% saves about £600 and may not.
Rules shape what you can do. Under MCOB 11.9 a British lender may run a lighter affordability check when you switch without borrowing more, provided the new deal is more affordable and you have not missed a payment in the past 12 months. In Australia, Moneysmart warns that switching with less than 20% equity may mean paying lenders mortgage insurance again, plus a break fee on a fixed loan and fees to close the old loan and open the new one. In the United States, refinancing a conventional loan with less than 20% equity can bring private mortgage insurance with it, as the CFPB notes.
To test a remortgage, enter your current balance, the new rate and the years left in the mortgage calculator, then compare the monthly payment and total interest with your current deal.