Stamp Duty Isn't Paid "Through" the Mortgage
Stamp duty itself must be paid as a cash lump sum on completion — lenders don't pay it directly. What buyers actually mean by "adding it to the mortgage" is borrowing a larger amount overall (increasing the loan) so that the cash they need to bring to completion is reduced by roughly the stamp duty amount.
Why This Costs More Than It Looks
Borrowing an extra amount to cover stamp duty means paying mortgage interest on that amount for the life of the loan — often 20-30 years. A stamp duty bill of a few thousand pounds paid in cash becomes a few thousand pounds plus decades of compounding interest if rolled into the mortgage instead.
It can also push your loan-to-value (LTV) into a higher bracket, which may mean a worse interest rate on the whole mortgage, not just the extra borrowed amount — so the true cost can be larger than the extra interest on the stamp duty portion alone.
When It Can Make Sense
- You have the deposit and affordability for the higher loan amount but limited additional cash reserves
- The alternative is delaying the purchase or missing out on a property
- You've compared the total cost (extra interest over the mortgage term) against the cost of raising the cash another way
Always run the numbers with a mortgage broker before deciding — see our mortgage comparison link, or start with your exact stamp duty figure on the main calculator.