• The government’s own calculator doesn’t include possible discounts
  • The software doesn’t differentiate between residential and “mixed use” properties

Homeowners and buyers in the UK could be owed as much as £2bn in repayments after overpaying their Stamp Duty Land Tax (SDLT), commonly known as stamp duty.

The overpayments have arisen because of failings in an online calculator provided on the HMRC website, according to reports.

The software apparently misses out a number of potential discounts. If a home has a separate annex, such as a “granny annex”, or has farmland attached to it, then it could be eligible for a discount.

Because these potential discounts are not being taken into account, it is being claimed that up to one in six calculations could be wrong. It is claimed that the calculator is not only being used by homebuyers but also by professionals such as lawyers who often rely on it to calculate tax due.

A rate of 12% stamp duty is paid on homes, while mixed-use properties are taxed at 5%, the Times reports.

It added that revenues from stamp duty had virtually doubled in the space of five years, hitting around £13bn last year.

A spokesperson from property tax specialists Thomtax said that HMRC was currently dealing with up to 900 cases every month.

They added that the tax calculator was up to the job and said that due to the volume of disputed cases, HMRC’s turnaround in responding to queries was often very long, taking up to 28 days.

HMRC said that its free online calculator should be used as a “guide only”. It added that “all reliefs are clearly signposted on stamp duty land tax returns”.