Although the temporary increase in the Stamp Duty Land Tax (SDLT) threshold to £500,000 was aimed at those buying their main residence, it also benefits those buying a second or subsequent property where there is a 3% supplementary charge. Thus, the rate of SDLT on a second home costing up to £500,000 is now 3%. Previously, the rate was 3% up to £125,000, then 5% up to £250,000 and then 8% up to £825,000. So the SDLT on a second home costing £400,000 is now £12,000 compared to £22,000 if the purchase had completed before 8 July 2020.
Note that there are different thresholds and rates of Land and Buildings Transaction Tax for properties located in Scotland and Wales.