Thousands of home buyers could be hit with an unexpected tax bill of up to £15,000 each if the stamp duty holiday ends as planned on 31 March, Rightmove has warned. The property website said that with the stamp duty deadline approaching, some sellers who had put their property on the market during the last few weeks were hoping to tempt buyers with a competitive price in an attempt to squeeze in a sale before the holiday ends. However, Rightmove said its latest analysis showed that it was taking 126 days – or just over four months – from the time an offer was accepted until legal completion. It estimates that 613,000 property sales agreed in 2020 are “stuck in the processing logjam” and awaiting legal completion. Of those, the site anticipates that about 100,000 buyers will miss out on their stamp duty saving and therefore have to find thousands of pounds to pay a tax bill they had not anticipated if the 31 March cut-off date remains in place. The site’s findings will fuel calls for the government to extend the stamp duty holiday, which allows buyers of homes of a value up to £500,000 in England and Northern Ireland to pay no stamp duty, or a reduced rate for homes above that. For someone buying a £500,000 property, the saving – and therefore the potential bill – is worth £15,000, though for an average-priced property the figure is between £2,000 and £3,000. Previously the stamp duty threshold for residential property purchases was £125,000. A number of commentators have called for the chancellor, Rishi Sunak, to take action, as he prepares to present the budget on 3 March. Economists at the consultancy Capital Economics have said ministers might be forced to extend the holiday “to avoid a damaging downturn”, while Yorkshire building society is among those calling for a stamp duty “taper”, which would give a three-month grace period – until 30 June – to sales already agreed and with a mortgage in place. The Yorkshire said that having the holiday come to a dead halt on 31 March “could even cause some transactions to fall through”. A spokesperson for the Treasury said on Sunday: “The temporary stamp duty cut is helping to protect hundreds of thousands of jobs which rely on the property market by stimulating economic activity. Its time-limited nature is what has encouraged people to take advantage of the scheme.” The housing market has been booming since last summer, partly as a result of buyers rushing to take advantage of the giveaway, but mortgage lender Halifax recently said growth was slowing, and now Rightmove has reported a 0.9% fall in average asking prices this month. This is the biggest monthly drop since December 2019. Home moving website Reallymoving said last week that if the 31 March deadline remained in place, sellers should prepare for an increase in “gazundering” where buyers reduce their offer just before exchange. It said that a large number of buyers would have factored in the stamp duty saving and, if they failed to complete in time, they would suddenly need to find a significant amount of money or renegotiate the price. Tim Bannister, Rightmove’s director of property data, said the challenge of processing so many house sales in less than three months “is made even tougher by the new lockdown restrictions, Covid-19 sickness and home schooling further reducing capacity in conveyancing, legal searches and mortgage lending”. The site – whose data covers England, Scotland and Wales – reported that the average asking price for a property coming on to the market in January was down by 0.9%, or £2,887, on December’s figure, at £317,058. As a result, the annual rate of asking price growth fell to 3.3%. In December it was 6.6%. Despite that, Rightmove said home buyer activity was strong. It reported that visits to the website were up 33% on the same period at the start of 2020 as people continued to reassess their housing needs and priorities following their experience of lockdown. Matthew Smith, sales and lettings director at estate agent Thornley Groves in Manchester, said: “The Manchester suburbs – places such as Sale and Altrincham – are absolutely flying. People are looking for more outside space, and there’s much more activity at the higher end of the market – properties being sold at £500,000 and over – compared to 12 or 18 months ago.”