Time is running out to escape the stamp duty surcharge on buy-to-let properties and second homes and the cost of failure is high.
Tax specialists have warned that many could be caught out because the deadline is 1 April, rather than the start of the new tax year on 6 April.
Missing the deadline by a day would increase the stamp duty on an average property in England and Wales, which costs £188,270, by a hefty £5,648.
In London it could add more than £15,000 to the cost of the average property purchase.
Chris Springett, director at accountancy firm Smith & Williamson, said anyone looking to buy a second home, residential buy-to-let or investment property must be sure they can complete by midnight on 31 March 2016.
This applies unless contracts were exchanged on or before 25 November 2015.
Springett said: “Missing the date by a few days could cost thousands of pounds and I fear many people could inadvertently miss out.”
The stamp duty charge on a property costing £188,270 is currently £1,265, but this will rise fivefold to £6,913 from 1 April, some £5,648 extra.
Stamp duty on the average London property costing £514,097 will more than double from £15,704 to £31,127.
Chancellor George Osborne is expected to confirm how the new rules will in his Budget on 16 March, days before the new charges kick in.
Springett added: “The changes are due to apply from April Fool’s Day, so anyone seeking to buy a second home, buy-to-let or residential investment property should keep this in mind.”