Latimer Hinks is urging people to be prepared for a review into residential property sales.
The Property Sales Campaign (PRC) will seeHM Revenue and Customs reviewing the tax records of those who have sold residential properties but not reported them, to collect tax on undeclared gains.
Martin Williamson, Head of Residential Property at Latimer Hinks, said: "From September HMRC will be reviewing tax records of anyone who has sold a property since at least April 2007 and has not declared any taxable gains,.
"It is understood that the tax authority will use National Insurance numbers on Stamp Duty Land Tax (SDLT) forms to tie these in with income tax records of individuals.
"While the campaign is primarily focusing on second homes,HMRC also plans to investigate sales of properties which could have been used for reasons other than residential accommodation, for example where a claim has been made for a tax deduction for home running costs.
People who come forward by August 9 and calculate and pay the tax due by September 6 may receive preferential treatment.
Those who sold property on or after 6 April 2012 should not face scrutiny, on the basis that they have until 31 January 2014 to declare any taxable gains.