Following the final decision of your tax investigation, you are often notified of how much you are required to pay in owed taxes and given a timeframe for HMRC to receive payment. However, you also have 30 days within which you can appeal their decision. The time it can take to get to a resolution can vary, from three to six months for an investigation of a single aspect of taxation, to an average of 16 months for a full tax investigation. However, as we have stated, other factors, such as an escalating investigation or appeals, can cause an investigation to take significantly longer. Source: https://www.fsb.org.uk/resources/how-long-does-a-tax-investigation-take