The effect of recent legislation is that what might appear to be a minor blemish in reporting to HMRC could lead to very significant penalties. Ad, when an overseas asset is involved, the penalties will include a charge equal to 10% of the asset\u2019s value. This is certainly to be avoided at all costs and there is a very limited time to make the necessary declaration. The new penalty regime refers to inaccuracies in the prior reporting of 'offshore matters' and 'offshore transfers'. So, if something happened outside the UK that had relevance to your UK tax affairs, and you have either not reported it to HMRC or not declared it at all, then the new penalties will apply unless HMRC receive a correction no later than 30 September 2018. The tax return inaccuracies being targeted are those arising in the 2016\/17 tax year (y\/e 5 April 2017) and all previous years. There are rules which determine how many past years can be examined by HMRC. However, given all the publicity in recent years concerning offshore disclosure facilities, HMRC are likely to take the view that a failure to correct amounts to at least 'careless' behaviour on the part of the taxpayer. Depending on the facts, HMRC might argue that the omission was more serious and amounted to 'deliberate' avoidance. 'Careless' behaviour allows HMRC to look back over ten years, but 'deliberate\u201d' behaviour opens up a\u00a020-year enquiry window. Given the mass of information which HMRC is in the process of receiving, the legislation allows HMRC until 5 April 2021 to open any enquiries. Correcting errors before the 30 September 2018 may still expose the taxpayer to penalties, but those will be raised under the 'old' rules rather than the much stronger new rules. If you think you have past errors in reporting to HMRC, then don't delay or it is likely to cost you significant penalties.