I am self-employed. Can I claim mileage allowance for the past five years? 

Arthur Weller replies:
If you have been entitled to claim mileage allowance but have not actually claimed it then you have overpaid income tax. So, in theory, you could claim ‘overpayment relief’. However, there are two points to note. Firstly, overpayment relief can only go back four years. Secondly, one of the exclusions from claiming overpayment relief is ‘Case A’ (see HMRC’s Self-Assessment Claims manual at www.gov.uk/hmrc-internal-manuals/self-assessment-claimsmanual/sacm12065), i.e. if the taxpayer failed to make a claim (SACM12075), and that is what seems to have happened in your case. But putting aside overpayment relief, you can now amend the tax return that should have gone in on 31 January 2019 for the tax year 2017/18.