See Am I liable to submit a tax return? for details on when a return needs to be completed. If you think that any of these circumstances apply and you have received a tax return from HMRC, or there has been some other change in your circumstances not covered here that makes you think you may be required to submit a return, you can contact HMRC about Self Assessment on 0300 200 3310 for advice.
If you have made a capital gain of more than £11,300 (2017-18), which is your tax-free allowance, or Annual Exemption Amount and/or started to receive income that is not taxed either entirely (or in part) before you get it (for example, rent payments to landlords or most income from National Savings & Investments accounts) and you have not received a tax return, you will need to contact HMRC by 5 October following the end of the year in which this happened – otherwise you may be liable to pay a penalty. If you did not submit a tax return for the previous tax year, but you need to submit one for the tax year that ended on 5 April, you must notify HMRC by 5 October.
If you have started a new business you must not wait until 5 October to tell HMRC. You should register within three months of trading, even if you already complete a Self Assessment tax return.
You should keep HMRC informed about any changes you think may affect your tax position. This applies even if this is not a change that requires a tax return, as it may affect the tax you pay through PAYE. When contacting HMRC you need to tell them your tax reference, which your employer will be able to provide, and your National Insurance number. You will also find these on your notice of coding.