If you or your partner receive child benefit and your annual adjusted net income is over £50,000 then part of that child benefit will be taxable. If you reach £60,000 then the whole of the benefit will be taxable. That's been the score since 7 January 2013 and you have to disclose the benefit to the taxman and, register for self-assessment, if not already registered, sending in a tax return. Failure to do so will land you with a bill for the tax you should have paid, interest on it - and PENALTIES!!!!
That's what happened in a case just before the first-tier tribunal of the tax chamber.* There, the taxpayer's partner had received child benefit for the tax years 2012/13 through to 2015/16. The taxpayer had not disclosed the benefit to the taxman because he claimed he did not know of his obligation to do so. In fact, some of the benefit would have been taxable. He did not find out about his obligation until the taxman wrote to him in August 2018 telling him about the the high income charge. He then made full disclosure and paid up everything required except for a penalty of £296.
The tribunal has cancelled the penalty. The taxpayer, a lorry driver, had a reasonable excuse for not disclosing the benefit because he had not known that he was obliged to do so. There had been a number of cases where tribunals had decided that ignorance of the law could be a reasonable excuse. It would depend on the nature of the law in question and the characteristics of the taxpayer. HMRC argued that it was not obliged to inform taxpayers of a change in the law but, nevertheless, there had been national campaign about the this scheme. The tribunal concluded that the taxpayer was completely unaware of the new legislation and noted that there had been no targeted campaign to write to all tax payers earning over £50,000 when it had come in.
If you are late in appealing a penalty in your case, you can have a go at asking the tribunal to extend your time for doing so, explaining why an extension is needed.
* Kevin Brazier v The Commissioners for Her Majesty's Revenue and Customs  UKTT 00185 (TC)