When is self assessment necessary?

Hello, I am employed and my employer normally takes care of reporting my income to HMRC, so I am not registered for self assessment. During the fiscal year 2022/2023, I had some additional income from bank savings account that exceeded my personal allowance of 500 pounds per year (I'm in the 40% tax bracket). HMRC has recently changed my tax code in order to collect the correct amount of taxes without me having to report anything. In the fiscal year 2023/2024 I will also have another additional income from investment dividends and capital gains. It is still unclear to me if this will also be automatically reported to HMRC or if I am responsible to report it. In general, after talking to an accountant and calling HMRC a few times, I haven't understood what are the conditions that make a self assessment necessary and when I can instead count on HMRC being notified and adjusting the tax code automatically. Thank you and have a good day.

Hi,

Only investment income from banks and building societies is reported. Any other interest or dividends that you receive must be reported by you.

For the capital gains, depending on what has been sold, you may need to file a return. You can see guidance here:

Report and pay your Capital Gains Tax

Thank you.