If you complete a self-assessment tax return, you should declare all streams of income, including any interest you’ve earned from your savings. This will be used to calculate any tax you owe.
If you’ve overpaid, you can reclaim tax in a couple of ways. You could do it on your self-assessment return, or by completing an R40 form – available at gov.uk.
Banks and other financial providers are not always required to deduct tax from any interest you earn, so it’s your responsibility to make sure you’ve paid any tax owed. What they will do though, is report any interest earned to HM Revenue & Customs (HMRC) at the end of each tax year. HMRC might use this information to change your tax code to collect any tax due throughout the year. They may adjust your tax code even if you complete a self assessment return.