The final declaration replaces the Self Assessment return and is due on the same date, 31 January after the tax year ends. Where the quarterly updates were rough totals from your business, the final declaration is where everything is trued up and everything else gets added: accounting adjustments, capital allowances, private-use splits, plus all your non-business income — employment, dividends, interest, pensions, capital gains. It is the point where you confirm the figures are complete and correct, and it carries the same legal weight your tax return always did. So the shape of the year changes but the destination does not. Four light-touch updates, then one proper year-end job. For most of our clients we handle the declaration and they only see it to approve it.