What replaces what
Under Self Assessment your figures reached HMRC once a year, in one return. Under MTD for Income Tax they arrive in four quarterly updates, each covering the tax year so far, and then in a year-end step that is due by 31 January following the end of the tax year.
Each update is cumulative, so it covers from the start of the tax year to the end of the update period rather than just the last three months. That is what makes corrections simple: if an expense was miscategorised in July, the October update corrects it and the earlier one is left alone.
Quarterly updates are summaries, not tax returns, and no accounting or tax adjustments are needed before you send one. The money side does not move with them either: the deadline to submit your tax return and pay any tax you owe is still 31 January. You cannot submit the year-end tax return until your quarterly updates are in.
The Self Assessment return for the year before you join
You still need to submit a Self Assessment tax return as you normally do for the tax year before you start using MTD for Income Tax. That is the last year in which your trading figures reach HMRC once, in a single filing. For tax years after that, you complete and submit your tax return using your MTD for Income Tax software.
New late submission and late payment penalties replace the current penalties that apply to your personal tax return once you are using the service. Those new penalties do not apply to Self Assessment returns for a non-resident company (SA700), a trust or estate (SA900) or a partnership, which keep the current penalties.
The year-end step is now called the tax return
If you read older material about Making Tax Digital, you will see the year-end step called the final declaration. HMRC's guidance for software developers was rewritten in August 2026 to remove that term in line with updated terminology guidance, and the wording on gov.uk is tax return. The old name still lives on inside the technical plumbing, so you may see final declaration in your software's own documentation or in an accountant's notes.
The deadline has not moved with the name. The year-end submission is due by 31 January after the end of the relevant tax year, and you can submit it earlier than that. Wherever you meet it, treat final declaration and tax return as the same step.
What the year-end return still has to cover
What has changed is where the trading figures come from. They now arrive during the year in the four quarterly updates, and the year-end return ties them together with everything else HMRC holds about you.
The submission remains the whole Self Assessment picture, not just your trading figures. HMRC adds some of it automatically, including employment and PAYE income, student and postgraduate loan plan types, state, private and occupational pensions, other taxable state benefits, CIS subcontractor deductions, Capital Gains Tax on residential property disposals, and Marriage Allowance claims.
You add the rest yourself: savings interest, your partnership profit share as an individual partner, dividends including dividends from your own company, and payrolled benefits in kind that are not subject to Class 1A National Insurance.
One thing does not move across to the new software. If you think you are due a tax refund, you cannot claim it in your tax return using MTD for Income Tax, so a refund claim stays outside the software.