The Final Declaration: How MTD Replaces the SA100 Tax Return

You've filed four quarterly updates — now comes the submission that actually finalises your tax for the year: the final declaration. It replaces the old SA100 Self Assessment return, and understanding it removes most of the anxiety around MTD's year-end.

Quarterly updates are estimates; the final declaration is the truth

Throughout the year, your quarterly updates are estimates-in-progress. They keep HMRC updated but don't finalise anything. The final declaration is where you:

1. Finalise each business's figures,
2. Add all your other (non-business) income,
3. Claim your reliefs and allowances,
4. Review HMRC's tax calculation, and
5. Declare that it's complete and correct.

That declaration is what replaces the SA100. Same information as the old return — new mechanism.

It's a client-level submission

Here's a structural point that trips people up. Quarterly updates are per business (each self-employment, UK property and foreign property files its own). The final declaration is different:

The final declaration is a single, whole-person submission. It pulls together all your businesses plus your other income into one year-end picture.

So even if you have three income sources filing separate quarterly updates, there's just one final declaration for you as an individual.

What you add at year-end

Your quarterly updates only covered self-employment and property. At the final declaration you add everything else that belongs on a tax return, such as:

• Employment / PAYE income and benefits (from your P60/P11D)
• Dividends — UK and foreign
• Savings interest
• Pensions — contributions and income
• Gift Aid and other reliefs
• Marriage Allowance
• Any adjustments, allowances and losses for your businesses

This is also where business annual adjustments (basis adjustments, averaging, capital allowances, losses and so on) are brought in — the things that don't belong in a routine quarter.

The crystallisation step

Finalising isn't a single button — it's a short, deliberate sequence often called crystallisation:

1. Intent to finalise — you tell HMRC you're ready to finalise the year.
2. HMRC produces its calculation — the software retrieves HMRC's official tax computation based on everything you've reported.
3. You review it — check the numbers, the tax due, and any payments on account.
4. You submit the final declaration — this locks the year in.

That review step matters: the final declaration is the legally significant filing, so you're confirming HMRC's calculation is right before you commit to it.

The deadline (and payment)

The final declaration is due by:

31 January following the end of the tax year.

For 2026/27, that's 31 January 2028 — the same familiar date as Self Assessment, and the same day your tax payment is due. Note that the first-year penalty easement for quarterly updates does not cover the final declaration, so this deadline is a hard one from year one.

How to make year-end painless

• Keep quarterly figures accurate as you go, so finalising is a review, not a rebuild.
• Gather your other income early — P60, P11D, dividend vouchers, interest statements, pension paperwork.
• Review HMRC's calculation carefully before declaring.
• Keep the receipt — your proof the year is finalised.

How ThisQuarter helps. ThisQuarter handles the whole year-end in one place: add employment, dividends, pensions, Gift Aid and Marriage Allowance, trigger and review HMRC's calculation, then make the final declaration — with the submission stored as proof. See how it works.

The bottom line

The final declaration is your SA100 replacement: one whole-person, year-end submission that finalises every business plus your other income. Do the quarterly updates well, add the rest, review HMRC's calculation, and declare by 31 January — that's the year closed.