Self Assessment in Tax Optimiser opens on 2025-26, the return now being prepared. The year before, 2024-25 (6 April 2024 to 5 April 2025), can be added to any client as a previous year. It is there for two jobs: filing a 2024-25 return that is late, and amending a 2024-25 return that has already been filed, whoever filed it.
The return was due by 31 January 2026, so anything filed now for the first time is late and HMRC may charge penalties and interest; Tax Optimiser does not work those out. A return already filed can be amended until 31 January 2027. After that date HMRC no longer takes an amendment online.
Adding the year
A previous year is never created for you. Open the client’s Self Assessment screen, which lands on 2025-26, and look under the Tax Year selector on the summary card:
- Press Start 2024-25 (previous year).
- Read the note that opens: the deadline that has passed, the amendment date, and what is not available for the year.
- Press Start blank.
The year opens with only the client’s name and National Insurance number, and asks which pages the return needs, exactly as a new 2025-26 return does. There is no Start and copy for a previous year: copying brings losses and details forward, and 2025-26 figures have no place in the year before. From then on the Tax Year selector switches between the two years, and each is saved, calculated and filed on its own.
Started it by mistake? Remove this empty year takes it away again while nothing has been entered.
Has a 2024-25 return already been filed?
The banner across the top of a previous year asks Has a 2024-25 return already been filed with HMRC?
- No (the default) — this is the first return for the year. It is filed as an original return, late.
- Yes — a return is already with HMRC, filed with other software, on HMRC’s own site or on paper. The next submission is sent as an amended return, which replaces the one HMRC holds. Enter the whole return as it should now read, not only the figures that changed.
Getting this right matters: HMRC rejects a second original return for a year it already holds one for. If the 2024-25 return was filed from Tax Optimiser the question is not shown — use Reopen for amendment on the accepted year instead, as described in amending an accepted return.
What is different on screen
The pages, the live calculation, the Return Preview, client review and submission all work as they do for 2025-26, with the 2024-25 rules behind them:
- Only 2024-25 boxes are shown. Boxes HMRC introduced for 2025-26 are hidden: the Winter Fuel Payment box, the foreign income and gains (FIG) regime boxes on the Foreign, Capital gains and Residence pages, and the close-company details for directors on the Employment page (boxes 7.1 to 7.4).
- 2024-25 rates and limits. The calculation uses that year’s Scottish bands, student loan thresholds, blind person’s and married couple’s allowances and Class 2 rate, and the inline limits follow the year (for example maintenance relief capped at £4,280 and Class 2 at £217.30).
- HMRC’s 2024-25 forms and rules. The Return Preview and the full return PDF are drawn on the 2024-25 SA100 forms, and the pre-flight checks run HMRC’s 2024-25 validation.
- Dates belong to the year. A date that has to fall in the return year is checked against 6 April 2024 to 5 April 2025.
Capital gains and the 30 October 2024 rate change
Capital Gains Tax rates on shares, cryptoassets and other property rose from 10%/20% to 18%/24% for disposals on or after 30 October 2024, in the middle of the year. HMRC’s 2024-25 calculation does not split the year: it charges those gains at 10%/20% throughout and expects the extra tax on the later disposals in box 51, Adjustment to Capital Gains Tax. Tax Optimiser follows HMRC exactly, so the figures agree when the return is filed.
Work the adjustment out with HMRC’s Capital Gains Tax adjustment calculator on GOV.UK and enter it in box 51 on the Capital gains page. For example, with pay of £30,000 and £20,000 of share gains all made after 29 October 2024: the £3,000 annual exempt amount leaves £17,000, charged at 10% = £1,700; the adjustment is the other 8%, £1,360, in box 51; Capital Gains Tax due £3,060. While box 51 is empty the calculation shows a reminder. Leave it empty only if every such disposal was before 30 October 2024.
Residential property gains are 18%/24% for the whole year and need no adjustment. Business Asset Disposal Relief and Investors’ Relief gains are charged at 10%, and carried interest at 18% within the basic rate band and 28% above it.
What is not available for 2024-25
A few parts of the 2024-25 return have no boxes in Tax Optimiser. If the client needs any of them, the 2024-25 return has to be filed another way:
- Furnished holiday lettings on the UK property and Partnership pages (the regime ended on 5 April 2025)
- the remittance basis and domicile boxes on the Residence page, and remitted foreign savings on the Foreign page
- Overseas workday relief on the Residence page
- offshore fund gains and benefits from overseas trusts (Foreign page boxes 41 and 42)
- attributed gains on the Capital gains page (box 18)
- the zero-emission goods vehicle allowance on the UK property and Foreign pages
- the pension contribution payment from HMRC box on the Employment page (box 3.1)
- the any other information box on the Residence page — use the box on the main return instead
The Lloyd’s underwriter and minister of religion pages are not offered for any year.
Submitting, and carrying figures into 2025-26
Submitting works as in submitting the return to HMRC: the pre-flight checks, the declaration, and HMRC’s answer on screen. The year locks once HMRC accepts it.
Losses and unused amounts from 2024-25 are not pushed into a 2025-26 return that already exists: key them into its brought-forward boxes. Where the client’s 2025-26 return has not been started (or is still empty and you remove it), open 2024-25 and Start 2025-26 with Start and copy to roll the previous year forward.
Common questions
Why does the firm’s returns list show so few clients for 2024-25? The Self Assessment returns list shows, for a previous year, only the clients the year has been started for. Everyone else filed 2024-25 elsewhere and is not counted as late.
Can I add 2023-24 or earlier? No. 2024-25 is the only previous year.
A box I filled in for 2025-26 is missing. It is a box HMRC added for 2025-26, so it cannot be filed for 2024-25.
HMRC rejected the return saying one has already been received. Answer Yes to the question on the banner and submit again as an amendment.
