Self Assessment season runs from the autumn records request to the 31 January online deadline, and practices get through it by moving the preparation work earlier. Collect records from September, draft SA100 returns in November and December, and keep January for review, sign-off and payment. CT600 year-ends and 60-day CGT returns land in the same months, so plan them on one calendar.
To survive Self Assessment season, separate preparation from review. Administrators chase records, populate draft returns and track deadlines from September onwards; qualified staff review, decide treatments and file. The 2025/26 online return and balancing payment are due by 31 January 2027, so most drafting should be finished before Christmas.
Key points
- Online SA100 returns for 2025/26 are due by 11:59pm on 31 January 2027; paper returns by 31 October 2026 (GOV.UK: deadlines).
- CT600 returns are due 12 months after the end of the accounting period, but tax is usually due 9 months and 1 day after it (GOV.UK).
- Capital Gains Tax on UK residential property must be reported and paid within 60 days of completion (GOV.UK).
- Most of the Self Assessment season workload is record collection and data entry, which can sit outside the review team.
- Clients who joined MTD for Income Tax in April 2026 still file a normal 2025/26 return this January.
Terms used in this guide
- Self Assessment season: the period from the autumn records request to the 31 January filing and payment deadline, when personal tax work peaks in most practices.
- SA100: the main Self Assessment tax return for individuals.
- CT600: the Company Tax Return filed with HMRC for each accounting period.
- 60-day CGT return: the separate report and payment for a gain on UK residential property, due within 60 days of completion.
- Payments on account: two advance payments towards next year’s bill, due 31 January and 31 July, each usually half of the previous year’s tax.
Self Assessment season deadlines for 2025/26 returns
Every date below was checked on GOV.UK in October 2026. Put them on one shared calendar so nothing in Self Assessment season depends on one person’s memory.
| Date | What is due | Source |
|---|---|---|
| 5 October 2026 | Register for Self Assessment if a 2025/26 return is needed and the client has not filed before (or did not need to for 2024/25) | GOV.UK |
| 31 October 2026 | Paper SA100 return for 2025/26 | GOV.UK |
| 30 December 2026 | Online return, if the client wants the balance collected through their tax code | GOV.UK |
| 31 January 2027 | Online SA100 return, balancing payment for 2025/26 and first payment on account for 2026/27 | GOV.UK |
| 31 July 2027 | Second payment on account for 2026/27 | GOV.UK |
| 12 months after period end | CT600 Company Tax Return | GOV.UK |
| 9 months and 1 day after period end | Corporation Tax payment (taxable profits up to £1.5 million) | GOV.UK |
| 60 days after completion | Report and pay CGT on a UK residential property sale | GOV.UK |
A client who registers after 5 October 2026 gets a filing deadline of 3 months from the date of HMRC’s notice, but the tax is still due by 31 January 2027. Payments on account are not needed if last year’s bill was under £1,000 or more than 80% of the tax was collected at source.
Why CT600 and CGT work collide with Self Assessment season
The SA100 itself is rarely the problem. The pressure comes from three filing cycles sharing one team. Companies with March year-ends need their Corporation Tax paid by 1 January, and those with December year-ends have their CT600 due on 31 December. Property disposals trigger 60-day CGT returns whenever they complete, including over Christmas. Every one of those jobs competes with SA100 drafting for the same qualified reviewers.
The fix is to give each cycle its own lane on the calendar and to start the data work early. A practice that knows in September which companies have autumn deadlines and which clients sold property in the year can plan Self Assessment season around them.
Preparation vs review: who does what
Very little of the time spent on SA100, CT600 and CGT returns is technical. Most of it is chasing, keying and tying out figures before anyone applies judgement. That is the work to move away from your reviewers.
| Task | Preparation (administrator) | Review (qualified accountant) |
|---|---|---|
| Records request and chasing | Sends requests, logs replies, chases gaps | Decides when to stop chasing and estimate |
| Income data | Keys P60s, dividend vouchers, interest certificates, rental summaries | Checks completeness against last year |
| Bookkeeping tie-out | Reconciles bank records to the ledger and lists queries | Approves adjustments |
| Draft return | Populates the return in the practice’s software | Decides treatments, reliefs and claims |
| Client approval and filing | Sends the return for signature and tracks it | Final review and submission under the practice’s credentials |
| Payments and penalties | Sends payment reminders and logs any penalty notices | Decides whether to appeal |
The same split works for SA100, CT600 and CGT preparation support and for the monthly bank reconciliation outsourcing that keeps business clients’ records ready before Self Assessment season starts.
A 7-step plan for Self Assessment season
- July to September: segment the client list. Flag new clients, property sellers, company directors and anyone joining MTD. Note every CT600 and Corporation Tax payment date between October and February.
- September: send the records request. One clear checklist per client type, with a return date in October.
- Early October: register new clients. Anyone who needs a return for the first time must tell HMRC by 5 October.
- October to November: chase and key. Log what has arrived, chase the rest weekly, and key data as it lands.
- November to mid-December: draft returns. Populate drafts and list queries for the reviewer and the client.
- December: review and approve. Clear the returns of clients who want tax collected through their code before 30 December.
- January: finish, file and pay. Keep January for late records, final reviews, payment reminders and penalty prevention.
Client records request checklist
- P60, P45 and any P11D for the year
- Bank and building society interest; dividend vouchers
- Rental income and expenses, with letting agent statements
- Self-employment income and expenses, or access to the bookkeeping software
- Pension contributions and Gift Aid donations
- Completion statements for any property or share disposals
- Child Benefit received, if income was over the charge threshold
- Any HMRC letters received since the last return
Building this checklist into your client onboarding checklist means new clients know from the first meeting what Self Assessment season will ask of them.
A worked example
A practice with 80 personal tax clients sends its records request on 15 September. By mid-November an administrator has keyed data for 55 clients and chases the other 25 every Monday. Drafts for the first 55 go to the reviewing accountant in batches of ten from late November. By 20 December, 60 returns are approved, and January is left for 20 late files and payments. The accountant spends January reviewing, rather than starting returns from a blank page.
Where MTD changes the 2026/27 Self Assessment season
MTD for Income Tax adds new deadlines without removing the old one. Clients who joined on 6 April 2026 still file their 2025/26 return under the current rules by 31 January 2027, and GOV.UK confirms that current penalties apply to that return (GOV.UK: MTD penalties). They also have a quarterly update due on 7 February 2027, a week after the January peak.
The 2025/26 returns you file this Self Assessment season also decide who joins next: clients with qualifying income over £30,000 for 2025/26 must use MTD from 6 April 2027. Flag them as you file and start the sign-up work in February. Our MTD quarterly deadlines calendar and MTD client onboarding checklist cover the next steps, and the MTD for ITSA guide explains the wider changes.
Penalties: what a missed 31 January costs
A late online return brings an automatic £100 penalty, with daily penalties after 3 months and further penalties at 6 and 12 months. Tax paid late attracts 5% penalties at 30 days, 6 months and 12 months, plus interest (GOV.UK: penalties). When a client does have a genuine excuse, our guide to appealing late filing penalties covers the routes and evidence, and our penalty appeals service can draft them for your review.
FAQs
When is the Self Assessment deadline for 2025/26?
The online return for the 2025/26 tax year must reach HMRC by 11:59pm on 31 January 2027, and any tax owed must be paid by the same date. Paper returns were due by 31 October 2026. If the client wants the balance collected through their tax code, the online return must be filed by 30 December 2026.
When is a CT600 due?
A CT600 is due 12 months after the end of the accounting period it covers. The Corporation Tax itself is usually due earlier, 9 months and 1 day after the period ends, for companies with taxable profits up to £1.5 million. Larger companies pay in instalments. A late return brings a £200 penalty from the first day.
Do I need to report capital gains separately from Self Assessment?
For UK residential property, yes. A gain must be reported and the tax paid within 60 days of completion, using HMRC’s online service. Other gains, such as shares, are reported on the Self Assessment return. If the client files a Self Assessment return, the property gain goes on it too, and the tax already paid counts towards the bill.
Can tax return preparation be outsourced safely?
Yes, when the split is clear. An outsourced team can chase records, key data and populate draft returns in the practice’s own software, using logins the practice issues and can revoke. The qualified accountant keeps treatment decisions, client advice, final review and submission under the practice’s agent credentials.
When should a practice start preparing for Self Assessment season?
In the summer. Segment the client list in July or August, send records requests in September, register new clients by 5 October and aim to have most drafts ready by mid-December. Practices that start chasing in January spend the month keying data instead of reviewing returns and advising clients.
Make January a review month
Our SA100, CT600 and CGT support takes on the chasing, keying and draft returns, working in your software from September, so your accountants spend the peak reviewing and advising. Book a consultation to plan this Self Assessment season with us.
