While for individuals, tax season often brings shudders, for CA firms in the UK it brings a monumental rise in workload. The HMRC deadline for self assessment is always in focus during this time. In the UK, the official window for filing your self-assessment tax return opens immediately after the end of the tax year 2025 on 5 April.
Although you can technically file your return from 6 April onwards, most filings happen towards the end — close to the UK self assessment deadline on 31 January.
The period from September to January is when self-assessment tax outsourcing becomes critical. For internal teams, this is a stressful stretch, with the workload increasing sharply. Knowing all the self assessment deadlines helps you plan effectively and ensures compliance for your clients.
In this blog, we will explore various aspects of the self-assessment tax season in the UK, while sharing all the key dates from September to January to help you get ready for the busy season.
When should you start preparing in September?
For self-assessment tax outsourcing and preparation, September is pre-season. This is the time to start preparing, ensuring you are not scrambling later in the year.
A good starting point for your preparation is pulling together last tax year’s records (from 6 April to 5 April), invoices, bank statements, pensions, and any capital gains documents. Whether you use spreadsheets, cloud-based bookkeeping software, or any modern practice management tools, you should start now to ensure that you don’t miss out on anything.
Why September matters for you
- September gives you enough time to chase any possible missing documents from clients.
- You can identify and scope out peak workload, which will give you an idea of whether outsourcing or temporary hiring is the path you should take.
- Preparing early also eliminates any potential for last-minute errors, which can result in penalties.
Key Takeaway: By starting your self-assessment tax preparation early, you gain the ability to convert any possible panic into a predictable workflow. It is also a good time to assess whether keeping everything in-house is the better option or if outsourcing UK Self-Assessment Tax makes more sense.
Do you need to register by 5 October?
As a UK chartered accountancy firm, if your client is new to Self-Assessment tax filing, you must register them with HMRC by 5 October following the end of the tax year.
If you miss this registration, it will change your tax filing window and can push you onto a third-month deadline from HMRC’s letter — but the tax still must be paid by 31 January. See HMRC’s official guidance:
https://www.gov.uk/self-assessment-tax-returns/deadlines. GOV.UK
Action checklist for 5 October
- Confirm who needs a Unique Taxpayer Reference (UTR).
- Gather details for GOV.UK Government Gateway sign-ups.
- If you have opted for self-assessment tax outsourcing make sure that your outsourcing partner has registered the clients and obtained UTRs on your behalf.
Key Takeaway: For the first-time self-assessment tax filers October 5 isn’t negotiable. However, by opting for the outsourcing route you can make sure that you do not spend all your time chasing forms.
If you have client(s) who still file self-assessment tax returns using paper, the deadline for that is midnight on 31 October after the conclusion of the tax year. While a majority of users files their taxes online, a considerable number of people still use paper to do so. For such users, it is recommended to start preparing early or convince them to switch to online filing via an outsourcing partner to avoid the tighter paper window. HMRC confirms the paper deadline as 31 October. (Source: GOV.UK)
Why do many firms move away from paper?
- Paper filing is error-prone and slow (postal delays).
- Online filing provides instant timestamps and significantly reduces transcription errors.
- Self-assessment tax outsourcing service providers typically file online and reconcile receipts digitally.
Key Takeaway: If possible, you must move away from paper-based tax filing, and by collaborating with an outsourcing service provider, you can do it with great ease.
For the taxpayers who want HMRC to collect any tax they are due through their PAYE tax code, they must submit an online Self-Assessment by midnight on 30 December following the tax year.
Missing this deadline will mean that they will not be able to use their PAYE collection. This option is only suitable for taxpayers who have an income tax liability under certain thresholds and are already on PAYE. See details: https://www.freeagent.com/guides/self-assessment/self-assessment-deadlines/ and HMRC SA150 notes.
Practical implications for your practice
- If your client wants to collect through PAYE, you need to submit by 30 December.
- Outsourcing partners can file on time and ensure the PAYE option is requested correctly.
Key Takeaway: For PAYE collection, December 30th is the cut-off date. Outsourcing can ensure timely online submissions and maintain open collection options.
For taxpayers 31st January is the big day. You can file and pay any tax due online by 23:59. This also includes the balancing of any payment for the previous tax year and the first payment on account for the following year where applicable. Missing this deadline can attract penalties starting from a £100 fixed penalty at first, then escalating interest/penalties. HMRC has listed 31 January as the online filing and payment deadline.
How outsourcing helps here?
- Outsourcing firms helps you scale your operation to meet the seasonal January workload without hiring external resources and prevent missed deadlines.
- Outsourcing service providers have experts that can run final reconciliations, prepare payment summaries, and set up client reminders.
- For CA firms, outsourcing reduces overtime pressure and error risk during the busiest stretch.
Key Takeaway: January 31st is non-negotiable deadline for tax payers. For the CA firms in the UK, outsourcing can help you manage workload and compliance by gaining access to dedicated resources.
Here is a compact timeline you can put in your firm’s calendar and hand to clients.
Timeline (September to January)
- September: Collate documents, decide outsourcing scope.
- By 5 Oct: Register new filers with HMRC. GOV.UK
- 31 Oct: Paper filing deadline (if relevant). GOV.UK
- 30 Dec: File online if you want HMRC to collect by PAYE. FreeAgent
- 31 Jan: Final online filing and tax payment deadline. GOV.UK
- Define whether you want full filing support or just reconciliation.
- Ensure NDAs and client data security protocols with providers.
- Agree on SLAs, turnaround times, and review responsibilities.
- Confirm your outsourcing partner aligns with your digital tools.
If you are outsourcing self-assessment, here are a few must-do steps for you:
- Define scope (data entry, reconciliations, filing, and client communications).
- Agree on security: NDAs, data encryption, and access controls.
- Set SLAs: turnaround times, review windows, and escalation paths.
- Map tech: ensure your partner works with your bookkeeping/portal tools.
- Client consent: get signed permission to act on the client’s behalf.
- Key benefits to measure: turnaround time, error rate, client satisfaction, and cost per return.
Key Takeaway: Outsourcing succeeds when expectations, security, and technology are agreed upon up front. You must treat it like a supplier relationship and not just as a quick fix.
We now have given a roadmap to navigate tax season successfully. Start in September, register by 5 October if needed, avoid paper after 31 October, meet the 30 December cut-off for PAYE collection where required, and never miss the 31 January filing/payment deadline. These dates are set by HMRC.
If the January peak season is squeezing your resources and time, it is time to look for a UK Self-Assessment Tax Outsourcing partner this September. You can kick start your preparation with our FREE TRIAL, or can just write in to us at marketing@datamaticsbpm.com and we will help you get started with your tax season preparation in a jiffy.
