The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. Whether you are self-employed, work under the Construction Industry Scheme (CIS), or have other income to declare, preparing your records early can make your Self Assessment easier and help you plan for the tax bill.
1. Key Self Assessment deadlines
- 5 October 2026: usual deadline to notify HMRC if you need to register for Self Assessment for the first time. If you register after this date, check the specific filing deadline HMRC gives you.
- 31 October 2026: deadline for most paper returns for 2025/26.
- 30 December 2026: deadline to file online if you want HMRC to consider collecting an eligible bill through your PAYE tax code.
- 31 January 2027: deadline to file your online return and pay the tax due for 2025/26, including any first payment on account for 2026/27.
- 31 July 2027: second payment on account for 2026/27, if applicable.
Different deadlines can apply in special circumstances. Always check the deadline in your HMRC correspondence. Check official HMRC deadlines.
2. What documents should you gather?
- Sales invoices, income records and relevant bank statements for the tax year.
- Receipts and invoices for business purchases and running costs.
- Employment P60 or P45 forms, P11D details and other income information where relevant.
- CIS deduction statements if you worked as a subcontractor.
- Rental income and property expense records if you let property.
- Details of pension contributions, charitable Gift Aid donations and other relevant tax information.
- Last year's return and HMRC payment records, particularly if you make payments on account.
You normally do not send receipts with your tax return, but you must retain supporting records in case HMRC asks to see them. HMRC record-keeping guidance.
3. Allowable expenses: what can sole traders claim?
Allowable business expenses can reduce taxable profit, but they must meet the relevant HMRC rules. Depending on your business and accounting method, examples may include:
- Office supplies, software and certain business telephone or internet costs.
- Business travel costs (not ordinary home-to-permanent-workplace commuting).
- Business insurance and qualifying professional fees.
- Working-from-home costs calculated using an appropriate method.
- Tools, equipment and other costs subject to the relevant tax treatment.
Only the business-use proportion of mixed personal and business costs is generally claimable. Buying something does not automatically make it tax-deductible, and capital allowances or other rules may apply. Read our home office expenses guide and HMRC allowable expenses guidance.
4. CIS subcontractors: check your deductions
If you work under CIS, gather your monthly deduction statements and check them against payments received. CIS deductions may be credited against your final tax and National Insurance position, depending on your circumstances. Missing statements and incorrect figures can delay an accurate calculation. See our CIS tax returns guide.
5. Don't forget payments on account
Some taxpayers must make advance payments towards the next year's bill. This can make the amount due in January higher than expected because it may include both a balancing payment for 2025/26 and the first instalment for 2026/27. Payments on account do not apply to everyone; eligibility depends on your previous liability and tax already collected at source. Understand payments on account.
6. Common mistakes to avoid
- Leaving everything until January: missing invoices and statements take time to find.
- Mixing personal and business expenses: use reasonable, supportable business-use calculations.
- Forgetting additional income: check employment, rental, savings and other income that may need declaring.
- Overlooking CIS statements: reconcile deductions before submitting.
- Assuming filing means payment: submitting the return and paying the bill are separate obligations.
- Ignoring HMRC messages: registration, payment and filing requirements can differ by situation.
7. A simple checklist before you submit
- Confirm the correct tax year and your filing obligation.
- Reconcile income against invoices and bank transactions.
- Review expenses and retain evidence.
- Check other income, reliefs and CIS deductions.
- Review the tax calculation and budget for the payment deadline.
- Keep your submission confirmation and supporting records.
Need help with your Self Assessment?
Precision Books supports sole traders, CIS subcontractors and small business owners with bookkeeping and Self Assessment preparation. If your records need organising or you want help understanding your tax return, get in touch to discuss your situation.
Sources: HMRC deadlines · Claiming expenses · Record keeping. General information only; tax treatment depends on individual circumstances. Information checked 11 October 2026.