Self Assessment & Invoicing for UK Sole Traders: Records, Deadlines & the Tax Year

Introduction

If you are a UK sole trader or freelancer, Self Assessment is how you report your income to HMRC and pay your tax. Good invoicing habits make that return quick and stress-free; poor record keeping turns it into a January scramble — and exposes you in a compliance check. This guide covers the UK tax year, the Self Assessment deadlines, exactly what records HMRC wants, how long to keep your invoices, and how to organise everything so your return practically files itself.

This is general guidance for UK sole traders, not personalised tax advice. For your situation, see HMRC's Self Assessment guidance or speak to an accountant.

Quick Answer: The UK tax year runs 6 April to 5 April. Sole traders file Self Assessment online by 31 January after the tax year ends, when any tax and Class 4 National Insurance is also due. HMRC requires you to keep all sales invoices and business records for at least 5 years after that 31 January deadline. InvoiNova's free UK invoice generator produces sequentially numbered PDF invoices you can archive for your records.

How the UK Tax Year Works

The UK tax year for income tax runs from 6 April to 5 April the following year. So the 2025/26 tax year covers 6 April 2025 to 5 April 2026. This odd-looking range is historical, and it catches out almost every new sole trader who assumes it follows the calendar year or their own accounting period.

As a sole trader, your business profit for a tax year is what you report on that year's Self Assessment return. From the 2024/25 tax year onward, sole traders are taxed on profits arising in the tax year itself (the "tax year basis"), so aligning your bookkeeping to 6 April–5 April keeps things simple.

Self Assessment Deadlines

Miss these and HMRC charges automatic penalties, so put them in your calendar:

Late filing penalties start at £100 for missing the 31 January online deadline, then escalate after 3, 6 and 12 months. Interest is also charged on late tax payments.

Payments on account

If your Self Assessment bill is over £1,000, HMRC usually asks for payments on account — advance payments toward next year's bill, each 50% of the current year's tax, due 31 January and 31 July. New sole traders are often surprised by this in their first January, when they effectively pay 150% of the year's tax at once. Budget for it.

National Insurance

Sole traders pay Class 4 National Insurance on profits above the threshold, calculated and paid through Self Assessment alongside income tax. Class 2 NIC has been largely reformed for recent tax years — most self-employed people with profits above the small profits threshold now get NI credits without a separate flat charge. Check the current year's rates on gov.uk.

The £1,000 trading allowance

If your gross self-employed income is £1,000 or less in a tax year, it is covered by the trading allowance and you generally do not need to register for Self Assessment for that income. Above £1,000, you must register and report.

What Records HMRC Requires

HMRC requires sole traders to keep records of all business income and expenses. Specifically:

Income records:

• A copy of every sales invoice you issue
• Records of all amounts received, and the dates
• Any other business income (interest, grants)

Expense records:

• Receipts and invoices for business purchases
• Mileage, use-of-home, and other allowable expenses
• Bank and card statements for the business

Why it matters: these records support the turnover and profit figures on your return. In a compliance check, HMRC's first request is almost always your sales invoices and bank records. If they do not reconcile, you have a problem. Keeping a clean, sequential set of invoices is your strongest defence — see invoice numbering for getting the sequence legally correct.

How Long to Keep Invoices

Sole traders must keep business records for at least 5 years after the 31 January submission deadline of the relevant tax year.

Worked example: for the 2025/26 tax year, the online return is due 31 January 2027. You must keep those records until at least 31 January 2032.

If you file late, or HMRC opens an enquiry, keep the records longer — until the enquiry is resolved. Digital copies are acceptable, so storing each invoice as a PDF in a dated, numbered folder satisfies the requirement without a filing cabinet.

Note for VAT-registered sole traders: VAT records must be kept for 6 years, and VAT-registered businesses must follow Making Tax Digital. If you are approaching the VAT threshold, see UK VAT registration for freelancers.

Organising Your Invoicing for an Easy Return

The difference between a five-minute return and a five-day one is how you invoice through the year:

1. Number invoices sequentially with no gaps — INV-2025-001, INV-2025-002, and so on. Gaps and duplicates raise questions.
2. Save a PDF of every invoice the moment you issue it, in a folder per tax year (6 April–5 April).
3. Record the payment date, not just the invoice date — your turnover is based on the tax year the income arises in.
4. Keep client details consistent so the same client is easy to total across the year.
5. Reconcile monthly against your bank, rather than once in January.

For the fields every UK invoice must legally carry, see what a UK invoice must include. For a first-time primer, see how to invoice as a freelancer.

How InvoiNova Helps at Tax Time

InvoiNova's free UK invoice generator is built around the record-keeping habits Self Assessment rewards:

• Sequential, professional PDFs — download each invoice and archive it per tax year.
• Invoice history — your last 20 invoices are saved locally in your browser, so recent invoices are one click away (see the invoice history feature).
• Saved clients and templates — bill recurring clients consistently, so your annual totals are clean.
• No account, browser-only — your data never leaves your device, which suits sole traders who handle their own books.

Use the UK freelancer tool for service invoicing, and store each PDF as you go — by 31 January your records are already in order.

Common Self Assessment & Record-Keeping Mistakes

• Assuming the tax year is the calendar year — it runs 6 April to 5 April.
• Missing the 31 January payment, not just the filing — both fall on the same date.
• Forgetting payments on account — your first January can be 150% of the year's tax.
• Not keeping copies of sales invoices — the first thing HMRC asks for.
• Throwing away records too early — keep them at least 5 years after the 31 January deadline (6 years if VAT-registered).
• Gaps in invoice numbering — makes reconciliation and HMRC checks harder.

Keep Clean Records From the First Invoice

Self Assessment is simple when your invoices are numbered, saved, and reconciled through the year. InvoiNova's UK invoice generator is free, needs no account, and gives you a sequential PDF for every job — ready to archive for your tax return.

Open the free UK invoice generator and start building tax-ready records today.