The Self Assessment deadline of 31 January arrives every year, yet millions of UK taxpayers still find themselves scrambling in the final weeks. Starting your document gathering now, in September, gives you four months to track down records, clarify queries with HMRC, and file your return without the January stress.

According to HMRC, around 12 million people must file a Self Assessment return each year (HMRC, 2026). The deadline applies to the tax year that ended on 5 April 2026, and your return must be submitted online by 31 January 2027. Gathering documents now means you can file early, claim any refund sooner, and avoid the website slowdowns that plague the final days of January.

Employment Income Records

If you were employed during the 2025-26 tax year, you need your P60 (the end-of-year certificate showing total pay and tax deducted). Your employer must provide this by 31 May following the tax year end. If you left a job during the year, you should have a P45 showing earnings and tax up to your leaving date.

Keep payslips for the full year, particularly if you had multiple jobs, started or left employment mid-year, or received bonuses, commissions, or benefits in kind (such as a company car or private health insurance). HMRC may query figures, and payslips provide the backup.

Self-Employment and Business Records

Self-employed individuals must provide a full account of business income and allowable expenses. Gather all invoices issued to clients, receipts for business purchases, and bank statements showing business transactions. If you use the cash basis (recording income when received and expenses when paid), your bank statements form the core record.

Allowable expenses include office costs, travel (excluding ordinary commuting), equipment, professional fees, and a portion of home costs if you work from home. The simplified expenses scheme lets you claim a flat rate per hour for home working or per business mile driven, but you must still record the hours or mileage (MoneyHelper, 2026).

Other Income Sources

Self Assessment covers income beyond employment and self-employment. Gather records for rental income (if you let property), dividend vouchers or statements (for dividends above the dividend allowance), and savings interest statements (for interest above the Personal Savings Allowance). If you received income from a trust, partnership, or foreign source, you need documentation of amounts received and any foreign tax paid.

Pension Contributions and Gift Aid

Contributions to a personal pension (including a SIPP) attract tax relief. Your pension provider should send an annual statement showing contributions made during the tax year. If you are a higher-rate or additional-rate taxpayer, you can claim extra relief through Self Assessment beyond the basic-rate relief added automatically to your pension pot.

Similarly, if you made Gift Aid donations, gather statements or receipts from the charities. Gift Aid extends your basic-rate and higher-rate tax bands, which can reduce your overall tax bill, particularly if you are close to a threshold.

Capital Gains Records

If you sold shares, investment property, or other chargeable assets during 2025-26, you must report capital gains above the annual exempt amount (6,000 GBP for 2025-26, falling to 3,000 GBP for 2026-27 as per HMRC rules). You need the purchase cost (including fees), sale proceeds (net of fees), and records of any allowable costs such as improvement works on property.

Read also: UK Income Tax Calculator: How to Work Out Your Take-Home Pay for 2025 to 2026

For shares, obtain contract notes or statements from your broker. For property, gather the original purchase completion statement, solicitor invoices, estate agent fees, and any receipts for capital improvements (not routine maintenance).

Student Loan and Tax Code Checks

If you have a student loan, HMRC uses Self Assessment to calculate repayments if you are self-employed or have multiple income sources. Check which plan you are on (Plan 1, Plan 2, Plan 4, or Postgraduate Loan) and note any voluntary repayments made directly.

Also review your tax code for any employment income. An incorrect code can mean you paid too much or too little tax during the year, which Self Assessment will correct.

Organising Your Records

Create a folder (physical or digital) for each income and expense category. Scan paper receipts or save digital copies in a cloud storage service so you have backup. HMRC can enquire into your return for up to 12 months after you file (or longer if there are significant errors), so keep all records for at least five years after the 31 January deadline (Citizens Advice, 2026).

Use a simple spreadsheet to list income and expenses by category. This makes completing the Self Assessment form faster and helps you spot missing items before you submit.

Filing Early and Seeking Help

Once you have all documents, you can complete your return in October or November rather than waiting until January. Early filing means HMRC can process queries well before the deadline, and if you are due a refund, you receive it sooner.

If your tax affairs are complex (multiple income sources, significant capital gains, or foreign income), consider consulting an accountant or tax adviser. The cost is often outweighed by accurate completion and optimised claims.

Starting now gives you time, reduces stress, and ensures you meet the 31 January deadline without penalties. Gather your documents systematically, and you will file with confidence.

Educational information only. This article provides general guidance on Self Assessment document gathering and does not constitute regulated tax advice. Nexzoe is not authorised by HMRC or the FCA. For complex tax situations, consult an HMRC-registered tax adviser or accountant. Tax rules and allowances change each tax year; verify current rates and deadlines with HMRC before filing.