Filing Self Assessment for the First Time: A Step-by-Step Walkthrough

Quick answer

Filing Self Assessment for the first time involves seven steps: check whether you need to file, register for Self Assessment, set up your Government Gateway account, gather your records, complete the online return, understand your tax bill, and pay by the deadline. The entire process can be done online at gov.uk and takes most first-timers between two and four hours once all records are assembled.

The 2025/26 tax year (6 April 2025 to 5 April 2026) must be reported by 31 January 2027. Before that date, you need to register — and registration must happen by 5 October 2026. If you miss registration, HMRC may still accept a late application, but you risk filing penalties stacking up from January.

Step 1: Check whether you need to file

Not everyone needs to file a Self Assessment return. HMRC's PAYE system handles tax for most employees automatically. You need to file for 2025/26 if any of the following apply:

If none of the above apply to you, you almost certainly do not need to file. If you are unsure, HMRC has an online tool at gov.uk/check-if-you-need-a-tax-return.

The Trading Allowance in practice

If your only "extra" income was casual work — selling items online, occasional odd jobs, or a small side project — and the total gross receipts did not exceed £1,000 in 2025/26, the Trading Allowance covers it entirely. No return required, no tax to pay.

If receipts exceeded £1,001, you must file. But you have a choice: either deduct actual expenses from income, or simply claim the £1,000 Trading Allowance as a flat deduction. The second option is only worthwhile if your actual expenses were less than £1,000.

Step 2: Register for Self Assessment

Go to gov.uk/register-for-self-assessment and follow the prompts. You will be asked to confirm your identity, provide your National Insurance number, and explain why you are registering (self-employment, rental income, untaxed income, etc.).

If you are self-employed, select the self-employment option. This registers you both for Self Assessment and as a sole trader. You will also need to register for Class 4 National Insurance, which HMRC handles automatically through the Self Assessment system.

What happens next:

1. HMRC creates your Self Assessment record.
2. Your 10-digit Unique Taxpayer Reference (UTR) is posted to your registered address — allow 10 working days within the UK, or 3 weeks if you are abroad.
3. An activation code for your Government Gateway account is posted separately — this can take 7–10 days.

Deadline: 5 October 2026 for 2025/26 income. Do not wait until late September. Postal delays are common, and if your UTR and activation code arrive after the deadline you will already be in penalty territory for late registration (up to £100).

If you have registered before but stopped filing, you may already have a UTR. Check old HMRC letters or log into your Government Gateway account. There is no need to re-register — simply file your return.

Step 3: Set up your Government Gateway account

If you have never used Government Gateway before, you need to create an account at gov.uk/log-in-file-self-assessment-tax-return. You will need:

• Your National Insurance number
• A recent P60 or payslip (to verify your identity)
• A mobile phone for two-factor authentication

Once you have created your account, you must add Self Assessment as a service and enter your UTR. The system then posts a second activation code, which can take 7–10 days to arrive. Log back in, enter the code, and your Self Assessment account is active.

Plan ahead on timing. If you register in October and the UTR arrives in mid-October, the Government Gateway activation code may not arrive until late October. For the online deadline of 31 January 2027 this is fine — but it illustrates why leaving everything until the last minute creates unnecessary stress.

Keep your Government Gateway user ID and password somewhere secure. You will need them every January.

Step 4: Gather your records

Pulling together the right paperwork before you start filling in the return saves significant time and reduces errors. For the 2025/26 tax year you need records of everything that happened between 6 April 2025 and 5 April 2026.

Income records

A note on bank interest: banks now report interest paid directly to HMRC, so HMRC may already have this figure. However, you are still required to enter it in your return. Cross-check your bank statements against what HMRC shows in your online tax account.

Expenses and reliefs

Collect evidence for:

• Pension contributions paid personally (not via salary sacrifice) — your pension provider's annual statement
• Gift Aid donations — total amount donated in the year (extends your basic rate band)
• Professional subscriptions approved by HMRC
• Self-employment expenses — receipts for materials, travel mileage logs, phone and broadband bills, software subscriptions, professional fees

For 2025/26, the HMRC approved mileage rate for cars is 45p per mile for the first 10,000 business miles and 25p per mile above that. If you use your own vehicle for business travel, multiply business miles by the appropriate rate and claim the total as an expense.

Step 5: Complete the return online

Log into your Government Gateway account and navigate to Self Assessment. You will complete the SA100 — the main Self Assessment tax return form — and any relevant supplementary pages.

Supplementary pages you may need:

HMRC's online system is form-based and walks you through each section with guidance notes. It will calculate your tax bill automatically once you have entered all income and expenses — you do not need to do the arithmetic yourself.

Tips for completing the return:

Work through the return in one session if possible, or use the save-and-return facility to come back. HMRC's system times out after 30 minutes of inactivity, which can lose unsaved work.

Enter all figures in whole pounds — HMRC rounds down to the nearest pound automatically. Do not include pence.

If you receive a pre-populated figure from HMRC (some banks and employers report directly), you can accept it or correct it. Always verify against your own records.

Check before you submit. The online system shows a tax calculation summary before you click submit. Review the total income, total deductions, and tax liability figures. If something looks wrong, go back and correct it before submitting — amending after submission requires a separate process.

Step 6: Understand your tax bill

Once submitted, HMRC calculates your bill and shows it in your online account. For a first-time filer, the bill has two components:

Balancing payment: the tax owed for the 2025/26 tax year. This is your actual liability minus any tax already paid (through PAYE, payments on account, or tax deducted at source).

Payment on account: if your 2025/26 tax bill (minus tax deducted at source) exceeds £1,000, HMRC requires two advance payments towards your 2026/27 bill. Each payment equals 50% of your 2025/26 bill.

Example — first-time self-employed filer:

This is the "January shock" that catches first-timers off guard. Budget for roughly 150% of your estimated tax bill when planning your January cash flow.

Class 4 National Insurance for the self-employed (2025/26):

• 6% on profits between £12,570 and £50,270
• 2% on profits above £50,270

Class 2 NI (previously a flat weekly charge for the self-employed) was abolished as a compulsory payment from April 2024. It can still be paid voluntarily to protect your State Pension entitlement if your profits are below the Small Profits Threshold (£6,845 for 2025/26).

Step 7: Pay your bill

The payment deadline for 2025/26 is 31 January 2027. Late payment attracts interest from 1 February 2027 at approximately 6.75% per annum (Bank of England base rate + 2.5%), plus a 5% surcharge on any tax still unpaid after 30 days.

Payment methods:

Important: if you pay by bank transfer, allow at least 3 working days before the 31 January 2027 deadline. A transfer initiated on 29 January may not clear by midnight on the 31st, making your payment technically late and triggering surcharges.

Your HMRC bank details for payment:

• Sort code: 08-32-10
• Account number: 12001039
• Account name: HMRC Cumbernauld
• Reference: your 10-digit UTR followed by K (e.g. 1234567890K)

If your UTR ends in a letter already, check HMRC's guidance — the reference format can vary. An incorrect reference can delay allocation of your payment.

Common mistakes first-time filers make

Being aware of these pitfalls before you start will save you from the most frequent errors HMRC sees:

Forgetting bank interest. Banks report savings interest to HMRC, but you still need to enter it. Many first-timers forget small amounts from instant access savings accounts. Check every account you held between April 2025 and April 2026, including cash ISAs (ISA interest is exempt — do not include it).

Claiming expenses that are not allowable. Self-employed people can only claim expenses that are "wholly and exclusively" for business purposes. Clothing (unless a uniform or protective gear), everyday meals, and the full cost of a home you use partly for work are common errors. For home working, you can claim either HMRC's flat rate (£6 per week with no receipts needed) or a proportion of actual costs based on the number of rooms used and hours worked.

Forgetting the payment on account. As shown in Step 6, your January bill is typically 150% of your first-year tax liability. Set aside money throughout the year — putting aside roughly 30–35% of each self-employment invoice into a separate account is a common approach.

Not registering by 5 October. The registration deadline is fixed. If you earned self-employment income above £1,000 in 2025/26 but have not registered by 5 October 2026, you are technically already in default. Register as soon as possible — HMRC takes a more lenient view of late registration where there is no intent to avoid tax, but a penalty can still be issued.

Using the wrong NI class. Self-employed people pay Class 4 NI (6% on profits between £12,570 and £50,270, 2% above). Employees pay Class 1 NI. If you have a mix of employment and self-employment, both sets of NI may apply — HMRC's system handles the calculation, but you must enter both income sources correctly.

Rounding errors on expenses. Enter all figures to the nearest pound. Some first-timers enter expenses to the penny, which can cause calculation errors. Others forget to include the VAT element on receipts when they are not VAT-registered (non-VAT-registered businesses can claim the full VAT-inclusive cost of purchases as an expense).

Record keeping requirements

HMRC requires you to keep records supporting your tax return for five years after the 31 January filing deadline for that year. For a 2025/26 return filed in January 2027, you must keep records until 31 January 2033.

Records to keep include: invoices and receipts, bank statements, mileage logs, payslips and P60s, contracts, and any correspondence with clients or HMRC. Digital records are acceptable — most accounting apps (FreeAgent, QuickBooks, Xero) retain records in the cloud, satisfying HMRC's requirements. Photographs of paper receipts stored in an app also count as valid records.

Failure to keep adequate records can result in HMRC estimating your income and expenses, often unfavourably. If HMRC opens an enquiry and you cannot produce supporting documents, penalties for inaccuracy can reach 30–70% of the underpaid tax, or higher if HMRC considers the error deliberate.

What happens after you submit

Once you submit your return online, HMRC sends a confirmation reference. Your tax calculation appears in your online account within a few minutes for straightforward returns, or within a few days if manual processing is needed.

HMRC enquiries: HMRC has 12 months from the filing deadline to open a routine enquiry into your return. For a 2025/26 return filed in January 2027, that enquiry window closes in January 2028. Keep all supporting documents until the enquiry window has passed — if no enquiry is opened, you can then apply the standard five-year retention rule.

Amending your return: if you spot an error after submitting, you can amend the return online until 31 January 2028. After that, contact HMRC in writing. Amendments that result in additional tax owed will attract interest from the original payment deadline; amendments that show an overpayment generate a repayment (plus interest from 31 January 2027 at approximately 3.25% per annum).

HMRC Self Assessment helpline: 0300 200 3310, Monday to Friday 8am to 6pm. Call volumes peak in January — if you have a question, call in November or December.