How to complete an online Self Assessment return

Scope: This walkthrough covers the return for 6 April 2025 to 5 April 2026 being filed during 2026 to 2027.

The short answer

Gather complete records for the tax year, use HMRC’s online service if your return type is supported, add every relevant income and gains section, claim only supported reliefs, review the calculation, then save the submission receipt. The service lets eligible users save an unfinished return and resume later; a UTR is needed to sign in and file.

Before signing in

Have your UTR and sign-in details ready. Build a year-wide checklist rather than copying only the documents that arrive first. Depending on your circumstances, records may include:

• employment P60s, P45s, payslips and benefits information;
• self-employment sales, expenses and capital purchases;
• UK and foreign property income and expenses;
• bank interest and dividend statements;
• pension and State Pension figures;
• disposals that may produce capital gains or losses;
• Gift Aid and pension contributions;
• student loan information; and
• payments already made towards the bill.

Reconcile totals to bank accounts, invoices and tax certificates. Keep an explanation for estimates and later replace them with final figures where required.

Choose all relevant sections

The online return begins with questions that tailor the later pages. Answer them carefully: hiding an irrelevant section is useful, but accidentally saying “no” can omit income that must be reported.

HMRC’s current guide identifies supplementary information for employment, foreign income, trusts and estates, capital gains and tax calculations. Online filing presents supported equivalents, while some circumstances require commercial software or other forms.

Use the official service’s eligibility list before assuming it supports partnerships, trusts, estates, non-residents or every specialised gain.

Enter figures from a controlled schedule

For each source, record the figure entered, its source document and the return field. This creates an audit trail and avoids adding the same amount twice. Distinguish gross income, tax already deducted, allowable expenses, capital items and personal costs.

Do not net several unrelated sources into one unsupported figure. For a sole trade or property business, follow the relevant accounting basis and HMRC categories. For employment, copy employer references and pay and tax figures from the correct year documents.

Review the calculation before submission

Check:

• each income source appears once;
• tax deducted at source has been entered;
• allowances and reliefs are supported;
• payments on account are not confused with the tax calculation;
• student loan and High Income Child Benefit Charge questions are correct where relevant;
• bank details for a repayment belong to the intended recipient; and
• the final declaration is true to the best of your knowledge.

The calculation is not a substitute for the statement of account. The statement can include payments already made, payments on account, interest or other amounts that are not presented in the same way in the tax calculation.

Submit and save evidence

Submit before the applicable HMRC deadline. Filing and paying are separate actions: submitting the return does not itself pay the bill.

Save or print the full return, calculation and submission reference. Then check the Self Assessment account after processing and follow the guide to pay the resulting bill. If HMRC requests supporting information, answer using the schedule and source records retained for the return.

Common mistakes

• Using calendar-year figures instead of the tax year.
• Omitting a small or second income source.
• Entering turnover after expenses rather than the requested gross figure.
• Treating private costs as business expenses.
• Assuming tax deducted means the income can be omitted.
• Waiting until the deadline to discover missing sign-in details or an unsupported return type.

What to do next

1. Read the records guide.
2. Reconcile all sources for 6 April 2025 to 5 April 2026.
3. Complete the tailoring questions and relevant sections.
4. Review the calculation against your schedule.
5. Submit, save the receipt and check the account.
6. Pay separately by the deadline.