How to File Self Assessment Tax Returns – Full Guide 2026

Team Nomi

Mar 20, 2026
8 minutes read

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Filing self assessment tax returns is important for individuals earning income outside of regular employment, such as freelancers and self-employed workers. It doesn’t have to be difficult to file your self assessment tax return in 2026. This process allows you to report your income, claim eligible expenses, and ensure you pay the correct amount of tax, helping you avoid penalties.

Before filing a self assessment tax return, you must register for self assessment with HM Revenue and Customs (HMRC).

Completing your self assessment accurately is important for meeting your tax responsibilities. It involves calculating your total income, deducting any allowable expenses, and determining any remaining tax liability.

Ways to File a Self‑Assessment Tax Return

When filing for self assessment, HMRC gives two options: offline (paper) and online. Both have their own timelines, advantages, and steps. Choosing the right method depends on your preference and the complexity of your income.

File a Self‑Assessment Tax Return Offline (Paper Filing)

Filing your return offline means completing the paper SA100 form 2026 and sending it by post to HMRC. Importantly, paper returns must be received by HMRC before the earlier deadline, typically 31 October 2026 for the 2026/27 tax year.

1. Request or Download the SA100 Form and Supplementary Pages

• • You can download a SA100 tax return form (for the tax year ending 5 April 2026) from the official GOV.UK website.
  • If you prefer, you can call HMRC and ask them to send a paper copy to your address.
  • Many people also need supplementary pages – for example: 
  • SA101: For individuals reporting less common types of income or claiming specific deductions.
  • SA102: For employees and company directors to report earnings.
  • SA104: Required for partners in a business partnership to declare their share of profits and losses.
  • SA105: Used by landlords to report rental income from property.
  • SA106: For individuals declaring foreign income or gains.

2. Fill in Your Details Carefully

On the paper form, you must provide accurate information about yourself and your income:

Personal information:

• • Full name, address, and contact details
  • Your Unique Taxpayer Reference (UTR) and National Insurance number

Income details:

• • Employment income
  • Self‑employment profits
  • Rental and property income
  • Interest and dividends
  • Any other income that isn’t taxed through PAYE

Allowances and reliefs:

• • Pension contributions
  • Gift Aid
  • Marriage Allowance
  • Other eligible reliefs

Expenses:

• • Only allowable business expenses if self‑employed or from property income

3. Complete Any Relevant Supplementary Pages

If you’ve received other forms (e.g., SA103, SA105), make sure you:

• • Fill them out alongside your main SA100 form
  • Clearly attach them in the correct order
  • Ensure totals on supplementary pages match the figures entered on the main SA100

Supplementary pages help HMRC understand specific income types like self‑employed profits or rental earnings.

4. Double‑Check Every Section Before Sending

Review your completed forms carefully to avoid errors. Mistakes can lead to delays, queries from HMRC, or even penalties if they affect the amount of tax due. Check that:

• • All boxes have sensible figures
  • You’ve signed and dated the form
  • All supplementary pages are included
  • You haven’t left essential sections blank

After checking, it’s a good idea to make a copy of everything for your records before posting.

5. Post Your Paper Return to HMRC

Once your SA100 tax return, supplementary pages, and any guidance notes are complete, send them by post to HMRC.

The deadline for paper returns is earlier than online, usually 31 October 2026 for the SA100 tax return for the 2026/27 tax year.

6. Wait for HMRC Confirmation

After posting your return, HMRC will process it. Processing times for paper returns are longer than online, in some cases, a few weeks or more. Once processed:

• • HMRC will send you a confirmation or tax calculation by post
  • If you owe tax, your bill and payment instructions will arrive separately

File a Self‑Assessment Tax Return Online

Filing online is now the most convenient and widely used method for completing the SA100 tax return 2026.

1. Sign in to HMRC Online Services

• • Go to the HMRC Self Assessment login page
  • Use your Government Gateway credentials.
  • If this is your first online return, you’ll need to register for online services first using your UTR (Unique Taxpayer Reference) and National Insurance number. 

2. Select the Correct Tax Year

• • Once logged in, choose the SA100 tax return 2026 for the 2026/27 tax year (6 April 2026 – 5 April 2027)
  • Make sure you select the correct year to avoid processing issues.

3. Enter Personal Information

• • Fill in your full name, address, and contact details
  • Confirm your UTR and National Insurance number
  • Provide details of your employment status (employed, self‑employed, director, etc.)

4. Complete Your Income Sections

The online system guides you through sections relevant to your income:

• • Employment income: P60, P45, benefits in kind
  • Self-employment income: Use the SA103 supplementary page if required
  • Property income: SA105 supplementary page for rental income
  • Dividends, savings, and interest: Include bank statements or certificates
  • Other income: Foreign income, capital gains, or miscellaneous income

The online system automatically calculates totals and highlights missing fields, reducing the risk of errors.

5. Claim Allowances and Reliefs

• • Enter pension contributions, Gift Aid donations, and other eligible reliefs
  • Include Marriage Allowance or other tax offsets you are entitled to
  • The system will adjust your tax liability based on the allowances entered

6. Review and Validate the Return

• • The online service runs a validation check, highlighting errors or missing sections
  • Carefully review each section for accuracy
  • Make corrections before submitting, as changes after submission require amendments and may delay processing

7. Submit Your Return

• • Once you are confirmed, click Submit
  • You will receive an instant confirmation and submission reference number – keep this for your records.

8. Pay Any Tax Due

• • After submission, HMRC calculates your tax bill
  • Pay online using bank transfer, debit card, or Direct Debit before 31 January 2026
  • Payments can also include any balancing payment for the previous year or payments on account for the next year.

Selecting the Best Software for Filing Self Assessment Tax Returns

Selecting the right software for your self assessment tax return can make the process easier and more efficient. Nomi’s self assessment software is a best option for filing for self assessment returns because of its powerful and practical features:

• • Complete Tax Return Management: Manage multiple income sources including self-employment, employment, property, partnerships, foreign income, and capital gains.
  • Direct HMRC Integration: Download taxpayer data from HMRC via API and submit returns directly through the platform.
  • Task and Workflow Management: Track progress with pre-filled workflow statuses and assign tasks to team members.
  • Digital Tax Questionnaire: Collect tax information in bulk through a digital questionnaire that clients can complete via email or mobile app.
  • Client eSignatures: Request client approvals quickly at no extra cost.
  • Document Management: Upload and securely store supporting documents such as working papers.
  • Mobile Application: Clients receive real-time notifications and can sign documents through the mobile app.
  • Secure Data Handling: Maintain GDPR compliance while managing sensitive client data.
  • Complete Compliance Support: Supports key forms from SA100 to SA110 and integrates with bookkeeping, payroll, and accounts modules.
  • Affordable Pricing: Flexible pay-as-you-go pricing with bulk submission credits available.

Overall, the software makes filing for self assessment simpler by streamlining data collection, integrating with HMRC, and validating tax calculations before submission. With its user-friendly interface and comprehensive features, managing your self assessment tax return becomes faster and more efficient.

Get a 30-day free trial to see the advantages for yourself, or schedule a free demo  to see its features and capabilities. Value for your investment is guaranteed by the competitive and transparent pricing of self assessment software. With Nomi’s built-in design and committed assistance, you can smooth your tax filing process, improve accuracy, and save time.

Frequently Asked Questions

1. What is the easiest way to file a simple tax return?

Ans: The easiest way to file a simple tax return is to submit it online through the HMRC Self Assessment system. Online filing automatically calculates your tax, highlights missing information, and guides you through each section. You can also use approved tax software to make the process even faster and easier.

2. How do I submit my own self-assessment?

Ans: You can submit your own self-assessment by logging in to your HMRC online account and completing the tax return step by step. Enter your personal details, income information, and any allowable expenses. Once everything is checked, submit the return online and pay any tax due before the deadline.

3. What are common self-assessment mistakes?

Ans: Some common mistakes include entering incorrect income details, forgetting to include additional income such as dividends or rental income, claiming expenses that are not allowed, and missing the filing deadline. Checking your return carefully before submitting can help avoid these errors.