How to file for self-assessment

Self-assessment has a reputation for being more complicated than it is. Most of the work happens before you touch a form — and the mistakes people make are usually about deadlines and record keeping rather than tax law.

1. Work out whether you need to file

You normally need to file a return if you were self-employed and earned over £1,000, received untaxed income such as rent or dividends, or need to report a capital gain. If every penny you earned was taxed through PAYE and you have nothing else to declare, you usually do not.

If you are unsure, HMRC has a short online tool that will tell you. Getting this wrong in either direction is the most common avoidable problem.

2. Register in good time

If this is your first return, you must register with HMRC before you can file. They will post you a Unique Taxpayer Reference, which can take a few weeks. Registering late is the single most common reason people miss their first deadline — through no fault of their own beyond leaving it to January.

3. Gather your records

Before you start, collect:

• Your UTR and National Insurance number
• P60 or P45 from employment, and payslips if you have them
• Records of self-employed income and allowable expenses
• Bank and building society interest statements
• Dividend vouchers or platform statements
• Rental income and expenses, if you let property
• Records of anything you sold that might attract Capital Gains Tax
• Details of pension contributions and Gift Aid donations

Keep these for at least five years after the 31 January deadline. HMRC can open an enquiry later, and being able to produce the paperwork settles it quickly.

4. Fill in the return

The online return is built from sections, and you only complete the ones that apply to you. Employment income, self-employment, property, dividends, capital gains and pension contributions each have their own part. If a section does not apply, you skip it.

This is where an adviser earns their fee, particularly around expenses. It is easy to claim something you are not entitled to, and easy to miss something you are.

5. Pay what you owe

The return calculates your tax. Payment is due by the same 31 January deadline. If the bill is large, remember that you may also have to make a payment on account towards the following year — for many people, the first self-assessment bill is larger than expected for exactly this reason.

Deadlines and penalties

The £100 penalty for a late return applies even if you owe no tax at all. It is a filing penalty, not a tax penalty.

Common mistakes

• Assuming PAYE means you are done. If you had any untaxed income alongside your salary, you probably still need to file.
• Forgetting the payment on account. Budget for roughly half of next year's bill being due at the same time.
• Claiming expenses you cannot support. If you cannot produce a receipt or a clear record, do not claim it.
• Missing the registration date. The 5 October registration deadline is separate from the 31 January filing deadline and catches people out every year.