What Is Self Assessment & Who Needs to Do It?

If you're self-employed, a subcontractor, or earn income outside of a regular PAYE job, chances are you need to complete a Self Assessment tax return. Yet for many people, the process feels daunting and confusing. This guide explains exactly what Self Assessment is, who needs to do it, and how to make the process as painless as possible.

What Is Self Assessment?

Self Assessment is HMRC's system for collecting Income Tax from people whose tax isn't automatically deducted from their earnings. Unlike employees on PAYE, where tax is taken at source by an employer, self-employed individuals and others with untaxed income must calculate and report their own tax liability each year.

The tax year runs from 6 April to 5 April the following year. Your Self Assessment tax return covers your income and expenses for that period.

Who Needs to Complete a Self Assessment Tax Return?

You'll need to register for Self Assessment and file a tax return if any of the following apply to you:

• You are self-employed as a sole trader and earned more than £1,000 in the tax year

• You are a subcontractor working under the Construction Industry Scheme (CIS)

• You are a partner in a business partnership

• You earned more than £10,000 from savings, investments, or dividends

• You have rental income from property

• Your income was over £100,000 in the tax year

• You received Child Benefit and you or your partner earned over £60,000

If you're unsure whether you need to file, it's always better to check with HMRC or speak to a bookkeeper — filing when you don't need to is far less costly than failing to file when you do.

Key Deadlines to Know

Missing Self Assessment deadlines results in automatic penalties, so it's essential to know the key dates:

• 5 October: Deadline to register for Self Assessment if you're filing for the first time

• 31 October: Deadline for paper tax returns

• 31 January: Deadline for online tax returns and payment of any tax owed

• 31 July: Deadline for the second payment on account (if applicable)

Late filing penalties start at £100 immediately after the deadline, rising to £900 or more if the return is three months or more overdue. Interest is also charged on any unpaid tax.

What Information Do You Need?

To complete your Self Assessment tax return accurately, you'll typically need:

• Your Unique Taxpayer Reference (UTR) number

• Records of all income earned during the tax year

• CIS deduction statements (if you work in construction)

• Receipts and records of allowable business expenses

• Details of any other income (rental, savings, dividends, etc.)

• P60 or P45 if you also had PAYE employment during the year

Self Assessment for CIS Subcontractors

If you work under CIS, your Self Assessment return is particularly important. The deductions made by your contractor throughout the year are advance payments towards your tax bill. When you file your return, HMRC will calculate your actual liability — and if you've had more deducted than you owe, you'll receive a refund.

Many CIS subcontractors are entitled to significant refunds, especially when allowable expenses such as tools, travel, and protective clothing are properly claimed. This is money you're owed — but only if your return is filed correctly.

How HES Bookkeeping Can Help

At HES Bookkeeping and Payroll Services, we take the stress out of Self Assessment for subcontractors and small business owners. We can help you:

• Register for Self Assessment with HMRC

• Keep your records organised throughout the year so filing is straightforward

• Identify all allowable expenses to reduce your tax bill

• Prepare and submit your tax return accurately and on time

• Claim back any CIS overpayments you're entitled to

Don't leave money on the table or risk a penalty for a late return. Get in touch with us today for a free consultation and let us handle your Self Assessment from start to finish.