Filing Your Tax Return: Step by Step

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‍‌​‌​​‌‌‌​‌​‌‌​​‌​​​‌​​‌‌‌‌​​‌‌​​‍# Filing Your Tax Return: Step by Step

Every self-employed construction worker needs to file a Self-Assessment tax return. Whether you're a sole trader subbie, a one-person limited company director doing a personal return, or a chippy with a bit of rental income on the side, this is the process. It's not complicated. It's just fiddly, and most people leave it too late.

Rule of thumb: start gathering your paperwork in April. File by October (paper) or January (online). Don't be the person panic-filing at 11pm on 31 January.

The Key Dates

Miss the 31 January deadline and you get an automatic £100 penalty. Miss it by 3 months and daily penalties start. Miss it by 6 months and HMRC adds 5% of the tax owed on top.

What You Need Before You Start

Gather all of this before you sit down to file:

• CIS statements (CIS301s). From every contractor you've worked for during the tax year. These show how much tax has already been deducted from your pay. They are gold.
• Bank statements. Your business account for the full tax year (6 April to 5 April).
• Receipts and records of expenses. Tools, materials, van costs, fuel, phone, PPE, insurance, training courses, trade memberships.
• Mileage log. If claiming mileage rather than actual vehicle costs.
• Any P60 or P45. If you also did PAYE work during the year.
• Details of any other income. Savings interest, rental income, dividends.
• Your UTR number. Your 10-digit Unique Taxpayer Reference. On any letter HMRC has sent you.
• Your Government Gateway login. If you've lost it, allow time to reset it. This always takes longer than you'd expect.

Tip for new starters: create a folder (physical or digital) at the start of every tax year. Chuck everything in as you go. Receipts, CIS statements, invoices. When filing time comes, it's all there instead of scattered across your van, your kitchen drawer, and three different email accounts.

The Forms You Need

SA100

This is the main return. Everyone files this.

SA103 (Self-Employment Supplement)

This is the bit where you put your self-employed income and expenses. If you're a sole trader, this is where the action is.

You'll fill in:

• Your total income (turnover)
• Your expenses broken down by category
• Your net profit

CIS Deductions Section

On the SA100, there's a section for CIS deductions suffered. This is where your CIS301 statements matter.

You enter the total CIS tax already deducted by contractors during the year. HMRC then credits this against your tax bill. If more was deducted than you owe, you get a refund.

This is where people lose money. If you forget to include your CIS deductions, HMRC calculates your tax as if nothing has been paid. You end up with a bill for the full amount when you actually owe much less, or HMRC owes you.

Walking Through the SA103

Income

Enter your total turnover for the year. That's the gross amount before CIS deductions and before expenses.

Expenses by HMRC Category

Construction-specific examples for each category:

Capital Allowances

Big purchases like a van or expensive power tools. The Annual Investment Allowance lets you deduct the full cost of qualifying equipment in the year you buy it, up to £1,000,000 (2025-26 rates). For most subbies, this means you can claim the full cost of your van in year one.

Common Mistakes

1. Forgetting to include CIS deductions already paid. This is the most expensive mistake. Your CIS301s show tax already paid on your behalf. Include them.
2. Claiming for work on your own house. That extension you built for yourself is not a business expense.
3. Mixing personal and business expenses. Your Netflix subscription is not an office cost.
4. Not claiming what you're entitled to. PPE, safety boots, tools, training. If you bought it for work, claim it.
5. Filing the wrong year's figures. The tax year runs 6 April to 5 April. Not January to December.
6. Forgetting Payments on Account. If your bill is over £1,000, HMRC will ask for advance payments towards next year's bill. Budget for it.

Tip for new starters: if this is your first return, use HMRC's free online filing. It walks you through each section with help text. It's not flashy, but it works and it's free.

What Happens After You Submit

• HMRC sends you a calculation showing what you owe (or what they owe you).
• If you owe, pay by 31 January. Bank transfer is fastest. The payment reference is your UTR followed by "K."
• If they owe you (common with CIS), the refund usually takes 4-12 weeks.
• If you owe more than £1,000, HMRC sets up Payments on Account for the following year. That means you'll pay half your estimated next year's tax in January and half in July.

Sources

• HMRC, "Self-Assessment tax returns guidance," 2025
• HMRC, "Construction Industry Scheme: CIS301 statements"
• gov.uk, "Expenses if you're self-employed"
• gov.uk, "Capital allowances and Annual Investment Allowance 2025-26"

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