If you work under the Construction Industry Scheme, HMRC is already taking 20% of your labour earnings every time a contractor pays you. But here's the thing — that 20% is rarely the exact amount you owe. Most CIS subcontractors overpay, and the only way to claim it back is through Self Assessment.

The problem is that the Self Assessment form looks intimidating the first time you open it. Dozens of boxes, unfamiliar terms, and no clear sign of where the CIS part even lives. This guide removes all of that uncertainty. We'll walk through every section that matters for a CIS subcontractor, box by box, so you know exactly what goes where.

Which Self Assessment Form Does a CIS Subcontractor Use?

Self Assessment isn't one form — it's a main form plus supplementary pages depending on your income sources. As a CIS subcontractor, you'll need:

• SA100 — the main Self Assessment tax return (everyone files this)
• SA103S — Self-Employment (Short) — the standard choice for most CIS subcontractors with turnover under £85,000
• SA103F — Self-Employment (Full) — required if your turnover exceeds £85,000, or if you have more complex income arrangements

If you have other income — rental property, PAYE employment alongside your CIS work, savings interest — you'll add the relevant supplementary pages. But for the majority of CIS subcontractors working exclusively in construction, SA100 + SA103S is the combination you need.

What You Need Before You Start

Gathering everything before you open the form saves hours of frustration. Here's the complete list:

• UTR number — your 10-digit Unique Taxpayer Reference (on any letter from HMRC)
• National Insurance number
• CIS payment and deduction statements — one per contractor, covering the full tax year (6 April to 5 April)
• Total gross income received — before the 20% CIS deduction was taken
• Total CIS deductions made — the actual amounts contractors withheld
• Records of all allowable expenses — tools, materials you supplied, vehicle costs, PPE, phone, training
• Materials costs — if any of your invoices included materials, separate these from labour
• Bank details for your refund (sort code + account number)

The Key Boxes on SA103S (Self-Employment Short)

The SA103S has around 40 boxes. Most of them won't apply to you. Here are the ones that matter for a CIS subcontractor:

Box	What Goes In Here	Notes
Box 9	Business description	Your trade: "Bricklayer", "Electrician", "General Construction", etc.
Box 15	Total turnover (gross income)	Enter the gross amount you were paid — before the 20% CIS deduction. Do not enter the net amount you received in your bank.
Box 17	Goods and materials	Cost of materials you supplied and included in your invoices. Only if materials are genuinely part of your income — not materials you expensed elsewhere.
Boxes 18–29	Allowable business expenses	This is where you claim your legitimate costs — tools, PPE, vehicle mileage, phone, training. Each category has its own box.
Box 34	Net profit (or loss)	HMRC calculates this: turnover minus allowable expenses. If you've claimed all your expenses properly, this figure should be significantly lower than your gross income.
Box 38	CIS tax deducted by contractors	This is the critical box. Enter the total CIS deductions withheld by all your contractors during the year. Get this from your deduction statements.

Filling In Your Return: Step by Step

Here's the full sequence, in the order you'll encounter each section online via HMRC's Self Assessment portal:

1. 1

   Log in and confirm your tax year

   Go to gov.uk and sign in to your HMRC Online Self Assessment account. Select the tax year you're filing for — for the 2025/26 year, this runs from 6 April 2025 to 5 April 2026. You can begin your 2025/26 return from 6 April 2026 onwards; the online deadline is 31 January 2027.

2. 2

   Complete the SA100 — your personal details

   This is the main form: name, address, NI number, UTR. It also captures any other income sources (employment income, savings, dividends, rental). Fill in what applies to you. If CIS construction is your only income, most of this section is straightforward.

3. 3

   Add the Self-Employment supplementary page (SA103S)

   When HMRC asks what types of income you have, select Self-Employment. This opens the SA103S. Confirm your trade or profession, your business start date, and whether your annual turnover was under £85,000 (which determines whether you use Short or Full).

4. 4

   Enter your gross turnover in Box 15

   Add up all the gross amounts from all your CIS deduction statements for the tax year. This is the labour income before the 20% was removed. If you also charged for materials on your invoices, those should be broken out separately — materials are not labour and are not subject to CIS deduction.

5. 5

   Claim your allowable expenses (Boxes 18–29)

   This is where most subcontractors leave money on the table. Each category of expense has its own box. The HMRC test: was this expense incurred "wholly and exclusively" for your CIS work? If yes, claim it. Common categories include motor expenses (or mileage at 55p/mile for 2026/27), tools and equipment, protective clothing, phone costs, and any professional subscriptions.

6. 6

   Enter your CIS deductions in Box 38

   Total the CIS deduction amounts from all your statements. Every statement shows two figures: the gross payment and the amount deducted. Add up all the deduction figures and enter the total in Box 38. This is the amount HMRC already holds on your behalf — it reduces your tax liability or triggers a refund.

7. 7

   Review the tax calculation and submit

   HMRC's online system automatically calculates your tax owed (Income Tax + Class 4 NIC) and subtracts what's been deducted under CIS. If the deductions exceed what you owe, you're in credit — HMRC will refund the difference. Check the figures make sense, then submit. Have your bank details ready to enter for the refund.

Expenses: What Can You Actually Claim?

Expenses directly reduce your taxable profit — and a lower profit means a lower tax bill. Yet HMRC data consistently shows that CIS subcontractors under-claim. Here are the main categories and what counts:

Vehicle and mileage costs

If you use your own vehicle for work — driving to sites, collecting materials, travelling between jobs — you can claim mileage at 55p per mile for the 2026/27 tax year (for the first 10,000 business miles). Keep a log of each journey: date, destination, purpose, and miles. You cannot claim mileage for commuting to a single fixed workplace, but CIS site work generally doesn't constitute a "fixed workplace" — so most site travel qualifies.

Tools and equipment

Hand tools, power tools, specialist equipment — any tool you purchased for your CIS work is claimable. Larger capital items (plant, machinery) may be handled via the Annual Investment Allowance rather than direct expenses, but for most subcontractors' day-to-day tools, you claim the full cost.

Protective clothing and PPE

Work boots, hi-vis jackets, hard hats, gloves, waterproofs specifically for site use — all claimable. Regular clothing that could be worn outside work is not.

Phone and communication

If you use your mobile for business — calling contractors, receiving job instructions — you can claim the business-use proportion. If it's 70% work, claim 70% of the cost.

Training and certifications

CSCS card renewal, trade-relevant courses, health and safety certifications — claimable if directly relevant to your current work.

What the Numbers Actually Look Like

Here's how the Self Assessment calculation works in practice for two CIS subcontractors with different income levels. Both are sole traders, basic-rate taxpayers.

* Income Tax calculated on profit above £12,570 personal allowance at 20%. Class 4 NIC at 6% on profit between £12,570 and £50,270. Figures are illustrative — actual amounts depend on your specific income and expenses.

Common Mistakes That Delay or Reduce Your Refund

Entering net income instead of gross

This is the single most common error. Your gross CIS income is what you were owed before deduction. Always use the top-line figure from your deduction statement, not the amount that arrived in your bank.

Forgetting to enter Box 38

If you file a Self Assessment without entering your CIS deductions in Box 38, HMRC doesn't automatically know to credit them against your bill. The deduction statements your contractors submitted should match — but the onus is on you to claim it on your return.

Missing the deadlines

Late online filing triggers a £100 fixed penalty — immediately, even if you don't owe any tax. Beyond three months, daily penalties of £10 begin to accumulate, up to £900. File by 31 January regardless of whether you think you owe anything.

Not registering for Self Assessment at all

If you've been working under CIS and haven't filed a Self Assessment, those years of overpaid tax are sitting with HMRC. You can claim back up to four tax years. The registration deadline for a tax year is 5 October following the end of that year.

What Happens After You File

Once you submit your return online, HMRC processes it and sends a tax calculation. If you're owed a refund:

• HMRC typically pays refunds within 5–10 working days of a successful online submission
• The refund goes directly to the bank account you provided in your return
• HMRC may need to verify your CIS deductions against what contractors reported — delays happen when there are mismatches
• If HMRC writes to you requesting clarification, respond promptly — delays in responding delay your refund
• You'll receive a tax calculation (SA302) confirming what was owed and what was refunded — keep this document

Frequently Asked Questions