How much CIS tax can a plumber claim back?

The refund amount depends on your total income, allowable expenses, and how much was deducted at source. Most subcontractor plumbers at the standard 20% deduction rate will receive a refund because the flat 20% applied to gross labour income often exceeds their actual effective tax rate once all business expenses are deducted. For example, a plumber earning £50,000 in subcontract labour with £15,000 in expenses has a taxable profit of £35,000. After personal allowance, the actual tax owed is around £5,800 — but if £10,000 in CIS was deducted, the refund is roughly £4,200. The more expenses you legitimately claim, the larger the refund.

How long does a CIS refund take from HMRC?

HMRC typically processes CIS refund claims within 4–8 weeks of receiving a completed Self Assessment tax return. Filing early in the tax year (April or May, shortly after the tax year ends on 5 April) usually results in faster processing. Filing close to the 31 January deadline means your return is processed during HMRC's busiest period and may take longer. Some accountants use the BACS repayment option which can be faster than cheque.

Do I need an accountant to claim a CIS refund?

You do not legally need an accountant — you can file your own Self Assessment tax return online through your Government Gateway account and claim the CIS refund yourself. However, an accountant familiar with CIS can often identify additional allowable expenses that increase your refund, and they can ensure the CIS deduction boxes are completed correctly to avoid delays. Many plumbers find the cost of an accountant (£300–£600) is more than recovered in the larger refund they achieve.

What if a contractor deducted more CIS than they should have?

If a contractor deducted CIS at 30% (the unregistered rate) when you are registered and should have been deducted at 20%, you can still reclaim the full amount through Self Assessment — the total deductions appear as a tax credit regardless of rate. However, you should ensure you are registered with HMRC so future contractors deduct at 20% rather than 30%. If a contractor cannot locate your registration, call the CIS helpline on 0300 200 3210 to confirm your registration status.

Can I claim a CIS refund if I also have other income?

Yes. Your CIS deductions are offset against your total income tax and National Insurance liability across all income sources. If you have both CIS subcontract income and private customer income (not subject to CIS), all income is reported on the same Self Assessment return. The CIS deductions are applied against the total tax bill. If deductions exceed the total liability across all income, the excess is refunded.

How to Claim Your CIS Tax Refund as a Plumber UK (2026 Guide)

Why CIS Overpayments Happen

Let's look at a concrete example to understand the mechanics:

Worked example — subcontractor plumber 2025/26

Subcontract labour income (gross)£55,000

CIS deducted by contractors (20%)£11,000

Allowable business expenses (van, tools, insurance, etc.)−£14,000

Personal allowance−£12,570

Taxable profit£28,430

Income tax on £28,430 (20% above personal allowance)£5,686

Class 4 NI (6% on profit above £12,570)£951

Total tax + NI liability£6,637

CIS refund due (£11,000 deducted − £6,637 owed)£4,363

That £4,363 is sitting with HMRC until you file your Self Assessment and claim it back. Every year you delay filing is another year that money stays with HMRC instead of in your account.

Documents and Records You Need

Gather these before starting your Self Assessment:

Monthly CIS deduction statements

From every contractor who paid you during the tax year. This is the official record of how much was deducted. If you're missing any, contact the contractor to re-issue.

Copies of all your invoices

Every invoice you issued to contractors. The total of these is your gross subcontract income. TraderInvoice stores these automatically.

Bank statements

Cross-reference against your invoices to reconcile what was actually received versus what was deducted.

Expense receipts and records

Van, fuel, tools, PPE, Gas Safe registration, insurance, phone, software — every allowable expense reduces your taxable profit and increases your refund.

UTR and Government Gateway login

Your Unique Taxpayer Reference and HMRC online account credentials.

How to Claim via Self Assessment — Step by Step

Go to www.gov.uk/log-in-file-self-assessment-tax-return and register. Do this by 5 October in the year after the tax year you are filing for (e.g., 5 October 2026 for the 2025/26 tax year).

Access HMRC online at gov.uk. File online rather than on paper — online returns are processed faster.

Complete the Self Employment pages (SA103)

Enter your total gross income, then your allowable expenses under each category. The difference is your net profit.

Enter your CIS deductions

On the Self Employment supplementary pages, there is a box specifically for CIS deductions suffered. Enter the total from all your monthly CIS statements. Do NOT enter this anywhere other than the designated CIS deduction box.

Submit and choose bank repayment

When HMRC calculates your tax and shows the refund due, select bank transfer (BACS) repayment. This is faster than cheque. HMRC will need your sort code and account number.

How to Maximise Your CIS Refund

The size of your refund is directly linked to the allowable expenses you claim. Every £1 of legitimate business expenses reduces your taxable profit by £1 — which reduces your tax liability and increases your refund.

Commonly missed expenses for plumbers:

Mileage (45p/mile for first 10,000 miles in 2025/26) if not claiming actual van costs
Home office costs if you do admin from home
Gas Safe registration renewal and certification fees
Professional development — trade magazines, courses, scheme memberships
Work-related phone costs (business proportion of your phone bill)
Bank charges on a business account
Accountancy and bookkeeping software fees

See the full list in our guide: What Can a Self-Employed Plumber Claim on Tax?

Refund Timeline and What to Expect

When you file	Typical refund timeframe
April–May (as soon as tax year closes)	4–6 weeks — fastest period, HMRC less busy
June–September	6–8 weeks — still reasonably fast
October–December	6–10 weeks — getting busier at HMRC
January (near the deadline)	8–12 weeks — peak filing period, longer processing

If your refund is delayed beyond 8 weeks from your filing date, you can chase it by calling HMRC Self Assessment on 0300 200 3310. Have your UTR and National Insurance number ready.

The 4-year deadline

You can claim a CIS refund going back up to 4 tax years. If you have not claimed for previous years (e.g. you did CIS work in 2022/23 and never filed), you can still file a late return and reclaim that money. This can result in a large lump-sum refund. Speak to an accountant if you have multiple unclaimed years.

Good records mean a bigger, faster refund

TraderInvoice keeps your CIS invoice history organised. Export your complete invoice record for your accountant in minutes. Free for up to 5 invoices/month.

Start Free Full CIS Guide →

Frequently Asked Questions

How much CIS tax can a plumber claim back?: The refund amount depends on your total income, allowable expenses, and how much was deducted at source. Most subcontractor plumbers at the standard 20% deduction rate will receive a refund because the flat 20% applied to gross labour income often exceeds their actual effective tax rate once all business expenses are deducted. For example, a plumber earning £50,000 in subcontract labour with £15,000 in expenses has a taxable profit of £35,000. After personal allowance, the actual tax owed is around £5,800 — but if £10,000 in CIS was deducted, the refund is roughly £4,200. The more expenses you legitimately claim, the larger the refund.
How long does a CIS refund take from HMRC?: HMRC typically processes CIS refund claims within 4–8 weeks of receiving a completed Self Assessment tax return. Filing early in the tax year (April or May, shortly after the tax year ends on 5 April) usually results in faster processing. Filing close to the 31 January deadline means your return is processed during HMRC's busiest period and may take longer. Some accountants use the BACS repayment option which can be faster than cheque.
Do I need an accountant to claim a CIS refund?: You do not legally need an accountant — you can file your own Self Assessment tax return online through your Government Gateway account and claim the CIS refund yourself. However, an accountant familiar with CIS can often identify additional allowable expenses that increase your refund, and they can ensure the CIS deduction boxes are completed correctly to avoid delays. Many plumbers find the cost of an accountant (£300–£600) is more than recovered in the larger refund they achieve.
What if a contractor deducted more CIS than they should have?: If a contractor deducted CIS at 30% (the unregistered rate) when you are registered and should have been deducted at 20%, you can still reclaim the full amount through Self Assessment — the total deductions appear as a tax credit regardless of rate. However, you should ensure you are registered with HMRC so future contractors deduct at 20% rather than 30%. If a contractor cannot locate your registration, call the CIS helpline on 0300 200 3210 to confirm your registration status.
Can I claim a CIS refund if I also have other income?: Yes. Your CIS deductions are offset against your total income tax and National Insurance liability across all income sources. If you have both CIS subcontract income and private customer income (not subject to CIS), all income is reported on the same Self Assessment return. The CIS deductions are applied against the total tax bill. If deductions exceed the total liability across all income, the excess is refunded.