Company Tax Return for Plumbers: Your Complete Guide
Running a plumbing business through a limited company means filing a company tax return with HMRC every year. This form — officially called the CT600 — reports your company's profits and calculates how much corporation tax you owe. This guide covers what plumbers need to know: allowable expenses, capital allowances on tools and vans, the Construction Industry Scheme, Gas Safe registration costs, and the common mistakes to avoid when preparing your return.
Who Needs to File a CT600?
If your plumbing business operates through a limited company, you must file a company tax return every year — even in a year where the company made no profit or broke even. Sole traders and partnerships do not file a CT600; they report income through Self Assessment instead.
Key deadlines:
- File the CT600: 12 months after your accounting period ends
- Pay corporation tax: 9 months and 1 day after your accounting period ends
Corporation Tax Rates
Corporation tax is charged on your company's taxable profits at rates that vary with profit level.
| Profit level | Tax rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001 to £250,000 | Marginal relief applies |
| Over £250,000 | 25% (main rate) |
Source: GOV.UK — Corporation Tax rates
If your company is connected to other limited companies (as an associated company), the thresholds are divided between them, which can move you into a higher rate band even with modest profits.
Allowable Expenses for Plumbers
Allowable business expenses are deducted from your income before calculating taxable profit. Identifying all your legitimate expenses is one of the most impactful things you can do to reduce your tax bill.
Materials and consumables
Pipes, fittings, valves, boiler components, radiators, cylinders, and all materials installed on jobs are fully deductible. Keep supplier invoices and link them to job records where possible — this is your evidence in the event of an HMRC enquiry.
Tools and equipment
Pipe cutters, soldering equipment, crimping tools, pressure testing kits, thermal imaging cameras, CCTV drain inspection systems, and rodding equipment all qualify as deductible business assets. Smaller tools can be written off as expenses directly; larger or more expensive items may benefit from capital allowances treatment (see below).
Van and vehicle costs
A van used entirely for business is fully deductible: fuel, insurance, MOT, servicing, tyres, and road tax. If you use a personal vehicle for some business journeys, you can claim the approved mileage rate rather than actual vehicle costs.
The outright purchase of a van qualifies for the Annual Investment Allowance, letting you deduct the full purchase price from your taxable profits in the year of purchase. See our capital allowances guide for the current annual limit and how to claim.
Gas Safe registration and licensing
Annual Gas Safe Register fees are a deductible business expense. The same applies to any mandatory licensing or industry registration costs required to carry out your work lawfully.
Uniforms and PPE
Overalls, steel-toed boots, gloves, knee protection, and safety glasses used on site are deductible. Regular clothing that could be worn outside of work does not qualify.
Insurance
Public liability insurance, professional indemnity, employer's liability (if you have employees), and tool insurance are all allowable expenses.
Training and certification
Renewing Gas Safe registration, legionella awareness training, unvented hot water cylinder qualifications, and first aid renewals are deductible as they maintain existing competency. Training that qualifies you in an entirely new discipline may be treated differently by HMRC.
Subcontractor costs
If you bring in other plumbers or heating engineers to help on larger jobs, their invoices are deductible as a business cost. If they are paid under the Construction Industry Scheme (see below), you will also need to deduct tax from their payments and remit it to HMRC.
Home office
If you use part of your home for quoting, invoicing, and administration, a proportion of home running costs — broadband, utilities — can be claimed against the business.
Capital Allowances for Plumbers
Capital allowances let you deduct the cost of business assets from your profits. The main allowance is the Annual Investment Allowance (AIA), which allows a full deduction for qualifying plant and machinery in the year of purchase.
Typical qualifying items for plumbers include:
- Vans and commercial vehicles
- Specialist tools and pipe-work equipment
- Pressure testing and drain inspection equipment
- Thermal imaging cameras and survey equipment
- Computers and tablets used for the business
- Any mobile equipment owned and used solely for work
If an asset has both personal and business use, only the business proportion qualifies for capital allowances. Document your usage to support any partial claims.
The Construction Industry Scheme (CIS)
Plumbers who work on construction projects or for main contractors frequently receive payments under the Construction Industry Scheme. CIS ensures tax is collected at source before profits reach your company.
Under CIS, the main contractor deducts tax from your gross payment before paying you:
- 20% if your company is registered as a CIS subcontractor
- 30% if your company is not registered
When completing your CT600, you declare the total CIS deductions suffered during the year as a credit against your tax bill. If the deductions total more than your corporation tax liability for the year, HMRC repays the difference.
Always chase your CIS statements. Missing them from the return means leaving tax credits on the table — or worse, paying tax you have already had deducted. If a contractor fails to issue monthly statements, they are legally required to do so. Chase them promptly.
If your company engages other subcontractors, you must also register as a CIS contractor and make the appropriate deductions from the payments you make.
Our CT600 guide for builders covers further practical detail for trades working in the construction industry, with close parallels to the plumbing context.
Common Mistakes Plumbers Make
Not claiming CIS deductions. This is the single most costly error plumbers make on their company tax return. If contractors deducted CIS tax at source and you fail to claim it on the CT600, you pay that tax twice.
Missing capital allowances. Boilers, specialist pipe equipment, vans, and inspection cameras can be significant purchases. Claiming AIA on these assets in the year of purchase reduces your taxable profit directly.
Including personal expenses. Anything personal paid through the company — personal fuel, groceries, clothing — must be removed from the accounts before calculating taxable profit. Keeping personal and business spending separate avoids this problem entirely.
Missing the payment deadline. Many company directors focus on the 12-month filing deadline and miss that corporation tax is due three months earlier. Set a calendar reminder for the payment date separately.
Classifying vehicles incorrectly. If the company owns a car (rather than a van), it cannot be deducted through AIA and carries benefit-in-kind implications if used privately. Make sure new vehicles are classified correctly at the point of purchase.
See our CT600 common mistakes guide for additional pitfalls that apply to all limited companies, with practical tips for avoiding them.
Summary
Filing a company tax return as a plumber follows the same core process as any limited company, but there are trade-specific elements that make a real difference to your tax bill: CIS deduction credits, capital allowances on tools and vans, Gas Safe and registration costs, and the full range of allowable business expenses. Get these right and you will pay the correct amount of corporation tax — nothing more.