CIS accounting

Specialist accountants for heating engineers.

Heating engineers working under CIS on new-build and commercial contracts supply both skilled labour and significant materials: boilers, cylinders, radiators, pipework, controls and system components. CIS deductions apply to the labour element only. The cost of materials you supply is excluded from the deduction base. On a boiler installation or full heating system contract, getting this split right can mean the difference between a modest refund and a substantial one.

Book a free call CIS refund service

20%

Deducted on labour (materials excluded)

~£2,000

Illustrative first-year CIS refund

100%

Gas Safe registration: allowable business expense

The specific challenges

What makes heating engineers accounting different.

Boilers, cylinders and system components excluded from CIS base

When you supply a boiler, unvented cylinder, radiators, expansion vessel or any other component as part of a contract, the cost of those materials should be excluded from the CIS deduction base. Only the labour element is subject to the 20% deduction. On a high-value boiler installation or a full heating system for a new-build development, the materials figure can be substantial. If your main contractor applies the deduction to the full invoice, the overpayment is significant and it is recoverable.

Gas Safe registration and annual professional costs

Gas Safe registration is a mandatory professional requirement for heating engineers working on gas appliances. The annual registration fee, individual card costs and any Gas Safe assessment fees are fully allowable business expenses. Many heating engineers pay these from a personal account and then fail to claim them, because the expense does not feel like a trade cost. It is, and it reduces your taxable income pound for pound.

Specialist equipment and diagnostic tools

Flue gas analysers, combustion analysers, leak detection equipment, pressure gauges, pipe bending machines and hand tools all represent capital expenditure eligible for capital allowances. The Annual Investment Allowance provides a 100% first-year deduction for qualifying plant and machinery up to £1 million. Heating engineers who invest in quality diagnostic equipment often underclaim because they expense it informally rather than through a formal allowance claim.

Van costs and mileage

A fully stocked van is essential for heating work. From 6 April 2026, the AMAP rate for cars and vans is 55p per mile for the first 10,000 business miles. Actual running costs (fuel, insurance, road tax, MOT, servicing, finance charges) are the alternative. We compare both for your mileage pattern and apply whichever produces the larger deduction.

How we help

What we do for heating engineers.

CIS refund including professional registration costs

We calculate your full refund entitlement, verify the labour/materials split on every deduction statement, include Gas Safe registration fees, tool costs and van expenses, and submit your Self Assessment return. Most heating engineers receive considerably more than they expect when the full expense picture is captured.

Gross payment status application

If your net CIS income (labour element, excluding materials) exceeds £30,000 per year, you may qualify for GPS and receive gross payments from main contractors with no deduction at all. We manage the application across all three qualifying tests and maintain the compliance record that keeps GPS in place.

Ongoing compliance and MTD preparation

Annual Self Assessment, CIS records management, and Making Tax Digital preparation if your gross income exceeds the relevant threshold. We handle the compliance so you can focus on the work.

View all services

Questions from heating engineers

I supply and fit the boiler on site. Should my contractor deduct CIS from the full invoice?

No. The cost of the boiler and any other components you supply is excluded from the CIS deduction base. Only the labour element of the invoice is subject to the 20% deduction. If your contractor is applying the rate to the full invoice including the boiler cost, you are overpaying on every installation. The overpayment is recoverable through your Self Assessment return.

Is my Gas Safe registration fee tax deductible?

Yes, fully. Gas Safe registration is a mandatory professional cost for your trade and is 100% allowable as a business expense. Annual renewal fees, individual card costs and any associated assessment fees all qualify. They should be included in your Self Assessment expense claim every year.

Do I have to file quarterly MTD updates if I work under CIS?

If your gross self-employment income exceeds £50,000 from April 2026, you must use MTD-compatible software and file quarterly digital updates. The threshold drops to £30,000 from April 2027. Importantly, the threshold is based on your gross income before CIS deductions, not the net amount you receive. A subcontractor earning £60,000 gross who receives £48,000 after deductions is tested on the £60,000 figure and is in scope from April 2026.

Get started

Talk to a specialist heating engineers accountant

Book a free call. We will talk through your CIS position, your deduction history and whether there is anything worth changing. No hard sell, no obligation.

✓

Specialist in CIS and construction accounting, not a generalist practice

✓

24-hour response guarantee

✓

Fixed fees, quoted before we start

Book your free call

Other trades we work with

Plumbers Electricians Joiners and Carpenters Groundworkers Roofers

See all trades we work with