CIS accounting

Specialist accountants for window installers.

Window and door installers working on new-build developments, commercial buildings and main-contractor refurbishment projects operate within CIS. The 20% deduction applies to the labour element of each payment only. Window and door frames, glazing units, sealed units, hardware and sealants that you supply for the job are excluded from the deduction base. On contracts where you supply and fit, the materials proportion of the invoice can be significant, making the correct materials split one of the most valuable things we check.

Book a free call CIS refund service

20%

Deducted on labour only (frames and glazing units excluded)

~£2,000

Illustrative first-year CIS refund

100%

FENSA registration: allowable business expense

The specific challenges

What makes window installers accounting different.

Frames and glazing units excluded from the deduction base

Window and door installations are materials-intensive. uPVC or aluminium frames, double or triple-glazed sealed units, glass panels, composite door slabs, ironmongery and sealants all represent materials costs that should be excluded from the CIS deduction base. Only the labour element is subject to the 20% deduction. On a block of new-build properties where you are supplying and fitting throughout, the materials figure is large. If your main contractor deducts from the full invoice, the overpayment is substantial and it is recoverable.

FENSA registration: an allowable expense that is often missed

FENSA registration is a practical requirement for window and door installers working on replacement glazing in domestic properties, providing Building Regulations self-certification. The annual FENSA membership fee and any associated registration costs are fully allowable business expenses against your taxable income. Many window installers pay FENSA fees personally and do not include them in their expense claims. They should be.

Domestic replacement work vs new-build site work: CIS scope

New-build residential, commercial glazing and main-contractor refurbishment contracts are within CIS. Replacement window or door installation carried out directly for a private homeowner, with no main contractor in the chain, is outside CIS. Window installers who work across both types must keep the income streams separate. Private domestic income is still taxable and must appear on your Self Assessment return, but no CIS deduction should have been taken at source.

Specialist installation equipment and vehicle costs

Frame-carrying cradles, glass suction lifters, power saws, drill sets, sealant guns and access equipment are capital expenditure eligible for capital allowances. Van running costs (fuel, insurance, road tax, servicing) or the 55p-per-mile AMAP rate from 6 April 2026 are both claimable routes for vehicle expenses. We compare both for your usage and apply the more beneficial option.

How we help

What we do for window installers.

CIS refund with correct materials exclusion

We review your deduction statements, verify that frames, glazing units and other materials have been excluded from the deduction base, include FENSA fees and all other allowable expenses, and submit your Self Assessment return. Window installers with high materials costs recover considerably more than a self-filed return typically produces.

Private and CIS income record-keeping

We structure your records to separate CIS site income from private domestic installations. Each income stream is allocated its own expenses, ensuring your Self Assessment return is accurate across both types of work.

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. We calculate the correct net figure (which can be significantly lower than gross invoiced value on supply-and-fit contracts), assess all three qualifying tests and manage the application.

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“Nobody had ever separated the frame cost from my labour. Three years of returns and the deduction had been on everything. Getting that corrected made a meaningful difference.”

Self-employed window installer, South West England

Composite snapshot based on client patterns. Name and figures anonymised. The tax mechanics are real.

Questions from window installers

I supply the frames and glazed units and fit them on new-build sites. Should the contractor deduct CIS from the full invoice?

No. The cost of window and door frames, sealed glazing units, glass panels and other materials 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 cost of frames and units, you are overpaying on every supply-and-fit contract. The difference is recoverable through your Self Assessment return.

Is FENSA registration tax deductible?

Yes. FENSA registration is a professional and regulatory cost directly related to your trade as a window and door installer. The annual membership fee and any registration costs are fully allowable as a business expense and should be included in your Self Assessment expense claim each year.

I also install replacement windows for private homeowners directly. Do those jobs fall under CIS?

No. Work carried out directly for a private homeowner without a main contractor in the chain is outside CIS because homeowners are not contractors for CIS purposes. No CIS deduction should be taken on those jobs. The income is still taxable, however, and must be included on your Self Assessment return. Keeping private domestic income and CIS site income in separate records prevents errors at year end.

How does the GPS turnover threshold work for a supply-and-fit window installer?

The GPS turnover test uses net CIS income, which excludes materials costs. If frames and glazing units account for a large proportion of your total invoiced amount, your qualifying net turnover will be considerably lower than your gross invoice figure. We calculate the correct net amount as part of any GPS eligibility assessment, because supply-and-fit window installers often have a misleadingly high gross figure that overstates their qualifying turnover.

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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.

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Specialist in CIS and construction accounting, not a generalist practice

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24-hour response guarantee

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Fixed fees, quoted before we start

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Other trades we work with

Plumbers Electricians Joiners and Carpenters Groundworkers Roofers

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