Window Fitter
Tax & MTD Guide

The tax position of a self-employed window fitter is shaped by two things most office-based trades never deal with: the Construction Industry Scheme taking a slice of your pay before it reaches you, and a genuinely tool-and-van-heavy cost base. Whether you install uPVC casements and composite doors for a builder on a new-build, or replace failed units directly for homeowners, the way you get paid drives the whole return. Get the CIS treatment and the expense list right and you usually finish the year owed money rather than chasing a bill.

This guide is built around how fitters actually earn and spend: subcontract labour with CIS deductions, materials you buy and recharge, and a long list of trade-specific costs from silicone guns to scaffold hire. Capture it properly as you go and the annual return stops being a January scramble.

How Tax Works for a Self-Employed Fitter

As a sole trader you pay Income Tax on profit, which is your total fitting income minus allowable expenses, not on the gross amount that lands in your account. For 2025/26 the personal allowance covers the first GBP 12,570, then you pay 20% to GBP 50,270, 40% to GBP 125,140 and 45% above, with the personal allowance tapering away between GBP 100,000 and GBP 125,140. Class 4 National Insurance is 6% on profit between GBP 12,570 and GBP 50,270 and 2% above, with Class 2 NIC settled through Self Assessment.

Scottish fitters pay Scottish Income Tax through six bands (19%, 20%, 21%, 42%, 45% and a 48% top rate) and carry an S-prefixed tax code, while National Insurance stays UK-wide. Welsh fitters have a C-coded tax code at rates currently matching the rest of the UK. If you also do a bit of PAYE work, or a previous employer left your record messy, your code can be wrong, so run it through the tax code checker.

CIS: The Deduction That Becomes a Refund

If you install windows and doors as a subcontractor for builders, developers or larger fitting firms, your work normally sits inside the Construction Industry Scheme. That means the contractor deducts a flat percentage from your labour before paying you and hands it to HMRC against your tax account.

• Registered for CIS: 20% deducted from labour
• Not registered: 30% deducted from labour
• Materials: the genuine materials element of your invoice should not be deducted, only the labour

The deduction is taken on your gross labour, ignoring your personal allowance and every expense you incur, so it almost always overshoots your real liability. At Self Assessment you report your full income, claim all your costs, work out the actual tax and NIC due, and offset the CIS already deducted. The result for most fitters is overpayment during the year and a refund after filing. Keep every CIS payment and deduction statement from each contractor, because those statements are the evidence for the tax you have already paid. Our CIS subcontractor guide goes deeper, and the CIS tax calculator shows roughly what refund to expect.

Note that work paid directly by a private homeowner is outside CIS, so no deduction is taken and you simply invoice and declare the full amount. Many fitters have a mix: builder work under CIS and homeowner work outside it. Both go into the same Self Assessment trade.

Allowable Expenses for Window Fitters

An expense is allowable when incurred wholly and exclusively for the business. A fitter's list is heavy on tools, the van, consumables and site access rather than office costs.

Van, Tools and the Methods Choice

Two choices matter most for a fitter. For the van, you either claim simplified mileage (45p per business mile for the first 10,000, then 25p) or the actual running costs plus a capital allowance, adjusted for private use. A fitter doing high mileage between merchant, yard and sites often does better on actuals, but mileage is simpler and needs only a journey log. You cannot switch methods mid-life for the same van, so choose once and record consistently.

For tools, most purchases are claimed in full in the year of purchase under the Annual Investment Allowance, from a GBP 30 silicone gun to a GBP 600 mitre saw. Keep the receipts; HMRC expects evidence for a tool-heavy trade. Materials you buy and recharge to a customer are deductible too, but remember you also declare the income you invoiced for them, so the two largely cancel out and only your margin is taxed.

What You Cannot Claim

The private share of a dual-use phone, van or broadband must be excluded. Everyday clothing is never allowable even if you only wear it on site; only protective gear and genuinely branded workwear count. Fuel for the school run or weekend trips is not business mileage. And fines, parking penalties on a job, or the cost of training into an entirely new trade are not deductible.

Worked Example: A CIS Window Fitter on GBP 52,000

Take a fitter who subcontracts mainly for two builders, with some direct homeowner work, invoicing GBP 52,000 of labour across the year and suffering 20% CIS on the builder portion.

Turnover (labour): GBP 52,000, of which GBP 40,000 was under CIS

CIS already deducted: GBP 40,000 at 20% = GBP 8,000 paid in advance

Allowable expenses:

• Power tools and consumables (AIA): GBP 3,200
• Van actual running costs and capital allowance: GBP 5,800
• PPE, workwear and access hire: GBP 1,400
• Insurance, CSCS and scheme fees: GBP 1,100
• Phone, software and accountancy: GBP 900
• Total expenses: GBP 12,400

Taxable profit: GBP 52,000 minus GBP 12,400 = GBP 39,600

Income Tax: GBP 39,600 minus GBP 12,570 = GBP 27,030 at 20% = GBP 5,406

Class 4 NIC: GBP 27,030 at 6% = GBP 1,622

Total tax and NIC due: GBP 7,028. Set against the GBP 8,000 already taken under CIS, this fitter is due a refund of around GBP 972. Run your own figures through the sole trader tax calculator to sanity-check the profit, then the CIS calculator to estimate the refund.

VAT for Window Fitters

You must register for VAT once taxable turnover passes GBP 90,000 in any rolling 12-month period. A fitter buying and recharging frames, glass and doors reaches this faster than a labour-only trade, because the materials you invoice count towards turnover even though much of it passes straight to suppliers. Watch the rolling total, not just the tax year.

Two wrinkles matter in construction. First, some energy-saving installations, such as certain window and insulation work in residential property, can qualify for reduced or zero VAT, so check the rate before you quote. Second, the VAT domestic reverse charge applies to most CIS-registered construction services between VAT-registered businesses: you do not charge VAT to the contractor, they account for it instead. If you work for builders and both of you are VAT-registered, this almost certainly affects you, so get it set up correctly in your invoicing.

MTD for Income Tax: What Changes for Fitters

Making Tax Digital for Income Tax replaces the once-a-year return with quarterly digital submissions and a year-end finalisation. The thresholds are based on gross income, not profit, and importantly on your turnover before any CIS deduction:

• April 2026: Combined self-employment and property income over GBP 50,000
• April 2027: Over GBP 30,000
• April 2028: Over GBP 20,000

This catches more fitters than they expect, because a CIS subcontractor invoicing GBP 55,000 of gross labour is over the GBP 50,000 line even after 20% is deducted at source. Instead of bagging up a year of statements each January, you record each invoice, CIS statement and material purchase digitally and send HMRC a summary every quarter. For a trade with steady site income and lots of small receipts, capturing it as you go is far easier than reconstructing it later. Our guide to MTD for sole traders walks through the quarterly rhythm.

Common Mistakes Window Fitters Make

Not registering for CIS. Staying unregistered means 30% is deducted instead of 20%, tying up more of your cash until you file. Registering is quick and cheap.

Losing CIS deduction statements. Each statement is proof of tax already paid. Lose them and you cannot reliably claim the refund you are owed.

Forgetting to claim the van and tools properly. The deduction the 20% CIS rate ignores is exactly what gets you your refund, so log every mile and keep every tool receipt.

Mixing materials into the labour deduction. CIS deductions should hit labour only, not the genuine materials element. Check contractor statements so you are not over-deducted.

Ignoring the reverse charge and VAT timing. A fitter recharging frames and glass can cross GBP 90,000 turnover quietly, and the domestic reverse charge changes how you invoice builders once registered.

People also ask

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