Window Fitters and Glaziers — CIS and Tax Guide

Whether you're fitting double glazing in new-builds, replacing sash windows in period properties, or cutting bespoke glass panels for commercial fit-outs, working as a self-employed window fitter or glazier puts you right at the heart of the UK construction industry. It's skilled, physical work with strong demand — but it also means dealing with some specific tax rules that don't apply to other trades.

The big one is the Construction Industry Scheme (CIS), which affects how you get paid when working for contractors. But there's plenty more to know about running your finances efficiently. This guide covers everything self-employed window fitters and glaziers need to understand for the 2025/26 tax year.

Registering with HMRC

Step one is to register as self-employed with HMRC. You can do this online, and you'll receive your UTR (Unique Taxpayer Reference) number within a couple of weeks.

If you're doing any work for construction contractors (which most window fitters and glaziers do at some point), you should also register for the Construction Industry Scheme (CIS). This is a separate registration that tells HMRC you're a subcontractor in the construction industry. More on CIS shortly.

You'll also want to ensure you have:

• Public liability insurance — essential for any trade working on client properties
• Employers' liability insurance — required if you employ anyone, even casually
• FENSA or CERTASS registration — for domestic replacement window installations, you need to be registered with a competent person scheme to self-certify compliance with Building Regulations
• Professional indemnity insurance — particularly useful if you provide surveys or specifications

The Construction Industry Scheme (CIS)

The CIS is probably the most important thing for window fitters and glaziers to understand, so let's break it down properly.

What is CIS?

CIS is a scheme under which contractors in the construction industry must deduct tax from payments to subcontractors and pass it to HMRC. If you're a subcontractor (which you are, if you're self-employed and doing work for a contractor), tax is deducted from your payments at source.

How much is deducted?

• 20% if you're registered for CIS
• 30% if you're not registered

This deduction applies to the labour element of the payment, not to materials. So if you invoice a contractor £2,000 for a job (£1,400 labour, £600 materials), CIS deductions apply only to the £1,400 labour portion.

Does it apply to all your work?

No. CIS only applies when you're working as a subcontractor for a contractor. It doesn't apply to:

• Work you do directly for private homeowners
• Work for commercial clients who aren't construction contractors
• Supply-only orders (selling windows without fitting them)

In practice, many window fitters do a mix of contractor work (where CIS applies) and direct domestic work (where it doesn't). You need to track these separately.

CIS and your tax return

CIS deductions count as advance payments of your income tax. When you file your Self Assessment return, HMRC totals up how much CIS tax was deducted during the year and offsets it against your actual tax bill. If more was deducted than you owe, you get a refund. This is actually quite common for window fitters who have significant expenses, because the 20% flat rate deduction often exceeds the tax they'd actually owe once expenses are factored in.

Make sure you keep all your CIS payment and deduction statements — you'll need them when filing your return. For the complete lowdown, read our CIS subcontractors guide.

Gross Payment Status

If you have a good compliance record with HMRC, you can apply for Gross Payment Status (GPS). This means contractors pay you without making CIS deductions, so you receive the full amount and pay your tax through Self Assessment instead. This helps enormously with cash flow. To qualify, you typically need to have filed your tax returns on time, paid your tax on time, and have a minimum annual turnover (currently £30,000).

Tax Rates for 2025/26

Your tax is calculated on your net profit (income minus expenses):

• Personal allowance: £12,570 — no tax on this amount
• Basic rate: 20% on profits from £12,571 to £50,270
• Higher rate: 40% on profits above £50,270

National Insurance:

• Class 2 NI: £3.45 per week
• Class 4 NI: 6% on profits between £12,570 and £50,270

Remember, CIS deductions are not an additional tax — they're advance payments towards this liability. When you file your return, you calculate your actual tax bill and deduct the CIS payments already made. The result is either additional tax to pay or a refund.

Use our sole trader tax guide to estimate your bill.

Expenses You Can Claim

Window fitters and glaziers have substantial business expenses. Claiming everything you're entitled to makes a real difference to your tax bill:

Materials and supplies. Glass, seals, gaskets, silicone, fixings, packers, trim, and all other materials used in your work. If you buy materials for a specific job and charge the client, you still record the purchase as an expense and the client's payment as income.

Tools and equipment. Glass cutters, suction cups, glazing tools, drills, measuring equipment, levels, and other tools are all deductible. Larger items like glass-lifting equipment or a van-mounted crane would be claimed through capital allowances.

Vehicle costs. Your van is essential to your trade. You can either claim mileage (45p per mile for the first 10,000 business miles, 25p after) or actual costs (fuel, insurance, servicing, road tax, finance). For a dedicated work van, actual costs usually give a higher deduction. Our mileage guide explains both options.

Workwear and PPE. Safety boots, hard hats, gloves, safety glasses, high-vis vests, and other protective equipment are deductible. Work trousers and branded clothing are also usually claimable for trades.

Insurance. Public liability, employers' liability, professional indemnity, vehicle insurance, and tools insurance are all deductible.

FENSA/CERTASS fees. Registration and membership fees for competent person schemes are a cost of doing business and fully deductible.

Training and qualifications. NVQ courses, CSCS card renewal, first aid training, and other CPD are deductible (though initial trade qualifications are not).

Waste disposal. Skip hire, tip fees, and waste removal costs.

Phone and communications. Business mobile phone costs, or a proportion of your personal phone bill if used for work.

Accounting software. The cost of tools like Accounted for managing your books and tax returns.

The full list of potential claims is covered in our sole trader expenses guide.

VAT Registration

You must register for VAT if your taxable turnover exceeds £90,000 in any rolling 12-month period. Many established window fitting businesses will be near or above this level.

There are some VAT complications specific to the construction industry that window fitters should be aware of:

Domestic Reverse Charge for Construction. If you supply CIS-notifiable services to another VAT-registered contractor (not the end user), you may need to apply the domestic reverse charge. This means you don't charge VAT on your invoice — instead, the contractor accounts for it. This only applies to B2B transactions within the construction industry where CIS applies.

Zero-rated new builds. Work on new-build residential properties can be zero-rated for VAT purposes, meaning you charge VAT at 0% rather than 20%. The work must be on a genuinely new dwelling, not a conversion or renovation.

Reduced rate renovations. Some renovation work on residential properties that have been empty for two or more years qualifies for the reduced VAT rate of 5%.

These rules are complex, and getting them wrong can be costly. If you're VAT registered or approaching the threshold, getting professional advice is well worth the investment.

Record Keeping and Digital Compliance

You need to keep records for at least five years, including:

• All invoices issued and received
• CIS deduction statements from contractors
• Bank statements
• Receipts for tools, materials, and other expenses
• Mileage logs if claiming vehicle costs

From April 2026, Making Tax Digital for Income Tax applies to sole traders with income over £50,000. This means quarterly digital submissions to HMRC. Penny, the AI bookkeeping assistant in Accounted, can help you stay on top of this by automatically categorising your bank transactions and flagging anything that needs your attention.

Even if you're below the £50,000 threshold, keeping digital records is simply good practice. It's faster, more accurate, and means you're not scrambling to find crumpled receipts when your tax return is due.

Cash Flow Tips for Window Fitters

Budget for CIS refunds. If you regularly receive CIS refunds, don't factor them into your monthly budget — treat them as a bonus when they arrive. Your cash flow planning should be based on what you actually receive each month.

Invoice promptly. The sooner you invoice, the sooner you get paid. For contractor work, this is particularly important because they have their own payment cycles.

Separate your tax money. Put 10–15% of your net pay (after CIS deductions) into a separate account for any remaining tax, NI, and payments on account. If your CIS deductions cover most of your tax liability, you may need to set aside less.

Plan for seasonal variations. Window fitting can slow down during adverse weather conditions. Having a cash buffer helps you through quieter months.

Related Reading

• Carpet and Upholstery Cleaners — Business and Tax Guide

• Locum Pharmacists — Tax, Expenses, and IR35 Guide

• Food Truck and Street Food Vendors — Tax and Expenses Guide

• CIS for Subcontractors — Complete Guide

• How to Claim Mileage When Self-Employed

• VAT Registration Threshold Guide

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