Choosing the right window installer can feel daunting, but asking the right questions separates a reliable professional from a costly mistake. Your new windows are a long-term investment in your home’s comfort, energy bills, and security, so taking time to vet the installer is essential.
The most important question to ask a window installer is whether they are registered with FENSA or CERTASS, as this ensures the installation complies with Building Regulations and is legally self-certified. You should also ask for written confirmation of the product’s energy rating, warranty terms, and whether the installer offers a deposit protection scheme.
What matters when you choose. When you choose a window installer, the questions you ask upfront determine whether your new windows will save energy, meet legal standards, and last for decades. Focus on verifying their registration, product performance, warranty cover, and payment protection – these four areas cover the most common pitfalls homeowners face. Always get written answers and compare at least three quotes before committing.
| Question | Why It Matters | What to Look For | Source to Verify |
|---|---|---|---|
| Are you FENSA registered? | Legal compliance for self-certification | FENSA or CERTASS membership number | FENSA / GOV.UK |
| What is the energy rating? | Energy bill savings | BFRC rating A++ to C | BFRC / Energy Saving Trust |
| What warranty do you offer? | Long-term cover | 10-year insurance-backed guarantee | TrustMark / FENSA |
| Can you provide references? | Installer reliability | At least 3 recent customer contacts | TrustMark / Checkatrade |
| Do you offer deposit protection? | Financial safety | Deposit protection insurance or credit card payment | TrustMark / GOV.UK |
| What is the installation timeline? | Project planning | Written schedule with lead time and duration | FENSA / TrustMark |
| Do you remove old windows and waste? | Legal disposal | Confirmation in quote | GOV.UK |
| What is the total cost including VAT? | Fixed price clarity | Itemised quote with VAT (0% for eligible energy-efficient installations) | HMRC / GOV.UK |
| Are your fitters Gas Safe registered? | Safety for gas pipework | Gas Safe Register ID card | Gas Safe Register |
| What security features are included? | Home security | Secured by Design certification, multi-point locks | Secured by Design / ONS |
1. Are you FENSA registered
This is the single most critical question. FENSA (Fenestration Self-Assessment Scheme) or CERTASS registration allows an installer to self-certify that their work meets Building Regulations. Without it, you must apply separately to your local authority for building control approval, which adds time and cost. A FENSA certificate also proves the installation is legal when you sell your home – estate agents and conveyancers will ask for it. Always ask for the installer’s FENSA or CERTASS membership number and verify it online.
- FENSA registration allows self-certification of window replacements under Building Regulations.
- Without it, you must apply separately to your local authority for building control approval (GOV.UK, 2026).
- Check the membership number on the FENSA or CERTASS website.
2. What is the energy rating
Window energy ratings are set by the British Fenestration Rating Council (BFRC) and range from A++ (best) to C. An A-rated window saves significantly more energy than a C-rated one. Upgrading from D to A-rated windows can save around £165–£200 per year on energy bills for a typical semi-detached home. Look for the BFRC label on the product – it shows the rating clearly. Remember that even within the same rating, U-values can vary, so ask for the specific U-value (a measure of heat loss) as well.
- Window energy ratings range from A++ (best) to C, with A-rated or above offering the highest thermal efficiency.
- Upgrading from D to A-rated windows can save around £165–£200 per year on energy bills (Energy Saving Trust, 2026).
- Ask for the BFRC label and the specific U-value.
3. What warranty do you offer
A solid warranty protects your investment. Most reputable installers provide a 10-year insurance-backed guarantee covering both the product and the installation work. Insurance-backed means that if the installer goes out of business, the guarantee is still honoured by the insurer. Also check whether the warranty is transferable – if you sell your home within the term, the new owners should benefit. Get the full written terms, including what is and isn’t covered (for example, glass breakage or hardware wear).
- Most reputable installers provide a 10-year insurance-backed guarantee covering both product and installation.
- Ensure the warranty is transferable if you sell your home within the term (TrustMark, 2026).
- Get written terms covering exclusions.
4. Can you provide recent references
Past customer experience is one of the best indicators of future performance. Ask for contact details of at least three customers from the past 12 months – ideally in your local area. Call them and ask about the installation quality, the installer’s punctuality, and whether any issues were resolved. Also check online reviews on independent platforms like TrustMark or Checkatrade. Be wary of installers who cannot or will not provide references.
- Ask for contact details of at least three customers from the past 12 months.
- Check online reviews on independent sites like TrustMark or Checkatrade (TrustMark, 2026).
- Call references and ask about installation quality and problem resolution.
5. Do you offer a deposit protection scheme
Deposit protection is your financial safety net. Never pay the full amount upfront – a typical deposit is 10–25% of the total cost. Insist on a payment plan that protects your deposit, such as paying by credit card (which offers Section 75 protection) or using a deposit protection insurance scheme. If the installer refuses to offer any form of deposit protection, this is a major red flag. Always get the payment terms in writing.
- Insist on a payment plan that protects your deposit, such as a credit card payment or deposit protection insurance.
- Never pay the full amount upfront – a typical deposit is 10–25% of the total cost (TrustMark, 2026).
- Get payment terms in writing.
6. What is the installation timeline
A clear timeline prevents frustration and helps you plan. Request a written schedule that includes the lead time for manufacturing (usually 4–8 weeks) and the expected installation duration (typically 1–3 days for a standard house). Delays beyond six weeks should be flagged in the contract with compensation terms – for example, a discount for each week of delay. Also ask how the installer handles weather-related delays, especially for larger projects.
- Request a written schedule including lead time for manufacturing and expected installation duration.
- Delays beyond six weeks should be flagged in the contract with compensation terms (FENSA, 2026).
- Ask about weather-related delay procedures.
7. Do you remove old windows and dispose of waste
Old window frames, glass, and sealants are waste that must be legally disposed of. Confirm whether the quote includes full removal of old materials and any associated rubbish. Improper disposal can incur fines under the Environmental Protection Act. Some installers may charge extra for disposal, so get this in writing. Also ask if they recycle old materials – many responsible installers recycle glass and aluminium.
- Confirm whether the quote includes full removal of old frames, glass, and any associated rubbish.
- Improper disposal can incur fines under the Environmental Protection Act (GOV.UK, 2026).
- Ask if they recycle old materials.
8. What is the total cost including VAT
Hidden costs are a common complaint. Ask for a fixed-price quotation that itemises materials, labour, and VAT. Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). Also check if scaffolding or specialist access (e.g., for upper-floor windows) is included in the price. Get everything in writing before you agree to anything.
- Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6).
- Check if scaffolding or specialist access is included in the price (HMRC, 2026).
- Get the full cost in writing.
9. Are your fitters Gas Safe registered
This question is critical if the installation involves any gas pipework – for example, moving a boiler flue or fitting a gas fire. Fitters must be Gas Safe registered to carry out such work legally. Unregistered work is illegal and can invalidate your home insurance. Ask to see the Gas Safe Register ID card for each fitter on site. If no gas work is involved, this question may not apply, but it is still worth asking to be thorough.
- If the installation involves any gas pipework (e.g., moving a boiler flue), fitters must be Gas Safe registered.
- Unregistered work is illegal and invalidates home insurance (Gas Safe Register, 2026).
- Ask to see the Gas Safe Register ID card for each fitter.
10. What security features are included
Windows are a common entry point for burglars. Ask if the windows meet Secured by Design standards, which include multi-point locking systems, toughened glass, and reinforced frames. Upgraded locks and laminated glass can reduce burglary risk by up to 50%. Check whether the handles have key-locking mechanisms and whether the glass is laminated or toughened. Even basic security upgrades can make a significant difference.
- Ask if windows meet Secured by Design standards, including multi-point locking systems and toughened glass.
- Upgraded locks and laminated glass can reduce burglary risk by up to 50% (Secured by Design, 2026).
- Check for key-locking handles and laminated or toughened glass.
Asking these ten questions will help you choose a window installer who is qualified, transparent, and trustworthy. Always get written answers and compare at least three quotes before making your decision. How to choose energy-efficient windows Understanding window U-values and energy ratings
Frequently Asked Questions
The most important question is whether they are registered with FENSA or CERTASS. This ensures the installation complies with Building Regulations and can be self-certified legally, as confirmed by GOV.UK.
New windows should have a BFRC energy rating of A++ to C. The Energy Saving Trust states that A-rated windows can save up to £195 per year on heating bills in a typical semi-detached home.
A reliable window installer should offer at least a 10-year insurance-backed guarantee. TrustMark recommends this to protect you if the company ceases trading.
Yes, always ask for at least three recent customer references. Checkatrade and TrustMark advise contacting these to verify the installer's reliability and quality of work.
Look for deposit protection insurance or pay by credit card for purchases over £100. GOV.UK explains that this gives you Section 75 protection if the installer fails.
