The short version
- Ask the same questions of every installer, and get the answers on the written quote.
- Check the company on Companies House and on its competent person scheme's register.
- Ask whether the windows are made to measure, and what cancellation period the contract gives.
- Walk away from a price that is only valid tonight, a large deposit before anything is made, or a "government grant" offer at the door.
FENSA publishes its own short list of questions to ask your window fitter: the cost, examples of other work, the options you can choose, how long it takes and whether the installer is FENSA approved. The 29 questions below build on that list.
What happens at the quote visit?
The salesperson looks at each window, takes rough sizes and prices the job, and a technical survey after you order takes the exact measurements. Some installers adjust the price at that survey, so ask at the visit what it can change and get the answer in writing.
Questions about the company
Company questions show whether the firm is still likely to be trading when you need the guarantee. Ask these seven:
- How long have you traded under this name? Check the answer on Companies House.
- Has the company been in administration or traded under another name? Companies House shows a company's previous names and any insolvency record.
- Is your salesperson paid on commission?
- Which competent person scheme are you registered with? GOV.UK lists six for replacement windows: Blue Flame Certification, CERTASS, Certsure, FENSA, NAPIT and ASSURE. Search the installer's name on the scheme's own website.
- Do you belong to a dispute resolution scheme? DGCOS is one example. Ask which scheme would handle a dispute with you.
- Do you make the windows or buy them in?
- Can I see recent work and speak to past customers? Citizens Advice suggests asking for 2 or 3 recent examples of similar work, with contact details, because written references are not always genuine. Our page on double glazing installers adds the survey and scheme checks to run on each firm.
Questions about the windows
Window questions pin down exactly what you are buying, so the quote compares with the others. Ask these four:
- What U-value or energy rating does the glass reach? Replacement windows in England must reach 1.4 W/m²K or better, or Window Energy Rating band B or better (Approved Document L).
- What frame material, colour and hardware are included? Get the locks and handles named on the quote.
- Are the windows made to measure or stock sizes? Get the answer in writing before you sign.
- Which windows open, and which are fixed? Opening panes change the specification and the price.
Questions about the fitting
Fitting questions decide who is in your home, when, and what state they leave it in. Ask these six:
- Are your fitters employed by you or subcontracted?
- How long from order to fitting, and what is the start date?
- What happens if the work runs late? Which? suggests asking about penalties for late work. Our guide to the best time of year to buy double glazing covers lead times and winter fitting.
- How are bad-weather delays handled?
- Who supplies the materials?
- Is removal and disposal of the old frames, and making good inside and out, included?
Questions about the guarantee
Guarantee questions decide what happens if something fails after the installer has been paid. Which? recommends a ten-year guarantee on frames, five to ten years on glass, at least two years on hardware, and a ten-year insurance-backed guarantee. Ask these three:
- How long is the guarantee on frames, glass and hardware?
- Is the guarantee insurance-backed, and who is the insurer? An insurance-backed guarantee pays out if the installer stops trading. FENSA verifies that every FENSA Approved Installer's guarantee is insured. Our guide on what happens if your double glazing company goes bust shows why the insurer matters.
- Does the guarantee pass to a new owner if I sell?
Questions about money and the contract
Money questions protect the payments you make before the windows are fitted. Ask these five:
- Can I have the full quote in writing, with the total including VAT? Citizens Advice says a quote is a fixed price, while an estimate is a rough guess. Check the contract itself against our double glazing contract checklist.
- How long is the price valid?
- How much deposit do you want, and how is it protected? DGCOS deposit cover for members' customers runs to 25% of the contract value, up to £5,000, for 120 days from signing, once confirmed in writing. Our guide on how much deposit to pay for double glazing covers the rest.
- What are the payment stages? Ask what has to be finished before each payment falls due.
- Can I pay by credit card? Section 75 of the Consumer Credit Act 1974 makes the card provider jointly liable with the installer for a single item with a cash price over £100 and up to £30,000.
Questions about cancelling
Cancelling questions tell you whether you can change your mind after you sign. Ask these four:
- What cancellation period does the contract give me, and on what date does it start?
- Do you offer your own cooling-off period, and for how long?
- Will you start work inside the cancellation period, and what do you need from me first?
- What would you charge if I cancelled after the cancellation period ends?
If you sign at home, you usually have at least 14 days to cancel, but made-to-measure windows can be an exception, so check the contract before you sign. The double glazing cooling off period guide explains each case.
Ask three installers the same questions
Tell us about your home and we pass your enquiry to local installers who cover your postcode.
Double glazing sales tactics to watch for
Watch for five double glazing sales tactics: an inflated first price, a price valid only tonight, a call to the manager, a budget question before any quote, and a demand that every owner signs at once. Each one has a question that puts the decision back in your hands.
| Tactic | What you hear | What to ask back |
|---|---|---|
| Inflated first price, then a big discount | "The list price is this, but I can take a lot off for you." | What is the price without the discount, in writing, and how long is it valid? |
| Price valid only tonight | "This price is only good if you sign tonight." | Can you put the price and its expiry date on a written quote? |
| Calling the manager | "Let me ring my manager and see what I can do." | Can you put that final price on a written quote for me to compare? |
| Asking your budget first | "What budget did you have in mind?" | Can you quote for my specification, item by item? |
| Every homeowner present to sign | "We need everyone on the deeds here tonight." | Can you leave a written quote for us to read together? |
A false claim that an offer is only available for a very limited time is always unfair under Schedule 20 of the Digital Markets, Competition and Consumers Act 2024. That covers a "tonight only" price that is not genuine.
Cold callers and doorstep sales
You do not have to talk to a double glazing cold caller or let one in. Schedule 20 also makes it always unfair to ignore your request to leave your home or not to return, and to make persistent, unwanted calls. Report a cold caller to the Citizens Advice consumer helpline on 0808 223 1133 (Welsh language: 0808 223 1144), whose advisers pass complaints to Trading Standards. Some councils go further: see our pages on double glazing quotes in Bromley and double glazing in Ilford for their doorstep rules.
Answers that should make you walk away
Walk away from any installer who gives one of these five answers:
- No written specification, only a total.
- A large deposit by bank transfer before anything is made.
- A guarantee with no length and no insurer named.
- Pressure to sign before you have other quotes.
- A "government grant" or "Help to Buy" offer as the opening line.
For the pricing side of these tactics, our sister site covers how to avoid being overcharged for double glazing.
Printable checklist
Print this table and fill in one column per installer. The quote with the most gaps is the one to question.
| Question | Installer A | Installer B | Installer C |
|---|---|---|---|
| Years trading, checked on Companies House | |||
| Competent person scheme, checked on its register | |||
| Glass U-value or energy rating | |||
| Made to measure or stock sizes | |||
| Fitters employed or subcontracted | |||
| Guarantee length and insurer | |||
| Deposit amount and protection | |||
| Price valid until | |||
| Cancellation period in the contract |
Get quotes you can put these questions to
The questions only work when you have more than one installer to ask. Get double glazing quotes from local installers who cover your postcode, free and with no obligation. Our guide on how to compare double glazing quotes shows how to line up the written quotes once they arrive.
Questions people ask
What should I ask a double glazing salesman?
Ask how long the company has traded, which competent person scheme it belongs to and who fits the windows. Then ask what U-value the glass reaches, whether the guarantee is insurance-backed, how the deposit is protected and whether the windows are made to measure.
Should I tell a double glazing salesman my budget?
You do not have to give a budget. Ask for a written, itemised quote for your specification instead, then compare it with quotes from two other installers.
How do I know a double glazing company is legitimate?
Check the company on Companies House and search its name on its competent person scheme's register. Ask for 2 or 3 recent jobs with customer contact details. Citizens Advice warns that written references are not always genuine.
What are the most important questions to ask when buying replacement windows?
The five most important questions cover the installer's competent person scheme, who fits the windows, who insures the guarantee, how the deposit is protected, and whether the windows are made to measure. Get each answer on the written quote.
What are people saying about Martin Lewis at Help to Buy windows?
Martin Lewis does not endorse Help to Buy windows. MSE says he never endorses products, and an advert using his name that does not link back to MSE is fake. The government's Help to Buy was an equity loan for buying a new-build home, and it is closed to new applications in England. "Help to Buy" window offers come from private companies. At least one says on its own site that its grant has nothing to do with the UK government, and that the grant is funded by selling enquiries to installers. Our pages on double glazing in Nottingham and Romford double glazing quotes show what local grant schemes actually cover.
