The short answer
Choose a window installer who is registered with FENSA or CERTASS, has solid independent reviews, gives you an itemised written quote, and offers a clear guarantee. Always get at least three quotes on the same specification, insist on a survey rather than a phone-only price, and check the registration yourself. Be wary of high-pressure “today only” discounts, large upfront deposits and quotes that are vague about what is included. See FENSA, CERTASS and Building Regulations for why registration matters.
Choosing the right installer matters as much as choosing the right windows — good windows fitted badly will still leak, draught and fail early. This guide sets out the checks worth running, what a good quote looks like, and the sales tactics that should put you on your guard.
Choosing an installer at a glance
- Registration FENSA or CERTASS
- Quotes At least three, same spec
- Format Itemised, in writing
- Survey Insist on one
- Guarantee Clear years & cover
- Red flags Pressure, big deposits, vagueness
The checks that matter
Start with registration: a FENSA or CERTASS registered installer can self-certify the work against Building Regulations and gives you the compliance certificate you will need when you sell. Then look at independent reviews across more than one platform, ask for examples of recent local work, and confirm any product and workmanship guarantees in writing. Check the registration on the scheme’s own website rather than taking a logo on a van at face value.
| Check | What good looks like |
|---|---|
| Scheme registration | FENSA or CERTASS, verified on their website |
| Reviews | Consistent, across more than one platform |
| Quote | Itemised and in writing, scope clear |
| Survey | On-site measure, not a phone price |
| Guarantee | Clear length and what it covers |
| Deposit | Reasonable, not most of the value upfront |
Get three quotes on the same specification
Always get at least three quotes, and make sure they cover the same windows — same frame material, glass spec, energy rating and number of openings — or you are not comparing like with like. A £500 difference can be entirely explained by one quote including a better glass spec or a longer guarantee. Line the quotes up item by item, adjust for anything missing, then weigh the less-quantifiable factors: communication, reviews and whether they did a proper survey. See how much double glazing costs for typical figures to sense-check against.
Deposits, contracts and guarantees
A reasonable deposit is normal, but be cautious of anyone wanting most of the value before work starts; consider deposit protection where offered. Get the agreement in writing with the spec, price, timescale and guarantee terms set out clearly. Check whether the guarantee is insurance-backed, which protects you if the company ceases trading. Taking your time and refusing to be rushed is the single best protection against a poor outcome. This is general guidance, not advice for your situation; always do your own checks before committing.
Compare double glazing quotes
Use our service to reach FENSA or CERTASS registered installers in your area, then apply these checks to choose well. Free to use, no obligation.
Frequently asked questions
How do I check a window installer is legitimate?
Confirm they are registered with FENSA or CERTASS on the scheme’s own website, read independent reviews across more than one platform, ask for examples of recent local work, and get an itemised written quote and clear guarantee. Verify the registration yourself rather than relying on a logo.
How many window quotes should I get?
At least three, all covering the same specification — same frame material, glass, energy rating and number of windows. This lets you compare fairly and spot quotes that are cheap only because they leave something out.
What are the red flags with double glazing salespeople?
High-pressure “today only” discounts, prices that fall sharply when you hesitate, demands for a large upfront deposit, no written quote, and reluctance to show FENSA or CERTASS registration. Reputable installers give you time and put everything in writing.
How big a deposit should I pay for windows?
A reasonable deposit is normal, but be cautious of anyone wanting most of the value before work begins. Look for deposit protection and an insurance-backed guarantee, and get the spec, price, timescale and guarantee terms in writing.
Sources & further reading
- FENSA / CERTASS — finding and checking registered installers
- Glass and Glazing Federation (GGF) — choosing an installer and consumer protection
- Energy Saving Trust — getting quotes for replacement windows
- GOV.UK / Building Regulations — standards for replacement windows
This is general information, not advice for your specific situation. Always carry out your own checks before committing. Replacement windows should be fitted by a FENSA or CERTASS registered window installer. We are an independent information and introduction service, not an installer.