How to Vet a Window Installer Before You Hire in the UK

A full set of replacement windows usually runs £3,000 to £8,000. That’s a lot of trust to hand a stranger who turns up in a van. Most of the horror stories that end up on consumer forums start the same way: no FENSA number, no paper trail, a fitter found through a Facebook ad and never checked.

Vetting an installer properly takes about fifteen minutes. Check three things before you agree to anything: their FENSA or Certass registration, their record on Companies House, and a written quote with a fixed price and a completion date. Skip any one of those and you’re trusting a four-figure job to guesswork.

Here’s the checklist worth actually using, including the two checks almost nobody writes about.

Key points

• Verify FENSA or Certass registration on the official register, not the installer’s own website badge.
• Run the company name through Companies House. Incorporation date and registered address tell you more than reviews do.
• Get a written, itemised quote before any deposit changes hands.
• A deposit over 50%, cash-only demands, or no contract are the three fastest ways to lose money.
• Ask specifically for an insurance-backed guarantee and the underwriter’s name.

Reading time: roughly 6 minutes.

Start with the FENSA or Certass number, not the sales pitch

Every legitimate double glazing installer in England and Wales should be registered with a Competent Person Scheme, almost always FENSA or Certass. That’s what lets them self-certify the job against Building Regulations without a separate council inspection.

Don’t take the badge on their van as proof. Go to fensa.org.uk (or certass.co.uk if that’s the scheme they quote) and search the company name or postcode yourself. It takes thirty seconds. If they’re not listed but claim to be, that’s the conversation over, not a follow-up question.

An installer who genuinely holds the registration won’t mind you checking. One who hesitates, or says “we’re waiting for it to come through,” is telling you something.

Check the company on Companies House, not just its reviews

This is the check almost every guide to hiring a window fitter skips, and it’s the one that actually predicts trouble. Search the trading name at find-and-update.company-information.service.gov.uk. It’s free and takes a minute.

Three things to look at:

• Incorporation date. A company formed three weeks ago, running Facebook ads and knocking on doors, is a classic pattern behind what trading standards call “phoenixing”: a rogue trader dissolves a company after complaints pile up, then reopens under a near-identical name. A firm trading under the same registered number for five or ten years has survived long enough that this isn’t the game.
• Registered office address. A residential address isn’t automatically a red flag on its own, small trades businesses often run from home, but paired with a brand-new incorporation date and heavy pressure to book quickly, it adds up.
• Filing history. Overdue accounts or a dissolution-then-reincorporation pattern under a similar name are both visible here, and neither shows up in a five-star Google review.

None of this replaces asking for references. It adds a layer that reviews can’t fake, because reviews can be bought and Companies House records can’t.

Ask for these three things before any money changes hands

1. A written, itemised quote. Price per window, glazing spec (double or triple, the U-value if they quote one), frame colour, lead time, and a completion date. Verbal quotes are worth nothing if the job runs late or the spec changes.
2. Proof of FENSA or Certass registration, checked against the official register yourself, not taken on trust.
3. Confirmation of an insurance-backed guarantee, in writing, with the underwriter named. This matters more than most homeowners realise: it’s what keeps your ten-year frame guarantee valid if the installer goes bust the following year.

Get quotes from at least two or three companies. Cheapest isn’t automatically worst, but a quote that’s 30-40% below the others usually means thinner frames, standard glazing sold as premium, or a company that’s not planning to be around for the guarantee period.

What a legitimate deposit looks like

A deposit of 10-25% to order materials is standard practice and reasonable. Full payment upfront, or anything over half the job cost before work starts, is not.

Cash-only requests are worth treating as a hard stop. Legitimate trades take card payments, bank transfer, or a cheque, all of which leave a record if something goes wrong. A trader who wants cash in hand and no invoice is often avoiding exactly that paper trail.

Quick recap

Deposit norms in one line: 10-25% is standard, over 50% or cash-only is a warning sign, and the balance should always be paid on completion, not before.

What you won’t find elsewhere

Most guides to hiring a window installer stop at “check FENSA and get three quotes.” That’s necessary but it misses the one check that actually catches rogue traders before they’ve taken your deposit: Companies House.

Call it the 3-Document Rule. Before any deposit leaves your account, you should have, in hand, three things: a written itemised quote, a registration number you’ve personally verified on the FENSA or Certass website, and written confirmation of the insurance-backed guarantee provider. If an installer can’t produce all three within a day of asking, that’s your answer.

Pair the 3-Document Rule with a Companies House search and you’ve done more due diligence than the vast majority of homeowners ever bother with, in under twenty minutes, for free.

Once you’ve narrowed down a shortlist, browse our UK window and glazing installer directory to cross-reference the company against its Companies House record by town and county, or go straight to get free quotes from FENSA and Certass registered installers covering your postcode. If you want to know what the traders who fail every one of these checks tend to have in common, our guide on how to spot a cowboy window fitter covers the pattern in detail.

Fifteen minutes of checking now is a lot cheaper than a half-finished job and a company that’s stopped answering the phone.