📅 Updated April 2026 · ⏱ 14 min read · ✍️ Findfitter Team
Choosing the wrong double glazing fitter can cost you thousands in overpriced quotes, shoddy work, or an installer who disappears mid-job. UK homeowners lose an estimated £15 million every year to rogue traders in the home improvement sector. This guide shows you exactly how to vet a fitter, what questions to ask, what to watch out for, and what to put in writing before you sign anything.
🔎 TL;DR — vetting a double glazing fitter in 8 steps
- Get 3–4 quotes minimum — spread between highest and lowest is typically £2,000–£4,000
- Only consider FENSA, Certass, or Trustmark registered fitters — verify on their official databases
- Check Companies House — avoid fitters trading under shell companies
- Read recent Google + Trustpilot reviews — look for detail, not just stars
- Never sign on the first visit — any legitimate quote is valid for 14–30 days
- Avoid cash-only demands — pay by card or bank transfer for chargeback protection
- Local installers are typically 20–40% cheaper than national chains for the same spec
- Get everything in writing — verbal promises don’t count
Why the fitter matters more than the windows
Two identical uPVC windows installed by different fitters can have completely different outcomes 10 years later. A poorly-installed window leaks, causes damp, loses its seal, and needs replacing within a decade. A properly-installed one lasts 30 years with no issues. The material is commodity — the craft is in the installation.
This is why FENSA and Certass certification exist. They’re the UK’s self-regulatory schemes for installers — and a fitter without at least one of these is not someone you should hire. Period.
The 10 red flags of a rogue fitter
Any ONE of these should make you pause. Two or more — walk away.
1. Mobile phone only — no fixed business address
Every legitimate UK installer has a registered office address you can verify on Companies House. A contact on a 07-prefix mobile with no address is usually a one-person operation with no insurance, no warranty protection, and no accountability if the job goes wrong.
2. No FENSA, Certass, or Trustmark registration
These aren’t optional. UK building regulations require replacement window installations to be notified to the local council — FENSA-registered fitters do this automatically and issue a certificate. Without it, you have no proof of compliance, which can invalidate your home insurance and cause issues when you sell.
Verify credentials directly:
- FENSA: fensa.org.uk
- Certass: certass.co.uk
- Trustmark: trustmark.org.uk
3. Cash-only payment demands
Pay by debit/credit card or bank transfer. Cash has zero chargeback protection — if the fitter vanishes, your money is gone. Credit cards give you Section 75 protection for amounts over £100, meaning the card issuer is jointly liable if the service isn’t delivered.
4. Pressure to “sign today” for a special price
This is the #1 trick of high-pressure sales visits. “Manager call-in discount”, “showroom offer”, “board price” — all designed to stop you getting competing quotes. A real price is valid for 14–30 days. If it expires at 8pm tonight, it was never a real price.
5. Quotes with no itemised breakdown
A legitimate quote must itemise: product cost, frame material, glass spec, scaffolding, removal + disposal, VAT, warranty period, and installation labour. “Total: £6,400” on a single line with no breakdown is a red flag for hidden costs or deliberate vagueness.
6. Reviews that feel too polished
Check for: specific customer names (first name + surname initial), location details, date ranges, and mentions of specific products. Fake reviews are usually short, too effusive (“5 stars, amazing!”), and cluster in short date windows. Real reviews include complaints about minor issues, which actually increases trust.
7. No insurance-backed warranty
FENSA and Certass provide insurance-backed guarantees — so if the installer goes out of business, your warranty is still honoured by the scheme. Without this, a “10-year guarantee” is worthless the moment the company closes.
8. Vague about product specifications
You should know: the frame supplier brand (Rehau, Deceuninck, Veka, Eurocell are common reputable UK suppliers), the energy rating (A++ / A+ / A), the glass type (argon-filled, low-e coating, toughened where required), and the locking mechanism (PAS 24 certified is the security standard).
9. No physical showroom or office
You can’t check physical showroom photos on Google Images or pop in to see actual samples. A serious installer has a showroom or at least a trade counter where you can inspect the actual frames before ordering.
10. Shell companies with no trading history
Check Companies House — free public database. Look at: incorporation date (avoid companies under 2 years old for a £10k job), filed accounts (shows financial stability), and director names (cross-check against struck-off company databases for patterns).
How to verify an installer is legitimate — 5-minute checklist
Before any quote visit, spend 5 minutes doing this:
- Check FENSA: enter the company name at fensa.org.uk — should show active registration, number, and coverage area.
- Check Certass if not FENSA: certass.co.uk has the same lookup.
- Companies House search: find the company, verify age, directors, filed accounts.
- Google the company name + “reviews”: look at Google Business profile (unverified companies don’t have one), Trustpilot, Facebook reviews.
- Google the directors’ names: look for any ties to previously liquidated companies. Rogue traders often change company names to avoid bad reviews.
- Ask for local references: any legitimate installer should give you 2–3 recent customers in your area you can call.
National chains vs local installers: which is better?
This is the biggest money-saving decision you’ll make. Here’s the real trade-off:
| Factor | National chains (Anglian, Everest, Safestyle) | Local FENSA fitters |
|---|---|---|
| Typical price | £6,000–£12,000 full house | £4,000–£9,000 same spec (20–40% less) |
| Marketing overhead | £500–£1,500 per sale (TV, direct mail) | Near zero — baked into price |
| Sales approach | Commission-based reps, high pressure | Owner-led, direct, no pressure |
| Product quality | Often branded frames from same UK suppliers | Often identical suppliers (Rehau, Deceuninck, etc.) |
| Installation quality | Subcontracted — varies by who they send | Usually the owner + 1-2 long-term staff |
| Warranty | 10-year insurance-backed via FENSA | Same 10-year insurance-backed via FENSA |
| Aftercare | Mixed — contact centres | Direct line to the fitter who did your job |
| Finance options | Strong — 0% finance widely available | Variable — ask upfront |
Verdict: for most UK homeowners, a well-vetted local FENSA installer is the better choice — you get the same quality product for less money, and better aftercare. Get 4 quotes mixing local + national and let the price gap speak for itself.
15 questions to ask during the sales visit
About the company
- Are you FENSA or Certass registered? What’s your membership number?
- How long have you been trading under this company name?
- Who will actually do the installation — your own team or subcontractors?
- Can you provide 2 or 3 local references I can contact?
About the product
- What frame brand are you using (Rehau, Deceuninck, Veka, Eurocell)?
- What’s the energy rating (A++, A+, A)?
- Is the glass argon-filled with a low-e coating?
- What’s the locking standard — is it PAS 24 certified?
About installation
- How many days will the installation take?
- Do I need to be home, and if so, which days?
- Who handles old window disposal — is it included?
- Who is responsible for making good internal plastering or external rendering?
About pricing and guarantees
- Is this quote valid for 30 days minimum?
- What deposit do you require, and is it protected?
- What does your 10-year warranty cover specifically — and is it insurance-backed?
What your written quote must include
Before you sign anything, the written quote must itemise:
- Company legal name, address, Companies House number, FENSA/Certass membership number
- Exact product specification — brand, model, colour, energy rating, glass spec, locking
- Each window location in your house (with rooms named or plan diagram attached)
- Opening vs fixed panes clearly identified
- Installation labour — itemised separately from product cost
- Removal and disposal of old windows
- Any scaffolding or access equipment
- Making good interior plaster / exterior render
- FENSA certificate (should be free — refuse installers who charge)
- VAT clearly shown (20% on standard work, 0% on qualifying energy-saving upgrades)
- Total price and payment schedule
- Deposit amount (should not exceed 25%)
- Start and completion dates
- Warranty terms and insurance-backed guarantee details
- Quote expiry date (minimum 14 days, 30 is standard)
How to compare quotes fairly (apples to apples)
The #1 reason homeowners get confused comparing quotes is that installers quote different specs. Standardise before you compare:
- Define one spec you want first — e.g., “uPVC, A-rated, Rehau frames, argon-filled, PAS 24”
- Ask every installer to quote against that spec exactly — no upselling, no downgrading
- Compare the totals on the same spec, not on different specs
- Then compare their recommended upgrades — A++ vs A-rated, laminated glass, etc. — separately
This method typically reveals a £2,000–£4,000 spread between the cheapest and most expensive quote for identical work — and helps you negotiate.
How many quotes should you get?
Minimum 3, ideally 4. Here’s why:
- 1 quote: you have no reference point at all — you’ll almost certainly overpay
- 2 quotes: you can tell if the first was inflated, but can’t tell which is fair
- 3 quotes: clear triangle — you can spot outliers on both sides
- 4 quotes: diminishing returns but still useful, especially if spec varies
- 5+ quotes: you’re wasting installers’ time and your own; convert one to an order