10 mistakes to avoid when buying windows
Buying replacement windows is a significant home improvement that affects your energy bills, security, and property value. Getting the details wrong can lead to costly repairs and legal issues.
The biggest mistake when buying windows is choosing the cheapest installer without checking FENSA or TrustMark registration, as non-certified installations can invalidate your building regulations compliance and reduce your home’s value by up to 5%.
What matters when you choose: Focus on certified installers, energy-rated glazing, and proper fitting – not just the upfront price. The table below helps you compare frame materials, but your biggest risks come from skipping checks on registration, guarantees, and energy performance.
| Frame material | Typical U-value (centre-pane) | Lifespan (years) | Average cost per window (supply + fit) |
|---|---|---|---|
| uPVC | 1.2–1.4 | 20–25 | £400–£700 |
| Timber | 1.2–1.5 | 30–40 | £700–£1,200 |
| Aluminium | 1.0–1.3 | 30–45 | £600–£1,100 |
| Composite | 1.0–1.2 | 25–35 | £800–£1,400 |
Source: Energy Saving Trust, 2026; FENSA, 2026
1. Skipping FENSA or TrustMark registration
Without FENSA registration, your installer cannot self-certify the work, meaning you must pay your local authority for a building regulations certificate – typically £200–£400 (GOV.UK, 2026). Unregistered installers also void the 10-year insurance-backed guarantee that FENSA members provide (FENSA, 2026).
- Always ask for a FENSA or TrustMark registration number before signing any contract.
- Check the number on the official register to confirm it is valid.
- Get the building regulations certificate in writing after installation.
2. Ignoring the energy rating window
Windows sold in the UK must display an energy label (A++ to E) – choosing a C-rated window over an A-rated one can add £80–£120 per year to your heating bill for a typical three-bed semi (Energy Saving Trust, 2026). The rating is based on the whole window (frame + glass), not just the glazing unit (British Fenestration Rating Council, 2026).
- Look for at least a B rating, ideally A or A+.
- Check the label shows the whole-window rating, not just the glass.
- Compare ratings between quotes – even one band difference matters for long-term savings.
3. Choosing the cheapest installer without checking references
A 2025 Which? survey found that 1 in 5 homeowners who chose the cheapest quote reported defective fitting within two years (Which?, 2025). TrustMark-registered installers must pass annual competency checks, reducing this risk (TrustMark, 2026).
- Get at least three quotes and compare the full specification, not just the price.
- Ask for references from the installer’s recent jobs in your area.
- Check online reviews on independent sites, not just the installer’s own website.
4. Overlooking the U-value of the glass
Building Regulations require a whole-window U-value of 1.6 W/m²K or lower for replacement windows in England (GOV.UK, Approved Document L, 2026). A double-glazed unit with a U-value above 1.6 will not meet current standards and may fail a future sale survey (Energy Saving Trust, 2026).
- Ask for the whole-window U-value, not just the centre-pane figure.
- Lower U-values mean better insulation – aim for 1.2 or below.
- Check that the window meets Approved Document L for your region (England, Wales, or Scotland have slight variations).
5. Forgetting about trickle vents
Since 2022, Building Regulations require trickle vents (or alternative background ventilation) in most replacement windows to prevent condensation and mould (GOV.UK, Approved Document F, 2026). Installing windows without vents where needed can lead to damp problems and void your warranty (FENSA, 2026).
- Check if your property needs trickle vents – most homes built after 2000 do.
- If you dislike the appearance, ask about acoustic or demand-controlled vents.
- Never block existing vents – this can cause condensation and mould growth.
6. Not checking the warranty length and cover
Most FENSA-registered installers offer a 10-year insurance-backed guarantee, but some non-registered firms only give 1–2 years (FENSA, 2026). A 10-year guarantee typically covers frame failure, seal failure, and glass breakage – always get the terms in writing (TrustMark, 2026).
- Ask for the full guarantee document before you pay a deposit.
- Check what is excluded – some guarantees do not cover condensation between panes after year five.
- Ensure the guarantee is insurance-backed, so it is valid if the installer goes out of business.
7. Choosing the wrong glazing type for your property
Triple glazing can save up to £55 per year on heating in a typical semi-detached home, but the extra cost (£200–£400 per window) often takes 10+ years to recoup (Energy Saving Trust, 2026). For listed buildings or conservation areas, you may need slim-profile double glazing or secondary glazing – check with your local planning authority first (GOV.UK, 2026).
- Triple glazing makes sense in very cold regions or near busy roads for noise reduction.
- For period properties, secondary glazing can improve energy efficiency without altering the original frames.
- Always get written confirmation from your local planning officer before ordering.
Guide to secondary glazing vs double glazing for period homes
8. Neglecting to measure the opening size accurately
A 5mm measurement error can leave gaps that cause draughts and reduce the window’s energy performance by up to 15% (Energy Saving Trust, 2026). Always get a professional surveyor from the installer to measure – never rely on your own tape measure for ordering (FENSA, 2026).
- Ensure the installer carries out a site survey before manufacturing the windows.
- Ask for the survey measurements in writing and keep a copy.
- Check that the quote includes any adjustments for uneven openings or non-standard sizes.
9. Assuming all double glazing is equally secure
Windows without multi-point locking systems are 3 times more likely to be forced open in a burglary (Secured by Design, 2026). Look for PAS 24:2022 certification on the window – this is the UK security standard for new windows (Secured by Design, 2026).
- Check that the window carries a Secured by Design logo or PAS 24 certification.
- Ask about hinge-side security – some windows are vulnerable if hinges are not reinforced.
- Consider laminated glass on ground-floor windows for extra break-in resistance.
Guide to window security standards and locks
10. Failing to factor in fitting costs separately
Fitting can add 30–50% to the total cost of windows – a £500 window may cost £750–£1,000 fitted (Energy Saving Trust, 2026). 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 from it (VAT Notice 708/6, checked August 2026). These simple checks protect your investment and ensure your new windows meet building regulations, improve energy efficiency, and add value to your home.
Frequently Asked Questions
The biggest mistake is choosing the cheapest installer without checking FENSA or TrustMark registration, as non-certified work can invalidate building regulations compliance and reduce your home’s value by up to 5% (FENSA, 2026).
Yes, you need a FENSA-registered installer to self-certify the work, or you must pay your local authority £200–£400 for a building regulations certificate (GOV.UK, 2026).
If your installer is not FENSA-registered, you will pay £200–£400 to your local authority for a building regulations certificate (GOV.UK, 2026).
Replacement windows should have at least an A rating to minimise heat loss; a C-rated window can add £80–£120 per year to your heating bill for a typical three-bed semi (Energy Saving Trust, 2026).
uPVC windows last 20–25 years, while timber windows last 30–40 years with proper maintenance (Energy Saving Trust, 2026).
Average costs range from £400–£700 for uPVC, £600–£1,100 for aluminium, £700–£1,200 for timber, and £800–£1,400 for composite, supply and fitted (FENSA, 2026).
Yes, aluminium windows typically have centre-pane U-values of 1.0–1.3, which can be better than uPVC's 1.2–1.4, but the whole-window rating depends on frame and glass (Energy Saving Trust, 2026).
