Replacing windows is one of the most common home improvement projects in the UK. Done well, it improves thermal performance, reduces noise, modernises the appearance of the house, and can have a positive effect on EPC rating. Done poorly — with the wrong frame specification, mismatched appearance, or an installer who can't certify their own work — it creates problems that persist for the lifetime of the installation.
FENSA and Building Regulations
Replacement windows in England and Wales must comply with Part L of the Building Regulations (thermal performance) and Part K (safety glazing in critical locations). You do not need planning permission to replace like-for-like windows in most properties, but the installation must be notified to your local authority.
FENSA (Fenestration Self-Assessment Scheme) allows registered installers to self-certify compliance and notify the local authority on your behalf. Using a FENSA-registered installer means you receive a FENSA certificate at the end of the job — essential for selling the property. CERTASS is an equivalent scheme.
If you use a non-registered installer, you must apply for a building regulations application yourself, pay the fee, and arrange an inspection. Most solicitors will ask for FENSA certificates when you sell, and the absence of them flags as a potential issue.
When Planning Permission Is Needed
Standard window replacement is usually permitted development. You need planning permission if:
- The property is listed — all window changes require listed building consent
- The property is in a conservation area and the windows face a highway — replacement must use materials that match the character of the area
- An Article 4 Direction is in place removing permitted development rights in your area
- You're changing the size or position of window openings (not just the frame within an existing opening)
Frame Materials
uPVC. The default choice for most UK homes. Low maintenance, good thermal performance, long lifespan (25–35 years), wide range of finishes including woodgrain foil wraps. Modern uPVC has improved significantly — slimmer sight lines and more sophisticated colours. Still not universally accepted in conservation areas. Cost: lowest of the three main options.
Aluminium. Slim profiles, excellent strength-to-weight ratio, and a contemporary aesthetic. Thermally broken aluminium performs well, though typically slightly below uPVC in whole-frame U-value. Powder coating is durable and available in any RAL colour. Increasingly popular on modern extensions and refurbishments. Lifespan 40+ years. Cost: typically 40–80% more than equivalent uPVC.
Timber. The original material and still the best choice for listed buildings, conservation area properties, and owners who want a traditional aesthetic. Requires maintenance (painting or staining every 5–8 years). Engineered timber (Accoya, Merbau) performs better than standard softwood and holds paint longer. Lifespan can exceed 60 years with maintenance. Cost: comparable to aluminium or higher for engineered timber.
Glazing Specification
Double glazing is the standard. Triple glazing offers better thermal and acoustic performance but adds cost and weight. For most UK climates, well-specified double glazing (low-e coating, warm-edge spacer bar, argon fill) achieves a centre-pane U-value of around 1.0 W/m²K — adequate for all but the most ambitious energy performance targets.
Whole-frame U-value (which includes frame, spacer, and glazing) is what matters for Part L compliance — it must be 1.4 W/m²K or better for replacement windows under the current regulations.
Safety glazing (toughened or laminated to BS 6206) is required within 300mm of a door, within 800mm of floor level in doors and side panels, and in all glazed areas below 800mm in height. Any reputable installer will specify this automatically.
Costs
| Window type | Typical installed cost (2026) |
|---|---|
| uPVC casement (standard) | £400 – £700 per window |
| uPVC sash | £700 – £1,100 per window |
| Aluminium casement | £700 – £1,200 per window |
| Timber casement | £800 – £1,500 per window |
| Full house (8–12 windows, uPVC) | £4,000 – £9,000 |
Getting Quotes
The window replacement industry has a poor reputation for high-pressure selling. Common tactics include inflated starting prices, large same-day discounts, and urgency pressure. Get at least three quotes on separate visits. Ask each installer for their FENSA or CERTASS registration number, their U-value specification, their warranty terms (frames and glass unit seal separately), and their waste disposal approach for the old frames.
The cheapest quote is rarely the best value. Glass unit failure (misting between panes) is the most common long-term problem — ask specifically what the sealed unit warranty covers and whether the installer will still be trading in 10 years to honour it.