Replacing windows is one of the most common home-improvement projects in the UK — and one where the process, paperwork, and costs are widely misunderstood. Using a FENSA-registered installer simplifies the Building Regulations element considerably. Budget £300–£1,500 per window fully installed; a typical three-bed semi with 10–12 windows runs £4,000–£12,000 depending on style, material, and glazing specification.
Why Replace Windows?
The most common reasons:
- Failed sealed units — condensation trapped between panes indicates the seal has broken; the unit must be replaced.
- Single glazing — pre-1980s homes often retain single-glazed metal or timber frames; replacement dramatically reduces heat loss and noise.
- Damaged frames — rotted timber, cracked uPVC, or corroded steel frames that can no longer be repaired economically.
- Upgrading thermal performance — switching from older double glazing (U-value 2.0+ W/m²K) to modern low-e argon-filled units (U-value 1.2 W/m²K) reduces heating bills noticeably.
- Security — older windows with single-point catches can be upgraded to multi-point locking as standard.
Building Regulations
Replacing a window in England is controlled building work under Part L (Conservation of Fuel and Power). The replacement unit must achieve:
- Whole-window U-value of 1.4 W/m²K or better, or
- Window energy rating (WER) of Band C or above (using the BFRC scale from G to A++).
All modern double-glazed windows with low-emissivity coatings and argon fill comfortably meet this standard. Compliance must be formally notified to the local authority — but there are two ways to do this.
FENSA and Competent Person Schemes
FENSA (Fenestration Self-Assessment Scheme) is the most widely used Competent Person Scheme for window and door replacement. A FENSA-registered installer can self-certify that their work meets Building Regulations and notify the local authority on your behalf. You receive a FENSA certificate within 30 days of work completion — this is the document mortgage lenders and conveyancers ask for when you sell the property.
Certass is the main alternative scheme, widely used by independent double-glazing companies.
If the installer is not registered with either scheme, the householder must apply for Building Control approval from the local authority (typically £100–£250 for residential window work) before work starts.
What to check before appointing
- Verify FENSA registration at fensa.org.uk using the company name or registration number.
- Ask to see evidence of their current registration certificate, not just a logo on their van.
- A legitimate registered installer will automatically include the FENSA notification in their standard service; you should not need to ask or pay extra.
The Replacement Process Step by Step
1. Survey
A pre-installation survey (usually free) checks:
- Opening sizes and whether existing frames are structural (rare in modern housing but common in older properties with steel or timber structural frames).
- Condition of lintels, reveals, and sills.
- Any planning constraints (conservation area, listed building, or Article 4 Direction removing permitted development rights).
2. Specification and Quote
The written quote should itemise:
- Frame material, colour, and profile specification.
- Glazing unit specification (thickness, coating, gas fill, U-value or WER).
- Hardware (handle type, trickle vent provision, hinge specification).
- Whether installation includes making good (internal plaster, external mastic, repainting reveals).
- FENSA or Certass registration (confirm this is included).
- Waste disposal.
3. Order and Manufacturing Lead Time
uPVC and aluminium windows are made to measure. Standard lead times:
- uPVC: 2–4 weeks from order.
- Aluminium: 4–8 weeks.
- Timber: 6–14 weeks (bespoke joinery takes longer).
4. Installation Day
A standard two-person team can replace 8–12 casement windows in a day. The process per window:
- Remove existing frame by cutting fixings and prising out the frame.
- Clean and measure the reveal; check lintel condition.
- Fix new frame into reveal using frame fixings at maximum 600 mm centres.
- Level, plumb, and square the frame before tightening fixings.
- Hang sash or fit glazed unit.
- Seal externally with low-modulus silicone mastic; fill internal gaps with expanding foam and make good with plaster or render as agreed.
- Fit handles, trickle vents, and any restrictors (required by Building Regs for windows above 1.1 m from floor level where a child could fall out — Part K).
5. Post-Installation
- Installer issues FENSA certificate (allow up to 30 days).
- Retain the certificate with your property documents.
- Check all seals, handles, and opening actions before the installer leaves.
Cost Breakdown
| Window type | uPVC (supply & fit) | Aluminium (supply & fit) | Timber (supply & fit) |
|---|---|---|---|
| Small casement (up to 0.5 m²) | £300–£550 | £500–£900 | £700–£1,400 |
| Standard casement (0.5–1.0 m²) | £400–£800 | £700–£1,300 | £900–£1,800 |
| Large casement / French casement | £600–£1,100 | £1,000–£2,000 | £1,200–£2,500 |
| Sash window (standard) | £600–£1,200 | £900–£1,800 | £1,500–£3,500 |
| Bay window (3-section) | £1,200–£3,500 | £2,000–£5,000 | £3,000–£7,000+ |
| Fixed/picture window | £200–£500 | £350–£700 | £400–£900 |
Prices include standard installation and FENSA certification. Significant lintel repairs, structural reveals, or listed building consent work are additional.
Whole-house estimates (three-bed semi, 10–12 windows):
- uPVC double-glazed: £4,000–£9,000
- Aluminium double-glazed: £7,000–£15,000
- Timber (softwood): £9,000–£18,000
Glazing Options
Double glazing remains the standard. A modern 4/16Ar/4 unit with low-e coating achieves a centre-pane U-value of approximately 1.1 W/m²K and a whole-window U-value (with uPVC frame) of around 1.2–1.4 W/m²K.
Triple glazing (4/14/4/14/4 with argon or krypton fill, double low-e coating) achieves whole-window U-values of 0.7–1.0 W/m²K. The additional cost is typically £80–£200 per unit over equivalent double glazing. The payback period is longest in milder UK climates; triple glazing is most justified for:
- North or east-facing rooms in England above the Midlands.
- Exposed coastal or upland locations.
- Homes targeting EPC A or near-Passivhaus performance.
Acoustic glazing uses asymmetric pane thicknesses (e.g. 6/12/10) or laminated glass to improve sound reduction. Relevant for properties within 50 m of a busy road (>60 dB Lden) or under a flight path.
Conservation Areas and Listed Buildings
If your property is in a conservation area, you may need planning permission to replace windows if:
- The existing windows are original single-glazed timber and you intend to change the material or profile.
- An Article 4 Direction has removed permitted development rights (common in terraced conservation areas).
Listed buildings require Listed Building Consent for any alteration. Replacement windows must normally match the original style, material, and profile — usually meaning secondary glazing (a second frame fitted inside the existing single-glazed window) rather than replacement, or a specialist slim-profile double-glazed timber unit manufactured to match the original.
Common Mistakes to Avoid
- Accepting verbal FENSA assurance without checking — always verify registration online before appointing.
- Ignoring lintel condition — a cracked or undersized lintel should be repaired before new frames go in; flagging this in the survey saves arguments later.
- Forgetting trickle vents — Building Regulations (Part F) require background ventilation in habitable rooms; most replacement windows include a trickle vent in the head or frame; make sure your specification confirms this.
- Choosing the cheapest quote without checking the glazing spec — a quote that doesn’t state the U-value or WER may be using sub-specification units.
- Skipping the snagging walk-round — check every window opens, closes, and locks before the installer leaves site.
