A window and door installer fits replacement windows and external doors in domestic and light commercial buildings. The work involves removing existing frames, preparing the opening, fitting and securing the new frame, sealing and finishing, and issuing a FENSA or Certass certificate. Because replacement windows must comply with Building Regulations Part L (thermal performance) and Part K (safety glazing), using a registered installer is important - they can self-certify the work and issue a compliance certificate that you will need when selling the property. Installers typically work with UPVC, aluminium and timber frames and can supply and fit, or fit only.
What they do
- Replacement double or triple glazing in UPVC, aluminium or timber
- External door replacement including composite, aluminium and timber
- Bi-fold and sliding door installation
- Sash window restoration and replacement
- New window openings in conjunction with a structural engineer or builder
- FENSA or Certass self-certification and compliance certificate issue
- Security and draught-proofing upgrades to existing frames
- Conservatory and orangery glazing work
When you need one
Any replacement window or external door installation in England and Wales requires compliance with Building Regulations. A FENSA or Certass-registered installer handles this for you.
Typical costs (UK, 2025)
| Job type | Typical cost |
|---|---|
| UPVC casement window (supply and fit) | £400 - £700 per window |
| Aluminium window (supply and fit) | £600 - £1,200 per window |
| Composite external door (supply and fit) | £1,000 - £2,000 |
| Bi-fold doors (supply and fit, per metre width) | £800 - £1,500/m |
| Sealed unit replacement (failed glass) | £80 - £200 per unit |
| Whole-house UPVC replacement (10 windows) | £4,000 - £8,000 |
Costs vary significantly based on frame material, glazing specification (U-value) and type of opening. Labour-only installation is typically 20-30% less than supply and fit. Get at least three quotes for large projects.
Qualifications to look for
FENSA (Fenestration Self-Assessment Scheme) is a government-authorised Competent Person Scheme. FENSA-registered installers can self-certify replacement windows and doors comply with Building Regulations Part L and Part K without the need for separate Local Authority approval.
Certass is an alternative government-authorised Competent Person Scheme with the same scope as FENSA. Both are equally valid - a registered installer under either scheme can issue a compliance certificate.
GGF membership indicates commitment to industry standards and a consumer protection scheme. GGF members have their work assessed and must carry professional indemnity insurance.
How to find a reputable window and door installer
Frequently asked questions
Do I need Building Regulations approval to replace windows?
Yes - replacement windows in England and Wales must comply with Building Regulations Part L (energy efficiency) and Part K (safety glazing). However, a FENSA or Certass-registered installer can self-certify the work without you needing to apply for approval separately. Always get a FENSA certificate from your installer on completion - you will need it when you sell your home.
What is the difference between FENSA and Certass?
Both FENSA and Certass are government-authorised Competent Person Schemes that allow registered window and door installers to self-certify compliance with Building Regulations. Either scheme is equally valid. An installer registered with either scheme can issue a certificate of compliance on completion of the work.
How long do replacement windows last?
UPVC windows typically last 20-25 years. Aluminium frames last 30-40 years or more. Timber frames, if properly maintained with regular painting or staining, can last considerably longer. The sealed double-glazing unit (the glass) typically has a warranty of 5-10 years against seal failure and condensation between the panes.