A window fitter installs replacement windows, doors, roof windows and glazed extensions - and must be registered with a government-authorised competent person scheme (FENSA or Certass) to self-certify that installations comply with Building Regulations Part L (energy efficiency) in England and Wales. Without FENSA or Certass registration, the homeowner must separately notify Building Control, obtain approval and pay the associated fees. A FENSA certificate is expected by solicitors and surveyors on property sale.
What they do
- Remove existing windows, doors and frames and make good the reveals
- Install uPVC, aluminium and timber replacement windows in casement, sash and tilt-and-turn styles
- Fit composite, timber, aluminium and uPVC external doors and frames
- Install bifold, sliding and French patio doors
- Fit roof windows (Velux and Fakro type) and flat rooflights
- Install secondary glazing for listed buildings and conservation areas
- Carry out draft sealing, mastic work and internal reveals
- Self-certify compliance with Part L and issue FENSA or Certass certificates
- Advise on energy ratings (BFRC Window Energy Rating)
- Fit cat flaps, letterboxes, handles and locking systems
When you need one
Always use a FENSA or Certass registered installer for replacement glazing - it is the only way to self-certify Part L compliance and protect your property records.
Typical costs (UK, 2025)
| Job type | Typical cost |
|---|---|
| uPVC casement window (supply+fit) | £400 - £800 |
| Aluminium window (supply+fit) | £600 - £1,500 |
| Timber sash window (supply+fit) | £800 - £2,500 |
| Composite front door (supply+fit) | £1,200 - £3,000 |
| Bifold doors (supply+fit) | £2,500 - £6,000+ |
| FENSA certificate | £30 - £50 |
Aluminium and timber prices vary significantly depending on profile system and glass specification. Triple glazing attracts a 20-40% premium over double glazing but significantly improves thermal performance. Always confirm the U-value of the specified units (1.2 W/m2K or better is the current Part L target for replacement windows).
Qualifications to look for
The main government-authorised competent person scheme for fenestration. FENSA registered installers can self-certify Part L compliance. Find a registered installer at fensa.org.uk.
The alternative to FENSA - also government-authorised for self-certification of replacement windows and doors under Part L. Check at certass.co.uk.
Membership of the GGF indicates commitment to industry standards and a complaints resolution process. Find members at ggf.org.uk.
How to find a reputable window fitter
Frequently asked questions
What is FENSA?
The Fenestration Self-Assessment Scheme - a government-authorised competent person scheme for replacement windows and doors in England and Wales. A FENSA registered installer can self-certify that installations meet Building Regulations Part L (energy efficiency) without a separate Building Control application. You receive a FENSA certificate on completion which is required for future property sale.
What does fenestration mean?
Fenestration is the industry term for the windows and external doors of a building - covering windows of all types (casement, sash, tilt-and-turn), external doors (composite, timber, aluminium, uPVC), roof windows and rooflights. The term comes from the Latin fenestra, meaning window.
Can I replace windows in a listed building?
Listed building consent is required for any alteration to a listed building, including replacing windows. You cannot simply use a FENSA-registered installer - a Listed Building Consent application must be made to the local planning authority. A conservation architect or heritage consultant should advise on appropriate replacement specifications, which will typically need to match the original character of the windows.