Replacing your windows is a notifiable building work in England, Wales and Scotland, and the rules changed materially with the 2021 update to Part L. This piece sets out what you now have to meet, how self-certification actually works, and what happens if a previous installer skipped the paperwork on a property you now own.
What Part L 2021 changed for replacement windows
The 2021 edition of Approved Document L Volume 1 (Dwellings) tightened the minimum thermal performance standards for replacement windows in England. The whole-window U-value threshold (Uw) moved from 1.6 W/m²K to 1.4 W/m²K. Wales published its own 2022 update to broadly similar levels, and Scotland’s Section 6 sits at 1.4 W/m²K for replacements. In practice, most reputable 2026 UK window ranges are manufactured to Uw 1.2 to 1.3 W/m²K, which comfortably exceeds compliance and gives some headroom against future tightening under the Future Homes Standard.
The regulation covers every replacement, whether the property is a Victorian terrace, a 1970s bungalow or a 2005 new-build. It does not cover repairs (replacing a sealed unit within an existing frame is not notifiable), and it does not cover secondary glazing, which is treated as an internal addition rather than a replacement.
What Part F requires on ventilation
Part L was accompanied by an update to Approved Document F (Ventilation) in 2021. The headline consequence for window replacements is that background ventilation must be maintained. In most existing UK homes, this means trickle vents are required on almost every replacement window unless the property already has adequate whole-house mechanical ventilation.
The practical rule: if the original windows had trickle vents, the replacements must have them. If the original windows did not, the installer must still fit them unless a competent person has certified that the property’s ventilation is adequate without them. That certification is rare in existing housing, so budget for trickle vents on every window as a default.
Current U-value requirements by element
Part L 2021 sets replacement thresholds not just for windows but for every fabric element you might upgrade. The table below is the compliance minimum for existing dwellings in England as of the 2021 edition, still current in 2026.
| Element | Maximum U-value (W/m²K) | Notes |
|---|---|---|
| Window (whole window, Uw) | 1.4 | 1.6 permitted on historic or listed buildings |
| Rooflight (whole unit) | 1.7 | Measured on the horizontal, not the slope |
| External door (over 60% glazed) | 1.4 | Treated as a window |
| External door (under 60% glazed) | 1.4 | Composite and solid doors |
| Roof windows | 1.4 | Uw at the manufacturer’s declared slope |
| Curtain walling | 1.6 | Where applicable to dwelling extensions |
Anything at or below these numbers passes. Salespeople sometimes quote centre-pane Ug figures, which will always look better; the number that satisfies Building Control is Uw, the whole window.
How FENSA and CERTASS self-certification works
Rather than notifying every window replacement to local authority Building Control, most UK installers are members of a Competent Person Scheme. The two dominant schemes are FENSA (Fenestration Self-Assessment) and CERTASS. Both are approved by the Department for Levelling Up, Housing and Communities and both certify that the installer works to Part L, Part F, Part Q and Part K requirements.
When a member installs your windows, they notify the completed job to their scheme, which passes the notification to Building Control and issues you a certificate within four to six weeks. That certificate is what a conveyancing solicitor will ask for when you sell the property. Keep it safe.
The cost of FENSA or CERTASS notification is embedded in the installer’s price, typically £15 to £35 per window. It is not usually a separate line, but a quote that appears materially cheaper than the market and does not list a scheme number is almost always excluding it.
What happens if the installer is not registered
You have two legal paths for a non-scheme installation. Either the installer notifies Building Control before starting the work (typical fee £150 to £300 per project, depending on window count and local authority), Building Control inspects on completion, and issues a Building Regulations completion certificate. Or the work is done unnotified, in which case you must retrospectively apply for a Regularisation Certificate.
The Regularisation route costs more (typically £250 to £600), takes months, and requires the installer to open up sections of the work so Building Control can verify the U-values, the trickle vents, the safety glazing and the fitting quality. It is the most common paperwork problem in UK property transactions. If you buy a house with windows that were replaced without paperwork, the seller either provides a Regularisation Certificate at their cost, or you negotiate a price reduction and take the risk on.
How Part L interacts with EPCs and property sale
The Energy Performance Certificate (EPC) is calculated from the fabric elements of the property including the windows. A property with A+ rated windows will score materially higher than the same property with G rated single glazing, typically enough to shift the EPC band up by one letter. From April 2028, the government intends to raise the minimum EPC rating for new rental tenancies from E to C, which puts pressure on landlords with older glazing.
On sale, the buyer’s solicitor will ask for FENSA or CERTASS certificates for any windows fitted since April 2002. Missing paperwork is one of the top three reasons UK residential transactions are delayed at exchange. If your certificate is lost, both schemes offer replacement copies for a small fee; if the original installer did not notify, you are in Regularisation territory.
Conservation and listed building carve-outs
Part L Appendix A permits relaxed U-value targets on historic and traditional buildings where meeting the standard would unacceptably alter the character or appearance. For listed buildings and buildings in conservation areas, the target relaxes to Uw 1.6 W/m²K, or is waived entirely where any thermal upgrade would be inappropriate. Approved Document L is explicit that heritage character can outweigh energy performance in these cases.
The practical effect is that timber sash replacements on a Grade II listed Georgian townhouse are not required to hit 1.4 W/m²K. Slimline 14 mm sealed units (the maximum that will fit into a traditional sash rebate) give Uw around 1.7 to 1.9 W/m²K, which is a valid outcome under the Appendix A relaxation.
Approved Document Q: security on new builds
Part Q sets minimum security standards on new dwellings. It requires all easily accessible windows and doors to be certified to PAS 24. Replacement windows in existing homes are not subject to Part Q, but PAS 24 remains a valuable specification and is offered as standard by most reputable manufacturers on ground-floor units. Insurers increasingly ask about it, and it is a modest cost add on any quote worth taking seriously.
What Building Control actually inspects
Where an installer is not on a Competent Person Scheme, Building Control will typically inspect twice: once at the survey stage to agree the specification, and once at completion. The inspector confirms three things. First, that the manufacturer’s data sheet shows a compliant Uw for the exact unit fitted, matched to the product identifiers on the invoice. Second, that trickle vents are present and correctly sized against Part F’s minimum equivalent area for the room. Third, that safety glazing (toughened or laminated to BS 6206 / EN 12600) is used in every critical location: within 800 mm of finished floor level, within 300 mm of a door edge, and anywhere in a door itself.
Photographic evidence during installation is often accepted in lieu of physical inspection on retrofits. If your installer is going the Local Authority route rather than FENSA or CERTASS, agree in writing whose responsibility the photographs are before work starts.
Compliance checklist before you sign a quote
Work through this list before authorising the order.
- Uw on the quote is 1.4 W/m²K or lower for every window (1.6 acceptable only on listed or conservation carve-outs)
- Trickle vents specified on each window unless a written ventilation waiver is attached
- Installer’s FENSA or CERTASS membership number stated in writing
- Insurance-backed guarantee provider named
- PAS 24 confirmed for ground-floor and accessible windows
- Deposit terms and insurance-backed deposit protection confirmed
What to do next
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