Part L Compliant Windows

What Is Part L?

Part L of the Building Regulations covers the conservation of fuel and power in buildings. For windows, it sets minimum thermal performance standards measured as U-values — the rate at which heat passes through the window. A lower U-value means better insulation. Part L applies to all replacement windows in England and Wales, with separate but similar standards in Scotland (Section 6) and Northern Ireland (Part F).

Current U-Value Requirements

Since June 2022, the updated Part L requirements for replacement windows in existing dwellings are a maximum whole-window U-value of 1.4 W/m²K. New-build dwellings have a notional target of 1.2 W/m²K, though the actual requirement depends on the SAP calculation for the whole building.

Exemptions for Period Properties

Listed Buildings and properties in Conservation Areas may be exempt from Part L thermal requirements where compliance would unacceptably alter the character or appearance of the building. This exemption is not automatic — it depends on the specific property, the local planning authority, and whether the conservation officer agrees that the thermal upgrade would harm the historic character.

In practice, most Conservation Area properties can achieve Part L compliance with standard 24mm double glazed units without any visible difference from the street. The exemption is primarily relevant for Grade I and Grade II* Listed Buildings where even slim heritage double glazing is not permitted and single glazing must be retained.

FENSA and Building Control

If your installer is registered with FENSA (or an equivalent competent person scheme like CERTASS), they can self-certify that the replacement windows meet Part L without involving Building Control. If your installer is not registered, you must obtain a Building Control application and pay the associated fee — typically £200–£400. We are a FENSA-registered installer.

What Happens If You Do Not Comply?

Non-compliant window replacements can cause problems when selling your property. Conveyancing solicitors will check for Building Control sign-off or FENSA certificates for any windows replaced after 2002. Missing certificates can delay or derail a sale and may require an indemnity insurance policy. Getting it right from the start is far simpler and cheaper than fixing it later.

All our windows exceed Part L requirements as standard. Get an instant price with our 3D configurator, or book a survey and we will handle the FENSA certification for you.