Replacing windows sounds simple until the paperwork starts.
U-values. Part L. FENSA. WER bands. It’s a lot of terminology for what is, at heart, a straightforward question: how well insulated does a new window need to be, and who checks that it is?
At Tailored Wood, we specify and fit timber windows across Hampshire, Wiltshire and the South of England, on everything from new-build extensions to listed buildings where the rules work very differently. This guide sets out what the current regulations actually require, in plain terms, with sources you can check yourself.
What U-value do windows need to meet UK regs?
Under current UK Building Regulations, a replacement window in an existing dwelling in England must achieve a maximum U-value of 1.4 W/m²K, or a Window Energy Rating of Band B or better. This standard has applied since 15 June 2022 and remains the current requirement as of 2026.
For new-build homes, the notional target is tighter, at 1.2 W/m²K, with a limiting value of 1.6 W/m²K that no individual window may exceed.
A separate set of rules, the Future Homes Standard, was published in March 2026 and comes into force on 24 March 2027. It applies to new-build homes, not to replacement windows in existing properties, and it keeps the same window U-value targets that already apply today. If you’re replacing windows in an existing house right now, the 1.4 W/m²K figure is the one that matters.
Listed buildings and properties in conservation areas work differently again, covered further down.
Understanding U-values: what the number actually means
A U-value measures how much heat passes through one square metre of a material for every degree of temperature difference between inside and outside, expressed in watts per square metre per kelvin (W/m²K).
The lower the number, the better the insulation. A window with a U-value of 0.8 W/m²K loses heat at roughly half the rate of one rated at 1.6 W/m²K.
For context, typical figures look like this:
- Single glazing: around 5.0 W/m²K
- Standard double glazing: roughly 1.2 to 1.6 W/m²K
- High-performance double glazing or vacuum insulated glass: as low as 0.8 W/m²K
- Triple glazing: typically 0.8 to 1.0 W/m²K
Crucially, the U-value that matters for compliance is the whole-window figure, not just the glass. It’s calculated to BS EN ISO 10077-1 and includes the frame, the spacer bar between panes, and the glazing itself. A window can have excellent glass and still fail to meet the standard if the frame performs poorly, which is one reason timber frame design and section thickness matter as much as glazing specification.
Part L explained: what it covers and what it requires now
Part L of the Building Regulations governs the conservation of fuel and power, and it’s the section that sets U-value requirements for windows, doors and rooflights.
The current version in force is Approved Document L, Volume 1 (dwellings), the 2021 edition incorporating 2023 amendments. It came into effect on 15 June 2022 and remains the applicable standard for existing homes as of 2026.
Under this document:
- Replacement windows in existing dwellings must achieve a maximum U-value of 1.4 W/m²K, or WER Band B
- New windows in new-build dwellings should meet a notional U-value of 1.2 W/m²K
- The limiting U-value, the hard cap no individual window may exceed, sits at 1.6 W/m²K
- A replacement window also can’t perform worse than the window it’s replacing, even if 1.4 W/m²K would technically be met
A new edition, Approved Document L: Energy and Greenhouse Gas Emissions, was published in March 2026 as part of the Future Homes Standard. It comes into force on 24 March 2027 for standard new-build work in England, with a further date of 24 September 2027 for higher-risk buildings. Its notional and limiting window U-values remain the same as the current 2021 figures. This update applies to new dwellings, not to replacement windows in existing homes, so it doesn’t change what’s required for a typical window replacement project today.
Part F and trickle vents
Alongside Part L, Part F covers ventilation, and it’s the reason most replacement timber windows now include trickle vents. Since June 2022, replacement windows generally need background ventilation built in, unless the property already has a system of continuous mechanical extraction. This applies to standard replacements, though heritage projects can sometimes secure an exemption where trickle vents would visibly alter a historic frame, agreed directly with building control.
FENSA and CERTASS: what self-certification actually means
Since 1 April 2002, replacing windows or doors in a dwelling has fallen within the scope of the Building Regulations. That means every replacement window installation needs to demonstrate compliance, one way or another.
There are two routes:
A competent person scheme.
FENSA (the Fenestration Self-Assessment Scheme) and CERTASS are both government-authorised schemes that let registered installers self-certify their own work. When a FENSA or CERTASS registered installer fits your windows, they notify your local authority directly and issue a certificate confirming the work meets Part L and related requirements. You don’t need to involve building control yourself.
Local authority building control.
If your installer isn’t registered with a competent person scheme, you’ll need to apply to your local council for building control approval before work begins, which involves inspection visits and fees.
A FENSA or CERTASS certificate confirms compliance, not workmanship quality. It’s a separate document from any product guarantee or installer warranty. It does matter at the point of sale, though. Solicitors routinely check for these certificates during conveyancing, and a missing certificate for windows fitted after April 2002 can hold up a sale or require retrospective compliance checks.
Listed buildings and conservation areas: how the rules change
This is where our own project experience comes in most directly, since a large part of our work involves exactly this situation.
Listed buildings and buildings in conservation areas are not automatically exempt from Part L. What they get is a specific allowance: under Approved Document L, Volume 1, paragraph 0.8, work to a listed building, a building in a conservation area, or a scheduled monument does not need to fully comply with the energy efficiency requirements where doing so would unacceptably alter the building’s character or appearance.
Paragraph 0.9 then adds the practical test: the work should still meet Part L standards to the extent that is reasonably practicable, and the local authority’s conservation officer is normally consulted on where that line sits.
In practice, this means:
- A single-glazed original sash in a listed building doesn’t have to be ripped out and replaced with a 1.4 W/m²K double-glazed unit if doing so would damage the building’s character
- Slimline double glazing or vacuum insulated glass, which improves thermal performance while keeping original sightlines, is often the accepted middle ground
- Listed Building Consent is still required for most window work on a listed building, even a like-for-like replacement, separately from Building Regulations compliance
- Building control will usually want to see the Listed Building Consent decision alongside any energy performance discussion, so the two processes run in parallel rather than one replacing the other
Historic England’s guidance, Energy Efficiency and Historic Buildings, sets out this approach in more detail and is worth reading directly if you’re weighing up options on a listed property.
What this means for choosing timber windows
A well-specified timber sash or casement window can meet the 1.4 W/m²K standard comfortably using slimline double glazing units within a traditional frame profile, without needing to look like a modern uPVC product. Vacuum insulated glass takes this further, achieving strong U-values in a genuinely narrow unit that fits period glazing bars without altering their appearance.
For listed and conservation area projects, the specification conversation usually starts from the building’s character rather than the regulation, then works out how far modern glazing can be brought in without compromising it. We handle this as part of every survey, working from CAD drawings and, where needed, a heritage statement to support a Listed Building Consent application.
Sources
- Approved Document L, Volume 1: Conservation of fuel and power (2021 edition incorporating 2023 amendments), UK Government
- Approved Document L: Energy and Greenhouse Gas Emissions (March 2026), UK Government
- Historic England, Energy Efficiency and Historic Buildings: Application of Part L of the Building Regulations to Historic and Traditionally Constructed Buildings
- British Woodworking Federation, guidance on U-value requirements for existing and new dwellings
- FENSA and CERTASS, competent person scheme guidance
Talk to Tailored Wood about compliant timber windows
We specify and fit timber windows across Hampshire, Wiltshire, Dorset and the wider South of England, covering standard Part L compliant replacements as well as listed building and conservation area projects where the rules need careful navigation. Every window is surveyed, designed and made in-house, with U-value calculations and, where needed, CAD drawings and heritage statements to support planning and building control.
If you want a clear answer on what your specific project needs to meet, get in touch for a free quote and survey.
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