Part L for a replacement rooflight

The requirement

A replacement rooflight in an existing dwelling must achieve a whole unit U-value no worse than 2.2.

That is not demanding. A modern double glazed unit with a low emissivity coating, argon fill and a warm edge spacer lands around 1.2 to 1.4.

Whole unit, not centre pane

The regulation uses the whole unit figure, including the frame and the spacer. Marketing often quotes centre pane, which ignores exactly the parts where heat escapes and is always the better number.

When comparing quotations, compare whole unit figures.

When it applies

Replacing a rooflight: yes, the new unit must meet the standard even where the work is otherwise like for like.

A new opening: yes, and the work is notifiable for structural reasons as well.

Repair rather than replacement, such as changing a handle or a seal: no.

Extensions are different

Where a rooflight forms part of a new extension, it is assessed as part of that extension’s overall performance rather than in isolation. A poorly performing rooflight then has to be compensated for by better performance in the walls or roof.

Getting the glass right first is cheaper than compensating elsewhere. There is also a limit on total glazed area as a proportion of floor area, so a very heavily glazed extension may need a calculation.

Who notifies

Where the work is notifiable we handle the notification and the completion certificate comes to you. That certificate is the document a buyer’s solicitor asks for.

The number, and what it applies to

For a replacement rooflight in an existing dwelling, Part L expects a whole unit U-value no worse than 2.2 watts per square metre kelvin.

That is a compliance threshold rather than a target. Any current double glazed unit with a low emissivity coating, argon fill and a warm edge spacer lands around 1.2 to 1.4, so the requirement is cleared with a wide margin and it rarely causes anybody difficulty.

The reason it appears on this site at all is not that it is hard to meet. It is that people are frequently unaware a replacement is notifiable at all, and that surfaces later at exactly the wrong moment.

Why a replacement is caught

A rooflight is a controlled fitting. Replacing one engages Part L even though you are not altering the structure, in the same way that replacing a window in a wall does.

The distinction the regulations draw is between repairing something and replacing it. Renewing a flashing, replacing a broken pane within an existing frame, or servicing an actuator is maintenance and is not notifiable. Taking the unit out and putting a different one in is a replacement of a controlled fitting and is.

That surprises people, because swapping a window feels like maintenance rather than building work.

Which U value the regulation uses

The point that matters most, and the one that makes quotations incomparable when it is fudged.

Centre pane measures the glass away from the edges. It excludes the spacer bar and the frame entirely, which is precisely where the heat gets out. It is always the better figure and it is what marketing prefers.

Whole unit includes glass, spacer and frame weighted by area. It is what the building experiences and it is what the regulations use.

Whole unit is always worse, frequently by 0.4 or more, and on a small rooflight with a lot of frame relative to glass the gap is wider still. A unit advertised at 1.0 and another at 1.4 may be identical performance described two ways.

Ask for the whole unit figure in writing, on the quotation, alongside the model and size code.

How it gets notified

Two routes, and it is the installer’s job either way.

Competent person self certification. Where the installer is registered with a relevant scheme, they certify the work against the regulations and notify the local authority afterwards, and you receive a certificate. No application, no council fee, no inspection visit. This is the usual route for a straightforward replacement.

Building notice or full plans. An application to the local authority or an approved inspector, with a fee and inspection visits at agreed stages, and a completion certificate at the end.

This is the route where the installer is not registered, or where the work is structural rather than a straight swap: forming or widening an opening engages Part A as well, and in a trussed rafter roof that means a designed trimmer and an engineer.

Where the rooflight forms part of an extension or a loft conversion already going through Building Control, it is covered by that application and nothing separate is needed. Tell your installer if that is the situation so you are not charged twice.

The other parts that come with it

Part L is rarely the only one engaged, and on a replacement it is worth knowing which others might be.

Part A, structure, wherever an opening is formed or altered. Adapting an opening by 40mm or 60mm to reach the nearest current size is normally straightforward; widening it significantly is a structural alteration and needs designing.

Part B, fire safety. In a habitable loft room the roof window frequently carries the means of escape duty, with a minimum clear openable area, minimum opening dimensions and a maximum sill height above the finished floor. Replacing a top hung escape window with a centre pivot unit can put a compliant room out of compliance.

Part B also limits unprotected area, which includes rooflights, within a certain distance of a boundary. That catches people out on narrow plots.

Part F, ventilation. An opening rooflight contributes to purge ventilation for a habitable room, calculated on free open area rather than on the size of the unit. A fixed replacement for an opening unit reduces that provision.

Part O, overheating, applies to new dwellings rather than to replacements or to extensions of existing houses, so it does not usually bite here.

Where the standard actually bites

Not on replacements. On extensions.

There the rooflights form part of the thermal calculation for the whole structure, so a poor unit forces better performance somewhere else in the build. There is also a limit on total glazed area as a proportion of floor area, which means a heavily glazed extension needs a calculation rather than an assumption.

Getting the glass right first is cheaper than compensating for it in the walls afterwards.

What the certificate does not cover

The quality of the installation.

A product certificate shows the unit meets the standard. It says nothing about whether the flashing kit was the correct profile for the covering, whether the underlay collar was lapped so water passes around the opening, whether the insulation collar was fitted, or whether the vapour control layer was sealed to the frame.

Those decide whether the installation performs anything like the certificate suggests. A 1200 by 1800 flat rooflight on an uninsulated kerb has close to a square metre of poorly performing envelope around its perimeter regardless of what the glass achieves, and an uncollared pitched unit runs a cold reveal that condenses every winter.

Compliance and performance are related and they are not the same thing.

Why it matters beyond the rules

The sale of the property.

A conveyancing solicitor will ask for evidence that notifiable building work was notified. The absence of a certificate produces a delay, an indemnity policy, or a retention, at exactly the point in a transaction where you least want any of the three.

Getting it notified at the time costs very little and frequently nothing beyond what is already in the price. Regularising it years later is considerably harder, because the installation is buried and nobody can inspect what is behind the lining or under the covering.

What to keep

The certificate itself, the written specification naming the unit, the glazing and the flashing kit, and the installer’s workmanship guarantee.

All three with the house paperwork rather than in a drawer, because in fifteen years nobody will remember which company did the work, and producing a certificate takes a minute where reconstructing one takes weeks.

What the requirement actually is

A rooflight is a controlled fitting. Replacing one is work to which the conservation of fuel and power requirements apply, and compliance is expressed as a maximum whole unit U value.

Three points follow from that wording, and each is worth holding onto.

Whole unit, not centre pane. The figure covers the complete assembly including frame and spacer. On a small rooflight those two carry a large share of the loss, so the whole unit figure is considerably worse than the centre pane one and it is the figure that has to comply.

The standard applying now, not when the original went in. A twenty five year old unit was built to a far weaker requirement, and that is irrelevant to the replacement.

The replacement standard, not the new build one. They are different figures.

Why it rarely constrains the choice

Any current mainstream double glazed roof window from a reputable manufacturer meets the replacement standard comfortably, frequently by a wide margin.

So in practice this is a box that gets ticked rather than a decision that shapes the specification. What shapes the specification is what you want beyond the minimum: a warm edge spacer, solar control, laminated glass.

Where it does become a conversation

On a listed building or in a conservation area, where the thermal requirement and the visual requirement pull against each other.

A low profile conservation unit may not reach the same figure as a bulky modern one, and there is provision for relaxation where full compliance would unacceptably alter the character of the building. That is discussed with building control rather than assumed, and it is worth raising early.

The part the regulation does not reach

The installation.

An insulation collar around the frame, insulation carried continuously around the reveal, an insulated kerb on a flat roof, and a vapour control layer taped to the frame are worth as much in the real building as the glazing specification, and none of them appear in the U value on the datasheet.

A compliant unit in a badly detailed opening performs worse than a modest unit in a well detailed one. Compliance is a floor, not a description of performance.

Notification and the certificate

The work is notifiable, either through building control directly or under a competent person scheme where the installer self certifies.

Ask which route is being used and make sure the certificate reaches you. It surfaces at sale, and obtaining it retrospectively is far harder than getting it at the time.