Changes to building regulations regarding
skylights and roof windows – 2022 update

From the 15th of June 2022, changes in building regulations have come into effect in
alignment with Government plans to deliver Net Zero. These regulations include guidance on
cutting carbon emissions in homes by 30% and other buildings by 27%.

What is Approved Document L?

This document outlines the new rules surrounding the energy efficiency of rooflights and
roof windows, both for homes and non-dwelling properties.

Comprised of two volumes, Document L has merged the previous four sections into
dwelling and non-dwelling properties, which include both existing and new developments of
rooflights and skylights.

Volume 1 – Dwellings: Rooflights and roof windows
Volume 2 – Buildings other than dwellings: Rooflights and roof windows
Summary

The below information is from Government documents, which are up to date as of
publication but are subject to change. For this reason, we recommend consulting a
professional when assessing your development options.

Limiting heat gains and loss from your rooflight

U-values should be calculated considering the whole structure, including any fittings, such as
kerbs or upstands. In the situation of rooflights and skylights, this will be both the glazing and
the structure holding it within the roof.
Insulating fabric elements such as rooflights in new dwellings must meet the standards
outlined below:
Element type  Maximum U-Value (1) W/ (m2.K)
All roof types  0.16
Wall  0.26
Floor  0.18
Party Wall  0.20
Swimming pool basin  0.25
Window  1.6

Rooflight  2.2
Doors (including glazed doors)  1.6
Air permeability  8.0m3/(h.m2) @ 50pa
1.57m3/(h.m2) @ 4Pa

Volume 1 – Dwellings: Rooflights and roof windows

U-values, target energy rates, target emissions rate and target
energy efficiency

In both the design and construction phases of installing skylights, the minimum standard of
total energy performance must be adhered to. In simple terms, this is the comparison of the
‘notional dwelling’ – the plans, and the ‘actual dwelling’ – which takes into consideration the
changes and developments after work has been done.

U-values seem confusing to the average homeowner, however, they needn’t be. The key is
to remember that similarly to golf, the lower the number the better. This just means that
the least amount of heat is escaping through the skylight, making it more energy efficient.
Target U-values for notional dwellings:

Roof windows (Vertical), U = 1.2W/(M2.K)
Rooflights (Horizontal), U = 1.7W/(M2.K)

The reference value used for both skylights and roof windows is 5m3/h.m2.
Full properties of the notional dwelling can be found in the Government’s Standard
Assessment Procedure (SAP) for energy rating of dwellings.

New rooflights & roof windows in existing dwellings, including
extensions

When adding rooflights and roof windows to buildings where people live, they must align with
the U-value regulations as stated above. It is important to remember that the whole structure
is used to calculate its energy efficiency. That means that the window and kerb or upstand
will be measured as a whole.

When you buy a rooflight that comes with a kerb, it is the supplier’s job to provide you with
the U-value of the whole fixture.

If you are updating an old skylight, ensure that the replacement is draught proofed and
insulated where appropriate, as well as complying with the minimum standards provided.
Roof windows and rooflights are not permitted to cover greater than 25% of the total floor
area within a dwelling.

This means that no more than a quarter of the room’s surface area
can be covered by skylights or roof windows. If you are considering covering more than 25%
of the floor with skylights, you would need to take further precautions to reduce energy loss.
More information on this can be found within the document.

A conservatory or porch does not apply to these rules due to the thermal separation,
however if there is no thermal separation, these developments must be treated as an
extension and comply with the new rules.

Changes to current rooflights & roof windows in existing
dwellings

If a building was previously not a dwelling but now is, there are specific requirements to
consider, called material change of use rules. A change to energy status similarly is when a
property previously had an exemption from energy efficiency requirements but now is not.
If your property has either changed material use or energy status, you must ensure that any
new developments of skylights on roof windows are in line with the new minimum energy
requirements as stated in the above table.

Volume 2 – Buildings other than dwellings: Rooflights and
roof windows

This section outlines the rules surrounding rooflights and roof windows on buildings that are
not dwellings.

U-values, target energy rates, target emissions rate and target
energy efficiency

Similarly to dwellings, in non-dwelling building developments, the U-values must be
calculated before the work starts and then revisited throughout and after completion.
The target for non-dwelling buildings is U=2.1W/9m2.K) when positioned horizontally
and the reference value used for air permeability for rooflights and roof windows is
5m3.h.m2.
Further information regarding this can be found within the SAP document under Reference
10.

New rooflights & roof windows in existing buildings, including
extensions

When extending on a non-dwelling building, there are requirements based on the maximum
amount of openings that can be put into the extension. These requirements are outlined
below:

Residential buildings where people
temporarily or permanently reside 

Building type  Windows and pedestrian
doors as % of exposed wall – 30

Rooflights as % of
the area of roof  – 20

Places of assembly (offices and
shops)

Building type  Windows and pedestrian
doors as % of exposed wall  – 40

Rooflights as % of
the area of roof  – 20

Industrial and storage buildings 

Building type  Windows and pedestrian
doors as % of exposed wall -15

Rooflights as % of
the area of roof  – 20

Vehicle access doors, display windows and similar glazing can be as large as the wall or
roof as required.
Any extension must have thermal separation from the building to not need to fit these
requirements. In situations where this is not the case, any new extensions must align with
requirements in the U-value table above. Rooflights and roof windows also need to comply
with the new regulations. Any walls, doors and windows should be insulated and draught
proofed to at least the same quality as the existing building.

Changes to current rooflights & roof windows in existing
buildings

When changing currently fitted rooflights and roof windows, replacement structures must
align with the new regulations and minimum u-values as shown in the first table.

If you would like further clarification on how this document could affect your upcoming
developments and how Sunsquare can help, get in touch.
Furthermore, download our whitepaper which explains in simple terms the building
regulations you may need to know about.

Summary

– Calculate the U-value based on the whole element, which includes any kerb or
upstand as well as the rooflight
– When calculating U-values,  for roof windows and windows calculate on the vertical
plane and for rooflights, calculate on the horizontal plane.
– You must calculate energy efficiency prior to building, and revisit it during the
development and reassess after the project is finished.