Approved Document L, F and O FAQ’s

The Government has announced changes to three building regulations for new and existing homes to help the UK deliver net zero emissions by 2050. This comes in the form of three Approved Documents; L, F, and O.

We have compiled some frequently asked questions on the new regulations to help you better understand the new requirements and what effect these changes will have on your windows and doors.

Why are the new regulations being introduced?

The new regulations are being introduced to ensure all new and renovation projects are on-spec for thermal efficiency, energy efficiency and ventilation capability.

The 2022 building regulation changes are seen as a stepping stone to the Future Homes Standard which is due in 2025.


What is the Future Homes Standard?

The Future Homes Standard is a set of standards that will ensure new homes built from 2025 will produce 75-80% less carbon emissions than dwellings built prior to 2022.

The built environment is responsible for 40% of UK greenhouse gas emissions. With 14% of this coming from homes in the UK. The Future Homes Standard is designed to bring these levels down. Helping to tackle climate change, reduce energy bills and reach the target of net zero by 2050.


When do the new regulations come into force?

The new regulations come into force on 15th June 2022.

However there is a transitional period for new dwellings, extensions and existing building, if a planning application or building control application has been submitted prior to 15th June 2022, and work will commence prior to June 15th 2023. These projects are exempt from the new regulations.

This applies to Document L, F and O.


Document L

What is Approved Document L?

Also referred to as Doc L, or Part L.

The Government is making changes to Doc L of the building regulations, which covers the conservation of fuel and power. With the aim of the new regulation to reduce the amount of carbon emissions produced by homes.

Doc L has two parts, Part 1, which applies to dwellings and Part 2 which applied to commercial buildings.

The full regulations can be downloaded here:

Conservation of fuel and power: Approved Document L


What does the changes to Document L mean for my windows and doors?

The new legislation means that the thermal regulations for windows and doors will change, to limit heat gain and loss.

Since the changes to the regulations were announced the System Houses who design the windows and doors have been busy optimizing and testing their products to ensure they fully comply with the new regulations.

The look and style of the external aesthetics of the products have stayed the same. While internally the frames have been modified to help achieve the new standards. Then rigorously tested to ensure they meet the new minimum U-values/Energy ratings set out in the legislation.


What are the new thermal regulations for my windows and doors under Document L?
New Dwellings U-value
Windows 1.6 W/m2K
Glazed Doors 1.6 W/m2K
Existing Dwellings U-value Window Energy Rating /
Door Energy Rating
Windows 1.4 W/m2K WER B
Glazed Doors
(more than 60% glass)
1.4 W/m2K DER C
Other Doors 1.4 W/m2K DER B

The above figures are a minimum level.

For new dwellings, the properties are assessed using U-values. Whereas for existing dwellings properties are assessed using Energy Ratings or U-values.

For existing dwellings the new product should be no worse than that of the element being replaced.


What is an U-value and how is it calculated?

Thermal performance is measured in terms of heat loss, and is expressed as a U-value.

The U-value is calculated based on the rate of transfer of heat through a structure (e.g. the window or door), divided by the difference in temperature across that structure. The units of measurement are W/m²K. The lower the value the more efficient the product.


What is Window/Door Energy Ratings?

Window Energy Rating (WER)/Door Energy Rating (DER) is a rating system that is based upon a scale of E to A++

A++ being the most energy efficient, similar to the household appliance scheme.

WER/DSERs can be used as a compliance method for existing dwellings and domestic type windows on other buildings under Document L.


Document F

What is Approved Document F?

Approved Document F outlines standards on building ventilation, air quality and the prevention of condensation for all buildings. To protect the health of the occupants. As windows now make homes more airtight, additional ventilation is required to circulate air in living spaces. Options include mechanical ventilation, extractor fans and trickle vents.

Document F applies to both new dwellings and existing dwellings.

The full regulations can be downloaded here:

Ventilation: Approved Document F


What do changes to Document F mean for my windows and doors?

New Builds and Extensions

A ventilation provisions must be demonstrated in any of the following ways:

a. Incorporating background ventilators (trickle vents) in the windows, equivalent to the following minimum equivalent area (EQA) (for dwellings with multiple floors):

i. Habitable rooms – 8000 mm2 EQA.
ii. Kitchen – 8000 mm2 EQA.
iii. Bathroom (with or without a toilet) – 4000 mm2 EQA.

b. If the dwelling will have continuous mechanical extract ventilation without heat recovery, windows in habitable rooms which are not wet rooms must have a minimum background ventilation area of 4000 mm2.

c. Other ventilation provisions, if it can be demonstrated to a building control body that they comply.

Replacing Existing Windows

Existing windows with background ventilators (trickle vents)

If existing windows have background ventilators, the replacement windows should include background ventilators. The new background ventilators should comply with both of the following conditions:

a. Not be smaller than the background ventilators in the original window.

b. Be controllable either automatically or by the occupant.

Existing windows without background ventilators (trickle vents)

Replacing the windows is likely to increase the air-tightness of the dwelling. If ventilation is not provided via a mechanical ventilation with heat recovery system, then increasing the air-tightness of the building may reduce beneficial ventilation in the building. In these circumstances, it is necessary to ensure that the ventilation provision in the dwelling is no worse than it was before the work was carried out. This may be demonstrated in any of the following ways:

a. Incorporating background ventilators in the replacement windows, equivalent to the following minimum equivalent area (EQA) (for dwellings with multiple floors):

i. Habitable rooms – 8000 mm2 EQA.
ii. Kitchen – 8000 mm2 EQA.
iii. Bathroom (with or without a toilet) – 4000 mm2 EQA

b. If the dwelling will have continuous mechanical extract ventilation without heat recovery, replacement windows in habitable rooms which are not wet rooms must have a minimum background ventilation area of 4000 mm2.

c. Other ventilation provisions, if it can be demonstrated to a building control body that they comply.

Please note:

  • Background ventilators are intended to be normally left open.
  • If the dwelling is close to an area of sustained load noise (e.g. a main road), then a noise attenuating background ventilator should be used.
  • Window night latches are not acceptable as a mean of background ventilation.

Document O

What is Approved Document O?

Approved Document O is a new building regulations which sets out overheating mitigation requirements for new residential dwellings.

The aim of Doc O is to:

  • Limit unwanted solar gains in summer,
  • Provide an adequate means to remove heat from the indoor environment.

Protecting the health and welfare of a building’s occupants by reducing the occurrence of high indoor temperatures.

The document holds two methods of compliance:

  • The Simplified Method: this method considers the size and orientation of glazing and compares the glazing area to the floor area. Window opening areas are also considered, to deliver suitable purge ventilation.
  • Dynamic Thermal Analysis (CIBSE TM59): this can be used to ensure dwellings comply with the document by predicting the risk of overheating and offering a range of strategies for reducing this risk.

The full regulations can be downloaded here:

Overheating: Approved Document O


How will Document O effect my windows and doors?

A reasonable provision must be made in a dwelling to limit unwanted solar gains and provide adequate means to remove the heat from the indoor environment.

Compliance will be demonstrated by the Designer/Architect. They will need to provide information on limiting solar gain and a means of removing excess heat.

This will likely limit the amount of glazing permitted in the building, unless overheating can be alleviated by other measures.

Examples of this would be by using shading and louvres, or solar control glass as well as glass with a lower solar gain.

Lower solar gain (g value) glass is typically darker to restrict light radiation – usually found in lower U-value glazing units with high performance low-e coatings and also triple glazing, so doesn’t necessarily always require solar control glass.