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Part L Transitional Provisions for Domestic Extensions 2022

The Government has issued a new Approved Document Part L with guidance on works to existing buildings. The new edition of the Building Regulations Part L will come into force on 15th June 2022.

The new 2022 Part L represents a significant step up from the current (2013) Edition, and includes an uplift in fabric insulation standards, improved U-values and increased thickness of insulation within extensions.

The document below has been produced by Quadrant Building Control to help builders and designers to understand what the new requirements will be for domestic extensions.

Typical construction details with insulation thickness have been included that should meet the U-values standards contained within the 2022 Edition of Part L. These construction details have been provided for some typical wall, floor and roof details often used within extensions.

Transitional provisions are available for schemes that will start on site before 15th June 2023, but a Building Regulation Application will need to be submitted before 15th June 2022.

U-values for Domestic Extensions

New thermal elements should not exceed the U-values listed in the table below.


Key Dates

Cavity Wall Construction

Mineral Wool Insulation
U-VALUE: 0.18 W/m2.k

Rigid Board Insulation
U-VALUE: 0.18 W/m2.k

Sloping Roof Construction

Rigid Board Insulation
U-VALUE: 0.15 W/m2.k

Flat Roof Construction

Rigid Board Insulation
U-VALUE: 0.15 W/m2.k

Floor Construction

Rigid Board Insulation
U-VALUE: 0.18 W/m2.k

It remains the responsibility of the designer to specify the construction of thermal elements that meet the U-value standards, and it is the responsibility of the contractor to construct the extension in accordance with the U-value standards. The details shown in this booklet are indicative and Quadrant Building Control hold no liability or responsibility for demonstrating compliance with the Building Regulations. Please complete U-value calculations for each thermal element before construction, to ensure the detail proposed meets the U-value standard.


Please contact Quadrant Building Control for further information regarding the transitional provisions, by calling 0345 557 2777, or find the contact details of your local office from our Contacts page.

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