Large sliding doors, glazed walls and rooflights can make an extension feel open and bright, but the amount of glazing affects Building Regulations energy compliance. The current Part L guidance provides an elemental opening-area route and alternative calculation methods for extensions.
The commonly referenced opening-area approach
Under the current Approved Document L guidance for an extension to a dwelling, the total area of new windows, roof windows, rooflights and doors should generally not exceed 25% of the extension floor area plus the area of qualifying existing windows and doors that disappear or cease to be exposed because of the extension.
What if the design has more glazing?
Exceeding that elemental opening-area approach does not automatically mean the design cannot comply. Approved Document L provides alternative routes, including an area-weighted approach and a Standard Assessment Procedure comparison route. These calculations can show whether compensating fabric or whole-dwelling performance is sufficient.
Why this matters early in design
A glazed wall, large roof lantern and wide sliding doors can use the available opening allowance quickly. Testing the proposal during design helps avoid discovering late in the Building Regulations stage that the glazing strategy needs additional insulation, better-performing components or another compliance method.
Current and future standards
As at 2 October 2026, the 2021 edition of Approved Document L incorporating 2023 amendments remains the relevant current guidance for ordinary existing-dwelling extension work unless a different transitional standard applies. New 2026 Part L standards are published but generally come into force for non-higher-risk work on 24 March 2027, subject to transitional arrangements.
What Archi Project can calculate
Where included in the quotation, Archi Project can undertake Part L / large-glazing design checks to help coordinate the opening sizes, thermal specification and Building Regulations drawing package. An EPC is not included in the standard package.
Frequently asked questions
Does the 25% figure mean I cannot have a highly glazed extension?
No. It is part of an elemental compliance route. Alternative calculation routes can be used where the opening area is larger.
Do rooflights count as openings?
Yes. The Approved Document L extension guidance includes windows, roof windows, rooflights and doors in the opening-area calculation.
Will the new 2026 Part L apply immediately?
No. The 2026 standards are published but the main non-HRB changes generally come into force on 24 March 2027, subject to transitional provisions.
Official sources
These are the principal official sources used for this guide.