Highly Glazed Extension Assessments — Part L Compliance Made Simple
If your proposed extensions' glazing area is more than 25% of the proposed floor area plus the area of any lost openings, it requires a specialist energy assessment under Building Regulations Part L, Section 10.10. TPLC Ltd provides fast, authoritative compliance reports accepted by building control authorities across the UK.
Building Regulations Part L · Section 10.7D
"The total area of windows, roof windows, rooflights and doors in extensions should not exceed the sum of the following.
i. 25% of the floor area of the extension.
ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension."
Approved Document L, Volume 1 — Dwellings (2021 edition)
Understanding the Regulations
What Is a Highly Glazed Extension?
Under Part L of the Building Regulations (Approved Document L, Volume 1), any extension to a dwelling where the combined area of windows, roof windows, rooflights, and glazed doors exceeds 25% of the extension's floor area plus the area of any lost openings is classified as a highly glazed extension.
This classification matters because the standard "elemental" compliance route — used for most extensions — is not permitted for highly glazed extensions. Instead, Section 10.10 requires a dedicated energy performance assessment to demonstrate that the extension meets the required carbon emission and energy efficiency standards.
In practice, this affects orangeries, garden rooms, large bifold-door extensions, glazed kitchen extensions, and any design where glass is a dominant feature. Without a compliant assessment, building control cannot sign off your project.
Why It Matters
Why You Need a Part L Assessment
Legal Requirement
Building Regulations Part L is a legal obligation. Without a compliant Section 10.10 assessment, your extension cannot receive building control sign-off.
Avoid Enforcement Action
Proceeding without the required assessment can result in enforcement notices, costly remediation works, or difficulties when selling your property.
Required by Building Control
Your local authority building control (LABC) or approved inspector will request this report as part of the completion process. We provide exactly what they need.
Professional Compliance Report
We produce a clear, detailed report demonstrating compliance with the energy performance requirements — formatted for direct submission to building control.
Fast Turnaround
We understand project timelines are tight. Our assessments are completed promptly so your build is not delayed waiting for compliance documentation.
UK-Wide Service
We work with homeowners, architects, and builders across England and Wales, providing assessments for projects of all scales and complexity.
The Process
How It Works
Submit Your Plans
Send us your extension drawings, floor plans, and glazing specifications. We accept drawings in any standard format.
We Carry Out the Assessment
Our specialists carry out the full Section 10.10 energy assessment, evaluating glazing ratios, thermal performance, and carbon emissions.
Receive Your Compliance Report
You receive a clear, professionally formatted compliance report demonstrating that your extension meets Part L requirements.
Submit to Building Control
Submit the report to your building control body alongside your other documentation. We are available to answer any queries they raise.
Ready to Get Started?
Ready to get your extension approved?
Send us your plans and we'll provide a fast, accurate Part L Section 10.10 assessment. No jargon, no delays — just a clear compliance report that building control will accept.