What We Do
Part L Section 10.10 Assessment Service
A complete, professionally produced compliance report demonstrating that your highly glazed extension meets Building Regulations Part L — accepted by building control authorities across England and Wales.
Our Core Service
Highly Glazed Extension Energy Assessment
When an extension's glazed area exceeds 25% of its floor area plus the area of any lost openings, Building Regulations Part L requires a dedicated Section 10.10 energy assessment. The standard notional elemental compliance route is not permitted for these extensions — a whole-house SAP calculation must be carried out instead.
We carry out that assessment from start to finish. Using the Standard Assessment Procedure (SAP), we model the existing dwelling and the proposed extension together, demonstrating that the combined dwelling primary energy rate, dwelling emission rate, and fabric energy efficiency rate do not exceed those of the dwelling with a notional extension.
The result is a clear, professionally formatted compliance report that your building control body will accept as part of the completion process.
What's Included
Glazing to floor area check for proposed extension
Full SAP whole-house energy calculation
Fabric energy efficiency rate (FEES) calculation
Dwelling emission rate (DER) and primary energy rate (DPER) outputs
Professionally formatted compliance report for building control submission
Follow-up support if building control raises queries
Extension Types
Extension types we assess
Any extension where glazing exceeds 25% of floor area plus the area of lost openings requires a Section 10.10 assessment. This includes a wide range of common designs.
Orangeries
Brick or stone-built extensions with large glazed roof lanterns and full-height glazed walls — almost always exceed the calculated threshold.
Garden Rooms
Fully glazed or predominantly glazed garden room extensions, including aluminium-framed structures with glazed roofs.
Bifold & Sliding Door Extensions
Rear extensions featuring large bifold or sliding door systems that form a significant proportion of the wall area.
Glazed Kitchen Extensions
Open-plan kitchen extensions with roof lanterns, skylights, or full-width glazed rear elevations.
Conservatories Converted to Living Space
Where a conservatory is being brought into the thermal envelope of the dwelling and used as habitable space.
Bespoke Architectural Extensions
Any architect-designed extension where glass is a dominant feature of the design and glazing ratios are high.
The Assessment in Detail
What the assessment involves
Step 01
Review of your plans and specifications
We begin by reviewing your extension drawings, floor plans, and glazing specifications. We will confirm the glazing ratio, identify any lost openings, and gather the thermal performance data for the proposed glazing units, frames, and roof elements.
Step 02
SAP whole-house calculation
Using the Standard Assessment Procedure (SAP 10.2), we model the existing dwelling and the proposed extension as a combined unit. This produces the dwelling primary energy rate (DPER), dwelling emission rate (DER), and fabric energy efficiency rate (FEES) for the extended property.
Step 03
Comparison against the notional extension
The calculated rates are compared against those of the dwelling with a notional extension — a reference extension built to minimum Part L standards. Compliance is demonstrated where the actual rates do not exceed the notional rates.
Step 04
Production of the compliance report
We produce a clear, professionally formatted report setting out the assessment methodology, inputs, outputs, and compliance conclusion. The report is accompanied by a compliance certificate and is formatted for direct submission to your building control body.
Step 05
Building control support
If your building control body raises any queries about the report, we are available to respond directly. We understand what building control officers need to see and can address technical questions promptly.
Getting Started
What we need from you
To carry out the assessment, we typically need the following. If you are unsure about any of these, get in touch and we can advise.
Extension floor plans and elevations (any standard format)
Glazing schedule or window/door specifications
U-values for proposed glazing units and frames (from supplier if available)
Roof construction details and any roof glazing specifications
Details of the existing dwelling (age, construction type, heating system)
Any lost openings created by the extension
Turnaround & Fees
Fast, transparent service
We understand that build programmes are time-sensitive. Once we have received all the information we need, we aim to deliver your compliance report promptly — typically within a few working days for straightforward projects.
Our fees are fixed and agreed upfront before we begin work. There are no hidden charges. If additional work is required — for example, if glazing specifications change during the project — we will agree any additional fee with you before proceeding.
To get a quote or discuss your project, use the enquiry form below or visit our contact page.
Ready to get your assessment?
Send us your plans and we'll provide a fast, accurate Part L Section 10.10 compliance report — no jargon, no delays.