Planning a heavily glazed extension? If your glazing exceeds 25% of floor area you need a compliance calculation. We handle the numbers — Area Weighted U-Value or full SAP — and produce a Building Control-ready report.
Send us your drawings and we'll confirm which method applies and turn around a report within 2–3 working days.
Thanks — we'll review your project and come back within one working day.
Part L of the Building Regulations sets limits on the amount of glazing in an extension to control heat loss and solar gain. The default rule is straightforward: glazing (including windows, doors, and rooflights) should not exceed 25% of the extension's floor area, plus the area of any openings removed from the existing building to create the extension.
This limit exists because glass has a significantly higher thermal transmittance than an insulated wall or roof — even the best triple-glazed units lose more heat than a well-insulated opaque element. In summer, large glass areas can also cause significant solar overheating, which is now assessed separately under Approved Document O.
If your proposed extension exceeds this threshold — as is common with open-plan kitchen extensions, garden rooms, orangeries, and roof lanterns — you cannot use the simple Reference Method and instead need either an Area Weighted U-Value calculation or a full SAP assessment. Both require a qualified assessor and produce a Building Control-ready report.
Approved Document L 2021 Volume 1 provides three methods. Which one applies depends on how much glazing you have and what other measures the design includes.
The Area Weighted U-Value method calculates a single overall thermal performance figure for your extension by weighting each element's U-value by its area. This overall U-value for your actual design is then compared against the same calculation applied to a notional compliant reference extension.
If your extension's overall U-value is equal to or lower than the notional reference, compliance is achieved — regardless of how much glazing you have. The key is offsetting the higher U-value of additional glazing by specifying better-performing walls, roof, or floor elements.
Common strategies include upgrading from double to triple glazing (reducing window U-value from ~1.4 to ~0.8 W/m²K), improving roof insulation beyond the Part L minimum, or specifying higher-performance wall panels in a structural glazing system.
U = U-value of each element (W/m²K)
Each wall, glazed area, roof, and floor section is assessed separately.
A = area of each element (m²)
All exposed surfaces of the extension envelope are included.
The notional reference extension uses the same formula but substitutes the minimum Part L1B U-values and limits glazing to 25% of floor area.
Pass condition: Actual overall U-value ≤ Notional overall U-value.
Where the Area Weighted U-Value method cannot achieve compliance — typically when glazing is very high or where upgrading the extension fabric alone is insufficient — a full SAP energy model of the extended dwelling is required.
SAP models the entire house including the proposed extension, calculating the combined Dwelling Emission Rate (DER). This must not exceed the emissions that would result from the same house with a notional compliant extension built to Part L minimums. The comparison includes carbon emissions from all energy uses — space heating, water heating, ventilation, and lighting.
The SAP method is more demanding to carry out, but it can be more flexible: improvements to the existing dwelling's fabric or heating system can offset the impact of extensive glazing in the extension. A boiler upgrade, loft insulation improvement, or solar PV addition in the existing house can create sufficient headroom to allow a heavily glazed extension to comply.
We carry out SAP assessments using Elmhurst's accredited SAP software and can model the interaction between your proposed extension and the existing dwelling to find the most cost-effective compliance route.
Most overglazed extension calculations are completed within 2–3 working days of receiving complete drawings and specifications. Engaging us early — before drawings are finalised — gives the most design flexibility.
The more complete the information provided at the outset, the faster we can turn around your report. We're happy to work from early-stage drawings and refine as the design develops.
"Build Envelope's calculation saved us weeks of back-and-forth with Building Control for our overglazed extension. They turned it around quickly, explained exactly what was needed, and handled all the follow-up questions. Highly recommended."
Extensions with more than 25% glazing need specialist compliance calculations. Our assessors understand local building control expectations and deliver reports that get approved first time.
Overglazed extension calculations for homeowners and architects across Birmingham, Solihull, Coventry, Wolverhampton, and the wider West Midlands. Area Weighted U-Value and SAP pathway assessments for Part L compliance.
Extension compliance calculations across Bristol, Bath, South Gloucestershire, and the surrounding South West. We advise on the most cost-effective route to compliance for your specific glazing ratio.
Based in Worcester — extension calculations across Malvern, Droitwich, Evesham, Bromsgrove, and Redditch. Our home county with the fastest turnaround and deepest local building control relationships.
Part L extension assessments across Cheltenham, Gloucester, Stroud, Tewkesbury, and the Cotswolds. We work with homeowners and architects to ensure glazing-heavy designs remain compliant.
Overglazed extension calculations across Oxford, Banbury, Bicester, Witney, and the wider Oxfordshire area. Both AWUV and full SAP pathways available depending on your project.
As Elmhurst-accredited assessors, our extension calculations are valid across the whole of England and Wales. Contact us wherever your project is located.
Looking for overglazed extension calculations near you? Whether you're an architect designing a glazed rear extension in Cheltenham, a homeowner with a conservatory conversion in Birmingham, or a builder working on a bifold-heavy kitchen extension in Bristol — we deliver compliant results with clear guidance on the most cost-effective route. Call 01386 365145 or email us to get started.
Traditional conservatories with a translucent or transparent roof that are thermally separated from the main house by walls and doors — and not heated — are generally exempt from Building Regulations entirely, and therefore do not require a Part L calculation.
However, if the conservatory is thermally connected to the main house (no separating wall and door, or under-floor heating connecting the two), or if it has an opaque roof, it is treated as an extension and Part L applies. If in doubt, check with your local Building Control body before proceeding.
The minimum requirement under Part L1B is a window energy rating of Band C or better, which typically corresponds to a whole-window U-value of around 1.4–1.6 W/m²K. However, for overglazed extensions, specifying performance above this minimum is usually essential to achieve compliance via the Area Weighted method.
Triple glazing with a U-value of 0.8–1.0 W/m²K is often the most practical way to offset excess glazing area without requiring major changes to wall or roof construction. We advise on the optimal specification once we've reviewed your drawings.
Yes. All glazed elements in the extension envelope count toward the total glazing area, including vertical windows, external doors (the glazed portion), rooflights, roof lanterns, and glazed roof panels. The 25% threshold applies to the combined area of all these elements relative to the extension floor area.
Roof glazing is often the element that pushes a design over the threshold — a relatively modest roof lantern on a 20m² extension can use up a significant proportion of the glazing allowance. We calculate the total as part of our initial review.
The Area Weighted U-Value calculation is relatively straightforward and can be carried out by a knowledgeable designer without accredited software. However, most Building Control bodies will want to see a clear, well-structured report demonstrating the methodology and results.
The full SAP method requires accredited SAP software and a qualified assessor. Our reports come with the backing of our Elmhurst OCDEA accreditation, which means they are accepted by Building Control without challenge — useful if there is any uncertainty about the calculation inputs or methodology.
No — these are two separate assessments. The overglazed extension calculation addresses energy performance under Approved Document L (heat loss and carbon emissions). Overheating is assessed separately under Approved Document O, which was introduced in 2021 for new dwellings.
Approved Document O applies to new build dwellings, not extensions — so if you are extending an existing house, an overheating assessment under Part O is not currently mandatory. However, if your extension has a large south-facing glazed area, it is worth considering shading and ventilation to avoid uncomfortable summer temperatures. We can advise on this as part of our review if needed.
For the Area Weighted U-Value method, we typically return a report within 2 working days of receiving complete drawings and specification. For the full SAP method, allow 3–4 working days, as we need to model the full dwelling including the existing structure.
If your drawings are not yet finalised, we're happy to carry out an initial review and advise on the likely compliance route and any specification changes needed — so that when the drawings are complete, the formal calculation can be turned around quickly.
Send us your drawings and glazing specification and we'll confirm the compliance route, provide a quote, and turn around a Building Control-ready report within 2–3 working days.
Send us your drawings and project details and we'll come back with a quote and confirm which compliance method applies — usually within one working day.
We'll review your details and come back within one working day.