Enter the areas and U-values for your proposed extension. See the proposed and notional comparison instantly, then choose whether to purchase a professionally presented calculation statement.
✓ Instant on-screen calculation✓ Proposed vs notional✓ Optional £30 PDF
01
Project & regulatory edition
Give the extension a reference and select the applicable guidance.
02
Proposed extension fabric
Enter the area and proposed U-value for each applicable element. Walls, roof and floor are required; leave unused opening types blank. Use net opaque areas and separate opening areas.
The proposed U-values below start at the applicable Approved Document L Table 4.2 figures. They are editable defaults, not verified values for your products. Replace them with the actual proposed specification before purchasing a report.
03
Notional fabric comparison
The comparison is calculated automatically from the entered extension geometry.
The reference uses the 2021 Approved Document L Table 4.2 values, 25% of extension floor area plus qualifying removed openings, and conservation of gross wall and roof areas. Removed openings affect only the notional allowance, not the proposed fabric heat loss.
🔒 Fixed reference U-valuesRead-only • W/(m²·K)
Walls 0.18Roofs 0.15Floors 0.18Windows / roof windows 1.40Doors 1.40Rooflights 2.20
These reference figures are locked for this calculator edition. You can change only your proposed construction U-values above. Special cases and energy-rating alternatives require professional review.
The notional opening allocation uses a proportional design-stage convention modelled on the supplied example. That convention and supporting evidence require independent technical review. This tool supports the 2021 edition incorporating 2023 amendments; ordinary commencement of the published 2026 guidance is 24 March 2027, subject to transitional provisions. Multiple roof constructions with roof openings are blocked pending explicit roof-plane assignment.
Optional detailed PDF statement
Download your calculation statement — £30
The PDF includes proposed and notional element schedules, the U-value and heat-loss calculations, numerical checks and methodological limitations. A separate purchase record is available after successful payment.
The sample is watermarked and uses illustrative figures; your paid document will contain your own entered project data.
Seller: SAP Company Network Limited (13181161), 15a Duke Street, Dartmouth, Devon TQ6 9PY. Telephone 07498 203953. One digital PDF for £30 total, inclusive of any applicable taxes; no subscription or additional delivery fee. Review the information below before purchasing.
Payment is collected securely by Stripe. Download links are valid for 30 days. Detailed calculation inputs and reports are scheduled for deletion after 30 days; backups and legally required transaction records may have different retention periods. A separate purchase confirmation and consent record is provided. Please review the terms and privacy information below.
Payment successful — your calculation is ready
Your links are available for 30 days after purchase. Save both files now. The purchase record includes your digital-delivery consent.
This self-service tool produces an automated Area-Weighted U-Value Calculation Statement from customer-entered extension fabric data. SAP Company does not verify the dimensions or product evidence, assess the wider project, certify compliance or guarantee acceptance by a Building Control body. This is not a whole-dwelling SAP assessment or a certificate.
Confirm project dimensions, declared product U-values and the notional comparison method with an appropriately competent professional before relying on the PDF. The notional opening allocation is a modelling convention requiring project-specific review. This edition applies England Approved Document L Volume 1, 2021 edition incorporating 2023 amendments; applicability and transitional provisions remain the customer's responsibility.
The £30 payment buys one digital calculation statement with a separate optional purchase record. The customer expressly requests immediate digital delivery and acknowledges the effect on the 14-day cancellation right before checkout. Statutory rights for faulty digital content remain unaffected. Nothing excludes or limits liability where doing so would be unlawful, including fraud or death or personal injury caused by negligence.
SAP Company Network Limited (registered in England and Wales, company number 13181161) handles report delivery and payment records; Stripe separately processes payment details. Calculation inputs and PDFs are scheduled for deletion after 30 days; the necessary transaction and consent records may be retained for lawful accounting and dispute purposes. See our Terms & Conditions and Privacy Policy, or contact SAP Company. These statements and policies should be checked together by a qualified adviser before public launch.
SAP Company Network Limited • Technical calculation aid • No independent certification • Return to SAP Company website
