Your streamlined solution for Part L Extension Compliance, ensuring your building projects meet regulatory standards with automated Average U-value calculations and clear compliance reports.

SAPPER is a free AI-powered SAP (Standard Assessment Procedure) compliance calculator for UK Building Regulations Part L. It automates data extraction from architectural PDF drawings, calculates thermal bridges, checks Part L compliance using Average U-value or Whole House methods, and generates professional compliance reports. Built by CoreLogic Ai Ltd.

What are the Part L 2021 U-value limits for extensions?

Part L 2021 (England, from June 2022) extension limiting U-values: External walls 0.18 W/m²K, Roofs 0.15 W/m²K, Floors 0.18 W/m²K, Windows 1.4 W/m²K, Doors 1.4 W/m²K, Rooflights 2.2 W/m²K. These are used as notional values in the Average U-value compliance calculation.

How does AI extract SAP data from drawings?

SAPPER uses a multi-pass AI extraction system: Pass 1 classifies drawing types (plans, elevations, sections) and detects scales. Pass 2 extracts dimensional data (floor areas, perimeters, storey heights) from annotations. Pass 3 analyses every elevation for windows, doors, rooflights with orientation detection. Results are validated with sanity checks and achieve 85-95% accuracy.

What are thermal bridge psi-values?

Psi-values (ψ-values) measure additional heat loss at building junctions in W/mK (Watts per metre Kelvin). Common UK junctions include E2 (lintels, accredited ψ=0.20), E3 (sills, ψ=0.04), E4 (jambs, ψ=0.04), E5 (ground floor, ψ=0.12), E10/E11 (eaves, ψ=0.04-0.06). RCD (Recognised Construction Details) values are 20-40% lower for brick/block masonry constructions.

What is the condensation risk f-factor?

The f-factor (fRsi) is the surface temperature factor used to assess condensation and mould growth risk at thermal bridges per BR 497. f ≥ 0.75 = low risk (acceptable), f 0.65-0.75 = medium risk (review needed), f < 0.65 = high risk (mould growth likely, design modification required). Calculated as fRsi = (Tsi - Te) / (Ti - Te).

Is SAPPER free to use?

Yes. SAPPER is completely free with no feature limitations, no credit card required, and no hidden costs. Part L calculator, AI extraction, thermal bridging, room in roof calculator, and PDF reports are all included at no charge.

SAPPER

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About SAPPER — AI SAP Compliance Platform

SAPPER (sapper.ai) is a free, AI-powered SAP (Standard Assessment Procedure) compliance platform for UK Building Regulations Part L. Developed by CoreLogic Ai Ltd, SAPPER automates three critical SAP assessment workflows:

Part L Extension Compliance — checks Average U-value method and Whole House method for dwelling extensions under Part L 2013 and Part L 2021.
AI Drawing Extraction — uploads architectural PDF drawings and uses computer vision to extract floor areas, perimeters, storey heights, window dimensions by orientation, and construction details with 85-95% accuracy.
Thermal Bridge Calculations — automatically identifies 12-15 junction types and calculates psi-values using BRE accredited values and RCD (Recognised Construction Details), with condensation risk assessment per BR 497.

Additional features: Room in Roof calculator (SAP 10.2 Table S6), AI-generated compliance narratives, professional branded PDF reports, BRE product database integration, and a self-improving AI system with correction feedback loops.

SAPPER is used by SAP assessors, architects, builders, developers, and homeowners across the United Kingdom. It is free to use with no feature limitations.

Key technical references: SAP 10.2 (2022), Approved Document L Volume 1 (2021), BR 497 (thermal bridging conventions), IP 1/06 (condensation risk), BRE RCD database.

Part L Assessment Wizard

This wizard has been replaced with the new improved Part L Calculator. Use the Calculator page to create your assessments.

SAPPER

AI-powered SAP compliance platform for UK Building Regulations. Part of the UKRAES ecosystem.

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Part L Calculator
Stair Master
SAP Extract
Room in Roof
Overglazing Checker

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For guidance only. Consult a SAP Assessor for official approval.