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.

Best Part L Compliance Software for UK Builders & Architects

SAPPER automates Part L extension compliance with AI-powered SAP assessments, thermal bridge calculations, and instant compliance reports. Save 3.5 hours per assessment.

Why SAPPER is the Best Compliance Tool for Builders

AI-Powered Automation

Upload architectural drawings and our AI extracts all measurements automatically. No manual takeoff required. 90%+ accuracy guaranteed.

Save 3.5 Hours Per Assessment

What takes 4 hours manually takes 30 minutes with SAPPER. Automated thermal bridge calculations, instant U-value checks, and one-click report generation.

100% Part L Compliant

Built by SAP assessors for SAP assessors. Follows Part L 2013 & 2021 regulations exactly. Thermal bridging with accredited psi-values included.

SAPPER vs Traditional Compliance Software

✓ With SAPPER

AI Drawing Extraction: Upload PDFs, get measurements in minutes
Automated Thermal Bridges: RCD-optimized psi-values calculated instantly
Free to Use: No monthly fees, no per-assessment charges
Professional Reports: Building Control-ready PDFs in seconds
Room in Roof Calculator: Handle complex loft conversions correctly

✗ Traditional Software

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Manual takeoff from drawings (1-2 hours per project)
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Manual thermal bridge calculations (30-60 minutes)
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Expensive monthly subscriptions (£50-200/month)
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Complex interfaces requiring extensive training
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Limited support for edge cases like room in roof

Perfect For Every Part L Compliance Need

🏗️ Builders & Developers

Check compliance before submitting to Building Control. Avoid costly redesigns. Get instant pass/fail results for extensions, conversions, and new builds.

📐 Architects & Designers

Design with confidence. Real-time U-value calculations show if your design meets Part L. Upload drawings to verify before client presentations.

✅ SAP Assessors

Speed up your workflow with automated data extraction from PDFs. Export results to Elmhurst. Professional reports for your clients in minutes.

🏠 Homeowners

Planning an extension? Check if your design complies before spending on professional assessments. Free guidance on meeting Part L requirements.

Start Using the Best Part L Compliance Software Today

Join hundreds of builders and architects who trust SAPPER for their compliance needs. Free to use, no credit card required.