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.

Automated Part L Compliance Reports in Seconds

Generate professional, Building Control-ready PDF reports automatically. What takes 2-3 hours manually now takes 30 seconds with SAPPER's AI-powered report generator.

Complete Compliance Documentation, Generated Automatically

Average U-Value Reports

•Project details and property information
•Element-by-element U-value breakdown
•Area-weighted calculations (shown step-by-step)
•Pass/fail status vs Part L 2013 & 2021 limits
•Compliance statement for Building Control

Full SAP Assessment Reports

•Extracted floor areas, perimeters, heights
•Window & rooflight analysis by orientation
•Thermal bridge heat loss calculations
•Condensation risk assessment (f-factors)
•AI-generated compliance narrative
•Professional formatting for Elmhurst import

From Data to Report in 3 Clicks

Enter Your Data

Use the calculator to input building elements manually, or upload drawings for AI extraction to auto-populate all measurements.

Review Results

See instant pass/fail results, thermal bridge analysis, and compliance status. Make adjustments if needed before generating the report.

Download PDF

Click "Generate Report" and download your professional PDF. Ready to submit to Building Control or share with clients immediately.

Professional Reports That Building Control Accepts

✓ What's Included

Professional Formatting: Clean, structured layout that meets Building Control expectations
Calculation Transparency: All calculations shown step-by-step for verification
Regulatory References: Cites Part L 2013/2021, SAP 10.2, and relevant guidance
Compliance Statement: Clear pass/fail with justification for submission
Thermal Bridging Details: Junction-by-junction breakdown with psi-values
Branded Output: Add your company logo and details (coming soon)

Time Savings Comparison

Manual Report Writing2-3 hours

Traditional SAP Software45-60 mins

SAPPER Automated30 seconds

Save 98% of report writing time

AI-Generated Compliance Narratives

SAPPER's AI analyzes your project data and writes professional compliance narratives automatically. No more struggling to explain technical details to Building Control.

Executive Summary: Project overview and compliance approach
Thermal Performance: Explanation of U-values and element choices
Thermal Bridging: Heat loss analysis and mitigation strategies
Compliance Statement: Clear declaration of regulatory adherence
Recommendations: Suggestions for optimization if needed

Example AI-Generated Text

"This 32m² single-storey rear extension has been designed to comply with Part L 2013 Building Regulations using the Average U-value method. The extension achieves an area-weighted average U-value of 0.27 W/m²K, which is comfortably below the regulatory limit of 0.30 W/m²K."

"Thermal bridging has been calculated using accredited psi-values from the BRE database for brick/block cavity wall construction. The Y-value of 0.08 W/m²K has been added to the average U-value calculation..."

→ Generated in 5 seconds from your project data

Multiple Export Formats for Your Workflow

PDF Reports

Professional PDF documents for Building Control submission and client presentations. Includes charts, tables, and branding.

✓ Available now

Elmhurst Format

Export data in Elmhurst-compatible format for import into official SAP software if full lodgement required.

✓ Available now

Word Documents

Editable .docx format for further customization or integration with your existing reporting templates.

Coming Q2 2026

Generate Your First Compliance Report Today

Stop wasting hours on manual report writing. Let SAPPER's AI do the heavy lifting while you focus on design and construction.

Explore More SAPPER Features

Average U-Values Calculator

Compliance Software Features

Pricing (100% Free)