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.

Modern UK house extension under construction

🇬🇧UK Part L1B Extension Compliance · £45 per assessment

SAP calculations for extensions.Part L 1B compliance in 15 minutes.

Calculate Average U-values for dwelling extensions and verify Part L regulations compliance. Professional-grade Part L assessment reports without the technical complexity.

See your verdict free — pay only for the report

15 minutes start to finish

PDF report for Building Control

SAP Calculations for Extensions

Calculate SAP and verify Part L compliance.All for £45.

Complete SAP assessment for dwelling extensions. One price, one project, clear compliance verdict backed by a professional report.

SAP U-value calculator

Select materials (Good · Better · Best) and our SAP tool computes W/m²K automatically. No spreadsheets needed.

Complete element coverage

Walls, roof, floor, windows, doors, rooflights — all building elements for SAP and Part L calculations.

Part L compliance reports

Professional PDF reports with SAP calculations, element schedules, Average U-value analysis, and Part L verdict.

Built for all experience levels

Homeowner-friendly guidance or Professional mode for architects and SAP assessors.

Stair Master · Maître Escalier

DXF to Full-Size PDF for Large Format Printing

Convert CAD drawings into print-ready 1:1 scale PDFs — or let us print and post them to you.

Convertissez vos plans DXF en PDF taille réelle prêts à imprimer — ou laissez-nous imprimer et vous les envoyer.

2D Nesting

Components packed automatically to minimise paper waste and calculate exact feed length.

1:1 Scale PDF

Custom page size with cut lines, corner marks, and a 50mm calibration square for verification.

Send for Printing

No large format printer? We print and post your plans for you — from €8/metre, postage included in France.

No CAD software required · Bilingual EN / FR · Works with HP DesignJet, Epson SureColor & Canon imagePROGRAF

One-time payment · No subscription · No sign-up to start

Your extension. Compliant.For just £45.

Try it free up to the results step. Only pay when you're ready to see whether you pass and download your report.