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.

What is SAP and why do I need it?

SAP (Standard Assessment Procedure) is the UK government's method for assessing the energy performance of buildings. It's required for Building Regulations compliance when constructing or significantly altering a dwelling. SAPPER helps you calculate Part L compliance for extensions.

What is Part L of the Building Regulations?

Part L covers Conservation of Fuel and Power. Part L1A applies to new dwellings, while Part L1B covers extensions and alterations to existing dwellings. Our calculator focuses on Part L1B compliance using the Average U-value method and Whole House comparison.

What is the Average U-value method?

The Average U-value method calculates the weighted average thermal transmittance of your extension's fabric (walls, roof, floor, windows). For Part L 2013, the average must not exceed 1.30 W/m²K. For Part L 2021, it must not exceed 1.20 W/m²K. SAPPER automates this calculation for you.

When do I need to use the Whole House method?

The Whole House method is required when the extension's floor area exceeds 50m² or has more than 25% glazing (conservatories, glass extensions). It compares the entire dwelling's CO2 emissions before and after the extension, ensuring they don't increase by more than 5%.

What drawings do I need for SAP extraction?

For accurate AI extraction, provide architectural drawings including: ground floor plan, first floor plan (if applicable), roof plan, all four elevations (front, rear, left, right), and building sections. Drawings should show dimensions, window sizes, and construction details. Scale 1:50 or 1:100 works best.

How accurate is the AI extraction?

Our multi-pass AI extraction system typically achieves 85-95% confidence. It classifies drawings, extracts dimensions, analyzes windows from all elevations, and performs sanity checks. Always verify extracted measurements against your drawings before submitting to Building Control.

What are thermal bridges and why do they matter?

Thermal bridges are areas where heat escapes more easily (e.g., around windows, floor junctions, corners). They're measured using psi-values (ψ) in W/mK. SAP requires thermal bridging to be calculated and included in heat loss calculations. SAPPER automatically calculates all junction types using accredited psi-values.

What are RCD psi-values?

RCD (Recognised Construction Details) psi-values are BRE-certified thermal bridging values for specific construction types (e.g., brick/block cavity walls). They're typically lower (better) than default accredited psi-values, improving your Y-value by 20-30%. SAPPER supports RCD optimization for compatible constructions.

What is condensation risk and why is it shown?

Condensation risk is assessed using the f-factor (surface temperature factor). BR 497 requires f > 0.75 to prevent mould growth and surface condensation. SAPPER calculates f-factors for all thermal bridges and flags high-risk junctions (f < 0.65) that may need design improvements.

Can I edit the extracted measurements?

Yes. After AI extraction, review all measurements on the SAP Dashboard. You can edit wall areas, floor areas, perimeters, heights, U-values, and add/edit windows. Always cross-check with your architectural drawings before generating the final report.

What U-values should I use?

U-values come from manufacturer certificates or Building Regulations notional values. For Part L 2013: walls 0.18, roof 0.13, floor 0.13, windows 1.40. For Part L 2021: walls 0.18, roof 0.11, floor 0.11, windows 1.20. Use certified values from your construction specifications when available.

What is a Room in Roof and how is it calculated?

A Room in Roof is habitable space within the roof structure (loft conversion, dormer). SAP calculates an equivalent storey height based on ceiling areas and pitch. SAPPER includes a dedicated Room in Roof calculator that handles mixed ceilings, vaulted ceilings, and gable walls using SAP 10.2 methodology.

How do I use the generated report?

SAPPER generates a comprehensive PDF report including project details, extracted measurements, thermal bridge calculations, and AI-written narrative sections. Submit this report to Building Control alongside your architectural drawings and construction specifications. The report is for guidance - final approval requires a registered SAP Assessor.

Is SAPPER suitable for official submissions?

SAPPER is a guidance tool to help you understand Part L compliance early in the design process. For official Building Regulations submissions, you must engage a qualified SAP Assessor who will use accredited software (Elmhurst, STROMA, etc.) and certify the assessment. Use SAPPER to optimize your design before formal assessment.

What file formats are supported for upload?

SAPPER supports PDF architectural drawings. For best AI extraction results, use vector PDFs (not scanned images). File size limit is 10MB per file. You can upload multiple drawings (plans, elevations, sections) in a single extraction session.

How are window orientations determined?

The AI analyzes elevation drawings to determine window orientations (North, South, East, West, etc.). Accurate orientation is critical for solar gain calculations in SAP. Always verify the AI has correctly identified which elevation faces which direction - check against your site plan.

What is a Y-value and how is it calculated?

The Y-value (thermal bridging coefficient) represents total additional heat loss from all thermal bridges per unit of exposed surface area (W/m²K). Calculated as: y = Σ(ψ × L) / Σ(A), where ψ = psi-value, L = junction length, A = total exposed area. Default y-value is 0.15 W/m²K. Using accredited psi-values typically gives 0.05-0.08. Using RCD values can achieve 0.04-0.06. A lower Y-value significantly improves SAP ratings.

What is the difference between Part L 2013 and Part L 2021?

Part L 2021 (effective June 2022 in England) introduced stricter standards than Part L 2013. Key differences: new build CO₂ target reduced by 31%, notional wall U-value tightened from 0.26 to 0.18 W/m²K, roof from 0.16 to 0.11, floor from 0.18 to 0.13, windows from 1.4 to 1.2. For extensions under L1B, the limiting U-values changed: roof from 0.16 to 0.15, floor from 0.22 to 0.18. Part L 2021 also requires SAP 10.2 instead of SAP 2012.

What is air permeability and why does it matter for SAP?

Air permeability measures the rate of uncontrolled air leakage through a building's fabric at 50Pa pressure difference, in m³/h·m². Part L 2021 sets a limiting value of 8 m³/h·m² for new builds, with a notional value of 5 m³/h·m². Lower permeability means less heat loss through air leakage. SAP calculations use the tested or design air permeability to determine ventilation heat loss, which significantly affects the DER.

How does SAPPER calculate thermal bridge heat loss?

SAPPER automatically identifies all non-repeating thermal bridge junctions (lintels, sills, jambs, ground floor, eaves, corners, party walls) and calculates their contribution using the formula: H_TB = Σ(ψ × L). It uses BRE accredited psi-values by default, with option to switch to RCD (Recognised Construction Details) values for compatible brick/block constructions. The total thermal bridge heat loss feeds into the Y-value calculation.

What SAP version does SAPPER use?

SAPPER supports both SAP 2012 (for Part L 2013 compliance) and SAP 10.2 (for Part L 2021 compliance). SAP 10.2 introduced updated carbon emission factors, new heating system efficiencies, revised weather data, and changed the primary energy metric. The Room in Roof calculator uses SAP 10.2 Table S6 methodology for equivalent storey height calculations.

What is the Average U-value formula?

The Average U-value (Uaw) is calculated as: Uaw = Σ(Ui × Ai) / Σ(Ai), where Ui is the U-value of each element and Ai is its area. The proposed Uaw must not exceed the notional Uaw calculated using Part L limiting U-values. For example, a 20m² extension with walls (28.8m² × 0.18), roof (20m² × 0.15), floor (20m² × 0.18), and windows (8.4m² × 1.4) gives Uaw = 0.305 W/m²K.

Can SAPPER handle conservatory extensions?

Yes. Conservatories are exempt from Part L if thermally separated from the dwelling, have no fixed heating, and floor area is under 30m². If these conditions are not met, the conservatory must comply via the Whole House method. SAPPER's calculator handles both scenarios and will guide you to the correct compliance route based on your project details.

Frequently Asked Questions

Everything you need to know about SAP assessments, Part L compliance, and using SAPPER

Quick Jump to Topic

General Regulations Calculations SAP Extract Thermal Bridging Advanced Reports

General

Regulations

Calculations

SAP Extract

Thermal Bridging

Advanced

Reports

Looking for detailed technical reference?

Visit our comprehensive Knowledge Base for U-value tables, psi-value reference, condensation risk thresholds, and complete SAP methodology

Open Knowledge Base →

Still have questions?

Visit the UKRAES system for more building regulations tools and resources

Visit UKRAES →