What is SAP 10? And How it Affects Your Building Project

SAP 10 is the current methodology used to assess the energy performance of new homes, conversions and certain extensions. Building Control uses it to decide whether a dwelling meets Part L.

Compared with SAP 2012, the new version is stricter, more detailed and far less forgiving of unclear specifications. Even small assumptions can now shift a project from a pass to a fail.

This guide explains what SAP 10 changed, why it matters and what you need to consider before finalising your Design SAP Calculation.

Why SAP 10 Replaced SAP 2012

SAP 2012 was outdated. Energy prices, heating technologies and carbon factors had moved on. SAP 10 updates the methodology so it reflects how homes are built and powered today.

The aim was simple: make calculations more accurate and push real improvements in energy efficiency before the Future Homes Standard arrives.

Key Changes in SAP 10

Updated Carbon Factors

Electricity is modelled as much cleaner than before, which means heat pumps and PV systems perform far better in SAP 10 than they did in SAP 2012.

A Stronger Notional Building

The benchmark house used for comparison now assumes:

• better U-values
• lower airtightness
• improved glazing
• stronger thermal bridging performance

This tightens compliance margins across the board.

More Weight on Fabric and Airtightness

Small differences in insulation or an air test result slipping from 5 to 7 have a bigger effect. SAP 10 responds quickly to weaknesses.

Better Modelling of Renewables and Ventilation

Heat pumps, MVHR and PV are treated more realistically, which shifts the balance of what’s considered “good enough” for compliance.

How SAP 10 Affects Your Specification

Fabric Needs to Match the Assumptions

Walls, roofs and floors must hit the U-values assumed at design stage. If insulation changes on site, the SAP result usually changes with it.

Thermal Bridging Matters More Than Before

Default Y-values are now so harsh that many designs cannot pass with them. Accredited junctions often make the biggest difference to compliance.

Airtightness Should Be a Planned Target

Assuming 5 is fine if the build is detailed around achieving it. If it comes back at 7 or 8, SAP 10 increases DER sharply.

Heating Choices Have Bigger Consequences

Switching late from a heat pump to a gas boiler, or fitting a boiler without the correct controls, can overturn a SAP Calculation pass unless fabric and PV compensate.

Glazing Balance Influences Both Heat Loss and Gains

Orientation matters. North and east glazing provide little useful gain under SAP 10, so large openings on these elevations may need improved performance elsewhere.

Why SAP 10 Makes Designs Feel “Tighter”

Under SAP 2012, many homes passed with headroom. SAP 10 removes much of that buffer.
The notional building is stronger, the modelling is more accurate and discrepancies add up faster.

It doesn’t mean your design is flawed, it means details now matter more.

Working Smoothly With SAP 10

• Lock in the heating system early
• Provide accredited thermal bridging details
• Use realistic airtightness targets
• Send datasheets before installation
• Review glazing layout early

Most SAP issues can be prevented with small adjustments made at the start rather than the end of the build.

Conclusion

SAP 10 sets a higher standard for energy efficiency and models homes more accurately than SAP 2012. Once you understand how it treats fabric, airtightness, thermal bridging and heating systems, compliance becomes much easier to manage.

If you want an early check on whether your design is on track, I can review your plans and highlight the areas that need improvement before construction begins.