SAP 10 was announced back in 2018 as an eventual overhaul of the ageing SAP 2012 methodology, with SAP 10.1 and 10.2 anticipated alongside the Part L 2021 uplift. That’s all settled history now: SAP 10.1 and 10.2 duly arrived with Part L 2021, and the current version is SAP 10.3, which also carries the notional specification for the Future Homes Standard. If you’ve found this post looking for what SAP 10 changed, here’s what actually changed along the way, and what’s live today.
August 2026 update: SAP 10.3 is now the current, in-force version – this post has been corrected from its original 2021 framing, where SAP 10 was still described as upcoming.
In this blog you’ll learn:
- What changed between SAP 2012 and SAP 10, and why
- Where SAP 10.3 sits today, and how it relates to Part L 2021 and the Future Homes Standard
- Where SAP itself is eventually heading
For the foundational question this post assumes is already answered – what a SAP calculation is, and whether your project needs one:
Why SAP 10 Happened at All
SAP 2012 had barely changed since its release in 2013, despite the UK’s construction methods, technology, and – critically – the carbon intensity of the electricity grid changing substantially over the same period. SAP 10 was the government’s response: a modernisation of the underlying methodology so that EPCs and compliance figures actually reflect how a dwelling performs, rather than assumptions that had quietly gone stale.
What Changed Between SAP 2012 and SAP 10
1. Changes to fuel factors/prices and carbon dioxide factors
Electricity’s carbon factor was revised to reflect a grid drawing far more heavily on renewables than it did under SAP 2012’s assumptions – meaning all-electric buildings are no longer penalised against a mains gas benchmark that no longer reflected reality.
2. Heating Patterns
Weekend and weekday heating patterns, previously assumed to differ, were aligned to a single 07:00–09:00 and 16:00–23:00 pattern across all days, reducing the default heating assumed at weekends. Heat network distribution loss factors were also increased, having previously understated real losses.
3. The Calculation of Lighting Energy
The calculation moved from a simple percentage of low-energy fittings to a reference lighting capacity based on floor area and potential solar gains – closer in approach to the non-domestic SBEM methodology, and requiring more detailed input data than before.
4. Thermal bridging
The default y-value was raised from 0.15 to 0.2 W/m²K, and accredited generic psi values were withdrawn – assessors now need to use established construction details or model junctions directly, with modelling the preferred route. Default psi values for existing junction types also increased, and a new junction type was added for roof and room-in-roof constructions not previously covered.
5. Thermal Mass Parameter
The indicative low/medium/high TMP bandings were removed. Every dwelling now needs its TMP calculated directly from element areas and kappa values, following BS EN ISO 13786.
6. Overheating
The summer overheating assessment began accounting for whether windows could realistically be left open during the day or night, factoring in noise or security constraints that might rule this out – directly informing which ventilation strategy the design could rely on.
7. Solar PV and storage
SAP 10 introduced battery storage and PV diverter options into the calculation, alongside improved overshading inputs using MCS assessment data where available. It also removed the ability for landlords of blocks of flats to claim CO2 savings from shared-inverter PV supplying multiple units – only dwellings with their own inverter can claim the benefit from that point on.
8. Hot water demand
The calculation began accounting for the actual number of showers and baths in a dwelling, including electricity used by instantaneous electric showers – a change that penalises designs with more sanitaryware and instantaneous showers specifically, compared with the flatter SAP 2012 assumption.
Where SAP Sits Today
SAP 10.3 is the current version, confirmed on the government’s official SAP guidance, and it’s the version carrying the notional building specification for the Future Homes Standard. It’s been updated with FHS-specific fabric and carbon factors on top of the SAP 10/10.1/10.2 changes above.
SAP itself is expected to eventually be replaced entirely by the Home Energy Model (HEM) – a half-hourly methodology rather than SAP’s monthly calculation. That transition isn’t tied to when the Future Homes Standard itself comes into force; see our Home Energy Model post for exactly where that stands and why the two timelines are separate. For what Part L 2021 – the regulation SAP 10.3 currently demonstrates compliance against – actually requires, see our Part L Building Regulations post.
What’s Next?
SAP’s evolution from 2012 to 10.3 has been a genuine overhaul, not a minor tweak – and it’s not finished, with HEM still to come. Whatever version applies to your project right now, getting the assessment right at design stage remains the same first step.
We carry out SAP calculations against the current SAP 10.3 methodology – if you’d like your project assessed, our team can talk you through what’s changed and what it means for your design.
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