— reviewed against the regs as they stand.
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Calculate the Ψ-value for the junction you have actually drawn
Start from one of 38 ready-made SAP junctions, change the layers to match your detail, and watch Ψ and the fᵣₛᵢ temperature factor update as you type — modelled to BS EN ISO 10211, validated against the ISO test reference case.
Free to model on screen, no sign-up. Building Control-ready PDF £25 +VAT a junction, or unlimited on Monkey Pro.
What SAP actually is
SAP (Standard Assessment Procedure) is the government’s methodology for rating the energy performance of dwellings — the engine behind Energy Performance Certificates, Building Regulations Part L compliance, and most planning conditions. A trained assessor builds a model of your dwelling — fabric, heating, hot water, ventilation, lighting, renewables — and SAP turns it into the numbers that decide whether you pass: the DER/DPER (dwelling emission rate) against the TER/TPER target (plain-English guide). SAP 10 is the current methodology; this page is about the changes it brought in 2021–22.
Who needs a SAP assessment
You’ll hit SAP in four places: Building Regulations compliance for new dwellings (Part L 1A) and material works to existing ones (Part L 1B); the EPC you must hand over on completion or sale; planning conditions on many developments (especially via energy statements); and increasingly mortgage/valuation and net-zero policies on new homes. Our SAP calculations service covers all of them — and the New Build Compliance Pack bundles SAP with the water, acoustics and air-tightness evidence most schemes need anyway.
The headline
Part L 2021 went live on 15 December 2021 and has applied to all developments registered after 15 June 2022. It targets a 31% improvement in CO₂ emissions over Part L 2013, achieved through better fabric plus renewables and low-carbon energy in the target calculation. The assessment tool is SAP 10, successor to SAP 2012.
A much tougher notional dwelling
The notional dwelling that sets your targets (TER/TPER) is far tighter in SAP 10:
| Element | Notional u-value (TER) | Maximum u-value allowed (DER) |
|---|---|---|
| Walls | 0.18 | 0.26 |
| Floor | 0.13 | 0.18 |
| Roof | 0.11 | 0.16 |
| Doors | 1.0 | 1.6 |
| Windows | 1.2 | 1.6 |
| Fully-glazed doors | 1.2 | 1.6 |
| Roof windows | 1.2 | 1.4 |
Plus: air test target of 5.0, waste water heat recovery to all showers, an 89.5%-efficient gas boiler, and 80 lm/W lighting. To pass, your DER and DPER must beat the targets — exceed any notional value and you’ll need to offset it elsewhere.
The big one: PV in the notional dwelling
SAP 10’s notional dwelling assumes PV is installed — for houses, kW of PV = 40% of the ground-floor area ÷ 6.5 (for flats, 40% of the flat’s floor area ÷ 6.5 ÷ the number of storeys in the block). That’s roughly 4kW for an average dwelling. In our experience fabric improvements alone won’t get most dwellings through SAP 10: the practical routes to a pass are solar PV or a heat pump. It’s a deliberate policy choice — the target assumes you’re generating — and it’s the change that surprises most people new to SAP 10.
Other changes worth knowing
Electricity’s carbon factor fell from 0.519 kgCO₂/kWh to 0.136 — below mains gas (0.210) for the first time, reflecting the greener grid. SAP 10.0 (2018) had cut it to 0.233; SAP 10.1 and SAP 10.2 took it to 0.136, applied month by month rather than as one flat figure, and SAP 10.3 — approved on 24 March 2026 as the Future Homes Standard methodology, while Part L 2021 work stays in 10.2 — uses a forward-looking 0.086 against 0.214 for gas, with electricity’s primary energy factor rising to 1.969 (what changed in SAP 10.3). All-electric designs, brutal to pass under SAP 2012, now have far more flexibility. It’s why heat pumps, MVHR and efficient electric hot water have become so common in new-build specs.
Thermal bridging options changed. SAP 10 removed Accredited Construction Details over accuracy concerns. That leaves manufacturer schemes — or bespoke calculated Ψ-values for each junction, which is exactly what our ΨMonkey psi calculator produces. The defaults those replace are tabulated in full. More on thermal bridging.
Heating pattern — now a consistent daily pattern instead of different weekday/weekend assumptions, reducing modelled energy use. Heat network loss factors increased, which matters for London planning carbon targets.
Lighting — proper lighting design now feeds the calculation (like SBEM), recognising high-efficacy LED products.
Hot water — shower flow rates are now entered, creating crossover with Part G water calculations.
PV configuration matters — flats only benefit from PV they’re directly connected to; landlord-supply-only arrangements no longer count for every flat. Battery storage and PV diverters can now be included, and MCS overshading data can replace defaults.
Fabric & services beyond the table — fabric energy efficiency, the energy hierarchy and fuel choice in SAP all interact with the target: better fabric shrinks the PV/heat-pump requirement; the right fuel choice changes the whole calculation.
What this means for you
Design for SAP 10 from day one: realistic fabric, PV or heat pump from the start, and calculated junction Ψ-values instead of defaults. Our SAP team can model your options.
Where it’s going: the Future Homes Standard
SAP 10 is the stepping stone to the Future Homes Standard — and that’s no longer a distant “someday.” Approved Document L 2026 was published on 24 March 2026 and comes into force on 24 March 2027, effectively ending new-build gas boilers and making on-site renewables a requirement, with SAP 10.3 the compliance methodology at launch (the Home Energy Model follows later). If you’re designing now, the SAP 10 decisions — fabric, PV, heat pumps, low-carbon heating — are exactly the decisions the FHS will require. The energy statements and PHPP worlds are converging on the same place.
SAP 10 vs SAP 2012 at a glance
| Change | SAP 2012 | SAP 10 |
|---|---|---|
| Electricity carbon factor | 0.519 kgCO₂/kWh | 0.233 in SAP 10.0 → 0.136 in SAP 10.2 (Part L 2021) → 0.086 kgCO₂/kWh in SAP 10.3 (Future Homes Standard) |
| PV in the notional dwelling | No | Yes (~4 kW typical) |
| Accredited Construction Details | Accepted | Removed — manufacturer schemes or calculated Ψ-values |
| Heating pattern | Weekday/weekend profiles | Consistent daily pattern |
| Shower flow rates | Not modelled | Entered into the calculation |
| Lighting | Default assumptions | Lighting design feeds in (like SBEM) |
| WWHR | Optional | Assumed on all showers in the notional dwelling |
| Air-tightness target | 5.0 m³/h/m² | 5.0 m³/h/m² |
Quick facts
- Carbon factors: electricity 0.136, gas 0.210 kgCO₂e/kWh in SAP 10.2 (Part L 2021 work); 0.086 and 0.214 in SAP 10.3 (the Future Homes Standard methodology, approved 24 March 2026), whose primary energy factors are 1.969 for electricity and 1.12 for gas. Do not mix these with the annual UK Government conversion factors — those are for reporting, these are for compliance, and a submission built on the wrong set will not reconcile.
- Notional PV for a house: 40% of lowest storey area ÷ 6.5.
- Air-tightness target in the notional dwelling: 5.0 m³/h/m².
- WWHR assumed on all showers in the notional dwelling.
- Calculated junction Ψ-values (BS EN ISO 10211) beat SAP defaults — often the cheapest route to a pass.
Further reading: BRE SAP 10 and Approved Document L.