Quick answer: SAP 10.3 is the approved methodology for the Future Homes Standard — schemes notified from 24 March 2027. Anything notified before then, and started before 24 March 2028, stays under Part L 2021 and may continue in SAP 10.2. The change that matters is the grid electricity factor pair: carbon 0.136 → 0.086 kgCO₂e/kWh and primary energy 1.501 → 1.969. Gas barely moves (0.210 → 0.214; 1.130 → 1.12). Electric heating gets a stronger carbon case and a weaker primary-energy one; a heat pump still wins both by a distance, a panel heater now loses the primary-energy test badly.
Which version applies to which job
The government’s Notice of Approval of 24 March 2026 approves “the Government’s Standard Assessment Procedure (SAP) for the Energy Rating of Dwellings, 10.3 edition” for new dwellings in England, superseding the June 2025 notice that approved 10.2. It takes effect on 24 March 2026 — but it carries the same transitional clause as the Future Homes Standard itself:
This approval does not apply in relation to building work on a particular building, where a building notice or an initial notice has been given to, or full plans have been deposited with, a local authority, in respect of that building, before 24 March 2027, provided that the building work on that building is started before 24 March 2028. In such cases, the earlier versions of the methodologies may continue to be used.
So the practical rule is the one you already use for the edition of Part L: a building notified before 24 March 2027 and commenced before 24 March 2028 is a Part L 2021 building and is assessed in SAP 10.2; a building under Approved Document L 2026 is assessed in SAP 10.3 — or, once it is approved, the Home Energy Model. For higher-risk buildings the cut-off is a valid Gateway 2 application before 24 September 2027. Protection is per building, not per site: the plot that starts late on a phased scheme moves to the new edition and the new SAP on its own. The edition checker works this out from three dates.
The fuel factors, from Table 12
These are read from the SAP 10.3 specification (version dated 12 June 2026), Table 12, set against the SAP 10.2 values from BRE’s technical paper on the factors. Both editions quote carbon as CO₂ equivalent — methane and nitrous oxide included.
| Fuel factor | SAP 10.2 — Part L 2021 work | SAP 10.3 — Future Homes Standard |
|---|---|---|
| Grid electricity, CO₂e | 0.136 kgCO₂e/kWh (annual average of monthly factors) | 0.086 kgCO₂e/kWh |
| Grid electricity, primary energy | 1.501 | 1.969 |
| Mains gas, CO₂e | 0.210 kgCO₂e/kWh | 0.214 kgCO₂e/kWh |
| Mains gas, primary energy | 1.130 | 1.12 |
Two details from the small print. The 10.3 electricity figure is a forward-looking average — the grid the dwelling will run on, not last year’s — and it is the same for every tariff. And the monthly variation SAP 10.2 introduced has gone: the footnotes to Table 12 say that in SAP 10.3 “the average figure given in this table is used directly in the SAP worksheet”, with the monthly tables kept “for comparison purposes only”. One number, applied flat.
What the arithmetic does
Divide the factors by the efficiency of the heating and you get the cost of a kilowatt-hour of heat under each version — which is what the carbon and primary energy targets are built from.
| Per kWh of heat delivered | SAP 10.2 carbon | SAP 10.3 carbon | SAP 10.2 primary energy | SAP 10.3 primary energy |
|---|---|---|---|---|
| Gas boiler at 89.5% (the 2021 notional) | 0.235 | 0.239 | 1.26 | 1.25 |
| Heat pump at SCOP 2.5 (the FHS notional) | 0.054 | 0.034 | 0.60 | 0.79 |
| Heat pump at SCOP 3.0 | 0.045 | 0.029 | 0.50 | 0.66 |
| Direct electric (panel heaters, electric boiler) | 0.136 | 0.086 | 1.50 | 1.97 |
kgCO₂e per kWh of heat, and kWh of primary energy per kWh of heat.
On carbon, everything electric gets better: a heat pump at SCOP 3 delivers heat at an eighth of a gas boiler’s emissions, and even a panel heater comes in at a third. That is the DER against TER test, and it moves in electricity’s favour.
On primary energy — the DPER against TPER test, the headline test since 2021 — it goes the other way. Every kilowatt-hour of electricity now costs 1.969 kWh of primary energy instead of 1.501, a rise of 31%. A heat pump still beats the boiler comfortably (0.66 against 1.25 at SCOP 3), but the margin is narrower than it was, and direct electric heating now costs more primary energy than gas (1.97 against 1.25) where under 10.2 it was already the wrong side of the line. A design that leaned on resistance heating and a lot of PV to scrape a 10.2 pass should not assume the same trick works in 10.3. The same 31% applies to every other kilowatt-hour of electricity in the calculation — lighting, fans, pumps, MVHR — which is why the notional dwelling’s fixed lighting jumps from 80 to 120 lumens per circuit-watt.
PV pulls less weight on carbon, because each kilowatt-hour it generates displaces 0.086 kg rather than 0.136 kg. That is not an accident: under the Future Homes Standard PV is a legal requirement in its own right (requirement L3), and the notional dwelling’s array grows from 40% of the ground-floor area ÷ 6.5 to ÷ 4.5 — about 45% more capacity for the same roof.
The notional dwelling in SAP 10.3
Appendix R of the specification defines the notional dwelling the Future Homes Standard targets come from — a single specification, with the Home Energy Model version differing only in how it sizes the notional heat pump. The fabric is the Part L 2021 notional’s: walls 0.18, floors 0.13, roofs 0.11, windows 1.2, doors 1.0, rooflights 1.7 W/m²K. What changes is everything around it: air permeability 4 m³/(h·m²) with decentralised mechanical extract, a heat pump at a SCOP of 2.5 in place of the gas boiler, waste water heat recovery on every shower except in single-storey dwellings, lighting at 120 lm/W, and the larger PV array above. The full set, dated and sourced, is in the building regulations reference.
Table R2 of the same appendix lists the junction Ψ-values the notional dwelling assumes — 0.05 for lintels, sills and jambs, 0.16 for a ground floor, 0.06 for eaves — and Appendix K’s Table K1 lists the defaults a junction gets when nothing is calculated: 1.00 for a lintel, 0.32 for a ground floor. With a tighter air permeability and a heat pump in the notional, there is less slack elsewhere in the calculation, and the gap between the default and a calculated value is often the pass. The reference page’s Ψ table now shows default, notional and calculated side by side for the common junctions.
What to do with this
If a scheme is already notified, check the commencement date against 24 March 2028 and carry on in SAP 10.2; nothing about SAP 10.3 reaches back into it. If a scheme will be notified from 24 March 2027, run the design-stage SAP in 10.3 from the start rather than converting a 10.2 model later — the notional, the factors and the PV rule all change together, and a design that passed in 10.2 is not evidence that it passes in 10.3. Either way, the levers are the ones they have always been: fabric, airtightness, calculated junction Ψ-values instead of defaults, and a heat pump that is sized and controlled to reach its SCOP.
Sources: SAP 10.3 specification, version 12-06-2026 (BRE) — Table 12, Appendix K Table K1, Appendix R Tables R1 and R2 · Notice of Approval of the methodologies of calculation of the energy performance of new buildings in England, 24 March 2026 (MHCLG) · SAP 10 Technical Paper S10TP-15: CO₂ and primary energy factors (BRE) · Future Homes and Buildings Standards: Building Circular 01/2026 (MHCLG)