If you produce EPCs on existing homes, the ground shifted slightly under you this month. The RdSAP 10 Conventions v12.2 — the MHCLG-approved rulebook that tells domestic energy assessors how to interpret what they find on site — came into effect on 6 August 2026, superseding v12.1 (which had run since August 2025). It’s not a new methodology, but the detail matters: the revision history lists 16 amended conventions, four brand-new ones, and the deletion of Appendix 4. Most of the changes are about squeezing out inconsistency between assessors, replacing “use your judgement” with a rule. Here are the ones worth reading twice.
Bungalows and self-contained annexes
The definition of a bungalow has been tightened: a property should only be recorded as a bungalow where all habitable accommodation is at ground-floor level. Chalet bungalows and anything with a room in the roof are now treated as houses.
There’s also clearer guidance on when an annexe counts as a self-contained dwelling needing its own EPC. It must have fixed cooking and bathing facilities — the cooking provision expected to include a fixed oven, a sink and either two fixed hot rings (for a one-bedroom dwelling) or four (for anything larger) — plus its own access, either from outside or via a communal corridor. An internal connecting door to the main house does not, on its own, stop the unit being self-contained.
Rooms in roof: Convention 2.06 rewritten
This is one of the biggest single changes, aimed at loft conversions and rooms in the roof:
- A new 25% rule for connected gable walls. Where only part of a gable wall is connected to another part of the dwelling, it’s treated as exposed if less than 25% is connected, and connected if more than 25% is.
- A new 20% dormer rule. Small dormers taking up less than 20% of the room-in-roof floor area can be ignored when measuring the roof. Larger dormers must be measured separately — vertical faces as stud walls, roofs as flat ceilings.
- Hidden insulation. Where there’s clear evidence that insulation runs from the rafters through to the eaves, an unobservable stud wall can now be assumed to share the same insulation level, rather than defaulting to uninsulated.
- Complex geometry can be merged, and in certain constructions you can now take U-values straight from Table 18 of the RdSAP 10 Specification rather than calculating each element.
That last point is tied to the removal of Appendix 4, which previously covered room-in-roof U-values — those values are now inferred from Table 18 where the stated conditions are met.
Conservatories with solid roofs
Convention 2.28 now spells out the definition: for RdSAP purposes a conservatory is a building part where 50% or more of the external heat-loss walls are glazed and 75% or more of the roof is glazed. Crucially, where a conservatory’s roof has been replaced with a solid roof (or a false ceiling added) so it no longer meets that test, it must be recorded as an extension, with clear rules for how to enter the roof depending on pitch and the evidence available. This finally standardises one of the most argued-over judgement calls in the job.
Heating, hot water and ventilation
Several amendments pin down how systems are recorded:
- Heat-as-a-Service gets its own new convention: where a provider owns or operates the heat generator, it’s treated as community heating regardless of where the generator sits — entered as a community heat pump (electric) or community boiler (otherwise), with documentary evidence required.
- Electric combi boilers (Convention 4.22) are now entered manually as a standard boiler, direct acting, fuel type electricity.
- Incomplete or condemned systems (Convention 4.03) now explicitly covers hot water as well as heating — a fixed but non-working hot water source still counts when deciding the primary heating and hot water type.
- Two mechanical ventilation systems get a new Convention 7.03e setting a clear order of precedence (MVHR first, then MV, MEV, PIV from outside, PIV from loft) where the software will only take one entry.
Chimneys, evidence and PV batteries
A few smaller but practical clarifications round it out. Decommissioned chimneys with air bricks (Convention 9.01) must now be considered: a ventilation opening under 30,000 mm² is treated as a blocked chimney (20 m³/hr), larger openings as open (80 m³/hr). On documentary evidence (9.02), where you can see insulation and have evidence of intent but can’t measure the thickness, you now record “insulated — insulation thickness unknown” rather than overstating or ignoring it. PV batteries over 5 kW (9.05) are split into a 5 kW entry plus a second battery for the excess. And an air-tightness certificate becomes invalid for RdSAP entry if material changes have been made to the property since the test (10.01).
What to do about it
None of this rewrites the RdSAP engine, but plenty of it changes the inputs — and inconsistent inputs are exactly what drives variation between assessors on the same house. The practical move is to read the full v12.2 document rather than rely on a summary, pay particular attention to the room-in-roof and conservatory rules if you survey a lot of older stock, and check that your software and accreditation scheme guidance reflect the new edition before you lodge.
If you’d like the modelled, new-build side of the picture handled properly — SAP 10.3, Part L compliance and PSI-value thermal-bridging work — that’s our patch. Get in touch and we’ll tell you how we’d approach your scheme.
Sources: RdSAP Conventions v12.2 (MHCLG-approved), BRE Group · Conventions (v12.1) for RdSAP 10, BRE Group · RdSAP Conventions v12.2: Key Changes for Energy Assessors, Energy Trust