RdSAP 10 — the updated version of the Reduced Data Standard Assessment Procedure used to produce Energy Performance Certificates and retrofit assessments — came into effect on 15 June 2025. The update represents the most significant revision to the methodology in several years, and has direct implications for assessors, retrofit coordinators and the organisations that rely on assessment data.

Why RdSAP Was Updated

The previous version of RdSAP was increasingly struggling to accurately represent the energy performance of modern homes and the range of technologies being installed under funded retrofit programmes. Heat pumps, battery storage, smart controls and advanced insulation systems were not well captured by the older methodology, leading to assessments that did not reflect real-world performance.

The update also responds to criticism that EPCs have historically been poor guides to actual energy costs and carbon emissions — partly because the underlying data collected during assessments was insufficiently detailed. RdSAP 10 substantially increases the data collected at each property visit, with the aim of producing more accurate and more useful outputs.

Key Changes Assessors Need to Know

Windows

Window assessment has changed significantly. Rather than a general description of glazing type across the property, assessors must now record data for every individual window — including total area, location and orientation, frame type, glazing specification and age. This is one of the most time-consuming changes for assessors, but produces considerably more accurate heat loss calculations.

Walls

The updated methodology allows assessors to define two alternative wall types per property, where previously properties with mixed construction types had to be managed through workarounds. This simplifies assessment for older or extended properties where wall construction varies.

Roof and Room-in-Roof

Two new room-in-roof construction types have been introduced — True Room in Roof and With Common Wall Room in Roof — each with expanded measurement requirements and a broader range of insulation specification options. Assessors working with converted lofts and dormer extensions will need to familiarise themselves with the new data capture requirements.

Lighting

Lighting data capture has been expanded to require a full count of all bulbs by type — LED, CFL and incandescent — rather than a general estimation. This increases the accuracy of the energy demand calculation for lighting, which can be a significant component of a property's total energy use.

Ventilation

Ventilation recording has been substantially expanded. Where previously only open fireplaces were required to be recorded, assessors must now count and classify all ventilation openings across the property — including flues, extract fans, blocked chimneys, passive stack vents and trickle vents in windows. This is particularly relevant for retrofit projects, where ventilation strategy is a core PAS2035 requirement and poor ventilation data has historically been a source of non-conformities.

Implications for Retrofit Coordinators

For Retrofit Coordinators working with PAS2035, the RdSAP 10 changes are broadly positive. The additional ventilation data captured during assessment is directly relevant to the Improvement Option Evaluation process, and the more detailed wall, window and roof data produces a more reliable foundation for measures modelling.

Assessment visits will take longer, and assessors should factor this into their scheduling and fee structures. The improved data quality will reduce the likelihood of significant discrepancies between the assessment findings and the eventual installation — a common source of delays and non-conformities in funded programmes.

All assessment bodies — Elmhurst, ECMK, Quidos and Property Tectonics — have updated their software to accommodate RdSAP 10. Any assessment conducted on or after 15 June 2025 should have been captured under the new methodology.

What This Means in Practice

Housing associations and local authorities commissioning retrofit assessments should be aware that assessment costs and timescales may have increased as a result of RdSAP 10. This is a consequence of more thorough data collection rather than assessor inefficiency. The improved accuracy of the resulting EPCs and assessment reports is worth the additional investment.

Retrofit Coordinators should verify that any assessments submitted for their projects post-June 2025 were conducted under RdSAP 10, and check that the ventilation data captured is sufficient to support the IOE and ventilation assessment requirements of PAS2035.