RdSAP (Reduced Data SAP) is the methodology currently used to produce EPCs for existing homes in the UK. It is being replaced by the Home Energy Model (HEM) with a reduced data methodology that uses a modular input framework instead of RdSAP's rigid defaults. The transition is now targeted to begin in the second half of 2027, with a legacy version of the existing rating shown on new EPCs until at least the end of 2029.
Why Is RdSAP Being Replaced?
RdSAP was designed as a practical compromise, a way to produce EPCs for existing homes without needing full construction drawings. But it has significant limitations that the Home Energy Model addresses:
- Monthly calculation: RdSAP averages everything over calendar months, missing how a home actually performs hour by hour. Heat pump efficiency, solar PV generation, and heating demand all vary dramatically within a single day
- Rigid age-based defaults: when an assessor cannot determine a construction detail, RdSAP applies a fixed default based on the building's age band. These defaults cannot account for individual improvements (e.g. a 1970s house with modern insulation still gets a 1970s default if the insulation type cannot be confirmed)
- Poor heat pump modelling: RdSAP uses fixed seasonal efficiency figures that do not capture how heat pump performance varies with outdoor temperature and flow temperature
- Simplified solar PV self-consumption: RdSAP applies a fixed assumed self-consumption fraction rather than modelling how much electricity is used on-site versus exported, because it lacks sub-monthly time resolution
- Fixed annual carbon factors: RdSAP 10 applies a single fixed electricity factor of 0.136 kgCO₂/kWh (already far below gas, and a big drop from SAP 2012's 0.519). HEM uses a lower, forward-looking 2025–2029 average of 0.086 and models energy half-hourly rather than as a monthly average
- Software inconsistency: different software providers can produce different results for the same property
What Changes with HEM?
| Feature | RdSAP (Current) | HEM Reduced Data (New) |
|---|---|---|
| Calculation | Monthly steady-state | Half-hourly dynamic simulation |
| Data collection | Fixed question set in rigid order | Modular: record what you observe |
| Missing data | Age-based lookup table defaults | Simplified inputs with updated default assumptions |
| Heat pump modelling | Fixed seasonal efficiency | Dynamic COP at each timestep |
| Solar PV | Annual generation only | Half-hourly generation + self-consumption |
| Battery storage | Self-use factor adjustment (RdSAP 10) | Charge/discharge modelled dynamically |
| Carbon factors | Fixed annual (0.136) | Forward-looking 2025–2029 (0.086) |
| Output | Single A–G Energy Efficiency Rating | Four metrics: Fabric, Heating, Smart, Cost |
| Software engine | Multiple third-party providers | Centralised ECaaS API |
The Modular Input Approach
The most significant practical change for assessors is the shift from RdSAP's rigid data model to HEM's modular input framework.
How RdSAP Works Now
RdSAP follows a fixed data collection process with predetermined questions in a set order. When information is unavailable (which is common for existing homes where original specifications are lost), the assessor selects from a limited set of age-based defaults. The same default applies regardless of any other evidence about the building's actual condition.
How HEM's Reduced Data Works
HEM's modular approach is fundamentally different. Assessors record what they can directly observe about each building element, and HEM's reduced data methodology supplies default values for anything that cannot be determined on site. Key differences:
- Observations drive the model: the more an assessor can observe and record, the more accurate the result
- Updated defaults: the government says default assumptions will use updated data, though their exact derivation has not yet been published
- Progressive detail: additional data from building surveys, thermal imaging, or retrofit plans can be added to improve accuracy without a full re-assessment
The government proposes that the initial release of HEM for existing dwellings will offer a single set of simplified inputs alongside the full inputs, and the assessor chooses between them for each part of the dwelling. Activating every simplification gives an experience equivalent to RdSAP today, which eases the changeover; recording more full inputs produces a more accurate result. The government has not yet settled on a name for this reduced-data form, with RdHEM (Reduced data HEM) and HEMEX (HEM for Existing Dwellings) among the options it consulted on. Over time, optional data sources such as smartphone LiDAR surveys, low-cost air-pressure tests, and smart-meter data may be added as admissible (but not mandatory) inputs.
Impact on Energy Assessors
The transition from RdSAP to HEM represents the most significant change to existing dwelling energy assessment since EPCs were introduced. Here is what assessors need to prepare for:
New Skills Required
- Understanding the four-metric framework and how each metric is derived from the HEM simulation
- Competence with the modular data collection approach, knowing which observations have the greatest impact on the final metrics
- Familiarity with the ECaaS platform and API-based workflow
- Understanding how HEM models technologies that RdSAP handles poorly, such as heat pumps, solar PV, battery storage, smart controls
- Ability to explain the new metrics to homeowners and advise on the most impactful improvements
Workflow Changes
- Data entry software will use the ECaaS API for calculations, replacing provider-specific engines
- Calculations run in under 2 seconds via the ECaaS platform, comparable to RdSAP, though the total assessment time is longer due to more detailed data collection
- Results include four separate metrics rather than a single rating, requiring more nuanced explanation to clients
- The modular input approach may change the order and structure of on-site data collection
For comprehensive preparation guidance, see our SAP Assessor Guide and Assessor Transition Guide.
Transition Timeline
| Phase | Period | What Happens |
|---|---|---|
| Consultation | January – March 2026 | HEM: EPC consultation open (closes 18 March 2026) |
| Preparation | 2026 | Government response, accreditation updates, assessor training |
| Launch | H2 2027 (revised target) | New HEM-based EPCs available alongside existing EER |
| Dual running | H2 2027 onwards | Both old (EER) and new (four-metric) EPCs in circulation |
| Legacy EER withdrawn | After the end of 2029 (earliest) | Legacy EER shown on new EPCs until at least the end of 2029, then eventually removed |
Frequently Asked Questions
When will RdSAP stop being used?
RdSAP will be replaced when HEM-based EPCs launch. The revised target is the second half of 2027; from launch, HEM replaces SAP and RdSAP. New certificates continue to show a legacy version of the EER, calculated by HEM, until at least the end of 2029. In the run-up to launch, assessors may need to be competent in both systems.
What is RdSAP 10, and is it the future?
RdSAP 10 is the current live methodology, used for existing-home EPCs since 15 June 2025. It is the last of its line: rather than updating RdSAP again, the government is moving to HEM. RdSAP 10 stays in use until HEM-based EPCs launch (targeted H2 2027), at which point HEM replaces it for domestic EPC assessments.
Will assessors need to collect more data on site?
Assessors will collect data differently rather than necessarily more of it. HEM's modular framework lets assessors record what they can observe, with default values supplied for anything not established on site. A thorough assessor who records more will produce a more accurate result. Additional data sources (building surveys, thermal imaging) can improve the assessment without a full re-survey.
Will existing EPC assessor accreditation still be valid?
Accreditation schemes will update their training requirements ahead of launch. Existing assessors will likely need additional training on the new methodology, the four-metric framework, and modular inputs. The exact requirements will be confirmed by accreditation bodies (Elmhurst, Stroma, ECMK, Quidos, etc.) as launch approaches.
Will assessor software change?
Yes. All calculations will run through the centralised ECaaS platform. Software providers will still provide the user interface and data collection tools, but the calculation itself moves to ECaaS. This eliminates inconsistencies between providers and means assessors can focus on data quality rather than software choice.