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 the existing system running alongside until at least October 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
- No solar PV self-consumption: RdSAP estimates annual generation but cannot model 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 | Calibrated defaults from building characteristics |
| Heat pump modelling | Fixed seasonal efficiency | Dynamic COP at each timestep |
| Solar PV | Annual generation only | Half-hourly generation + self-consumption |
| Battery storage | Not modelled | 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 applies calibrated defaults 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
- Calibrated defaults: defaults consider multiple building characteristics rather than just age band
- Progressive detail: additional data from building surveys, thermal imaging, or retrofit plans can be added to improve accuracy without a full re-assessment
- Conservative defaults: where no observation is possible, the simplified inputs tend to give a more cautious (less favourable) result than the full inputs, which rewards thorough data collection
In the initial release, 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 |
| Full transition | 1 October 2029 (target) | HEM compulsory for all EPCs; existing EER discontinued |
Frequently Asked Questions
When will RdSAP stop being used?
RdSAP will be phased out as HEM-based EPCs launch. The revised target is the second half of 2027 for new HEM-based EPCs, running alongside the existing system until at least October 2029. After that, all EPCs use HEM only. During the transition, 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) and runs alongside them until HEM becomes compulsory for all EPCs on 1 October 2029.
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 calibrated defaults for unknowns. 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.