The Home Energy Model (HEM) is replacing both SAP and RdSAP as the methodology for domestic energy assessment in England. For domestic energy assessors this is the most significant change to working practice since SAP was introduced in 1993, but it does not arrive as a single event. New-build assessors (OCDEAs, who use SAP) and existing-dwelling assessors (DEAs, who use RdSAP) face two different transitions, on two different timelines, through two different routes. Non-domestic SBEM assessment is separate and is not replaced by HEM.
Two Assessor Roles, Two Transitions
HEM replaces SAP and RdSAP, but the two jobs move to it separately. Which transition applies to you depends on the work you do:
| New-build (OCDEA) | Existing dwellings (DEA) | |
|---|---|---|
| Current method | SAP (full) | RdSAP (reduced data) |
| What HEM is for | Part L / Future Homes Standard compliance | Domestic EPCs (sales, lettings, MEES) |
| HEM form | Full HEM via ECaaS | Reduced-data HEM (provisionally RdHEM / HEMEX) |
| Driven by | Future Homes Standard | EPC reform |
| Timeline | SAP 10.3 sole route now; HEM delayed 8 Jun 2026, then 24-month dual running | New EPCs from H2 2027; HEM compulsory for all EPCs 1 Oct 2029 |
| Your deep-dive | Assessor Transition Guide | RdSAP to HEM Transition |
Both paths share the same HEM engine and the same ECaaS platform, but the data you collect and the dates you work to differ. New-build work uses the full input set (detailed below); existing-dwelling work uses a simplified, modular input set designed to keep surveys practical.
The rest of this page covers the new-build (SAP) path in detail, where HEM lands first and changes the workflow most. If you assess existing dwellings, the RdSAP to HEM transition guide is your path; for a step-by-step new-build preparation guide, see our Assessor Transition Guide.
What's Changing for New-Build (SAP) Assessors
HEM replaces SAP as the primary methodology for demonstrating compliance with Part L of the Building Regulations for new dwellings. The changes affect every aspect of how assessments are carried out:
- Calculation methodology: HEM uses half-hourly dynamic simulation based on BS EN ISO 52016-1, replacing SAP's monthly steady-state approach. This is not simply a SAP update; it is a fundamentally different calculation engine.
- Data requirements: HEM needs far more detailed input data than SAP. You will need specific product make/model information for all heating, ventilation, and renewable energy equipment, plus detailed building geometry and individual hot water outlet specifications.
- Calculation platform: ECaaS (Energy Calculation as a Service) replaces the current model of multiple third-party SAP software engines. Every assessment will use the same centralised calculation engine, accessed through software provider interfaces.
- Default values: Where data is missing, HEM applies punitive defaults that are significantly more severe than SAP's assumptions. Incomplete assessments will produce substantially worse compliance outcomes.
- Calculation time: HEM calculations run in under 2 seconds via ECaaS, comparable to SAP. However, the significantly more detailed data entry requirements mean overall assessment time is longer.
Assessment Time and Workflow
The shift from SAP to HEM represents a step change in assessment complexity. The table below compares the typical workflow for a standard house type:
| Aspect | SAP Assessment | HEM Assessment |
|---|---|---|
| Data collection time | ~20 minutes (from drawings) | ~1 hour 40 minutes (detailed specs required) |
| Calculation run time | Near-instantaneous | Under 2 seconds (via ECaaS) |
| Iterative adjustments | Quick: change input, instant result | Fast calculation, but more data to update per change |
| Product data needed | Generic categories (e.g. 'gas boiler') | Specific make/model from PCDB |
| Geometry detail | Floor area + basic dimensions | Detailed geometry with precise measurements |
| Hot water outlets | Not individually specified | Each tap, shower, and bath individually specified |
| Missing data impact | Moderate (reasonable defaults) | Severe (punitive defaults) |
The longer assessment time is a direct consequence of HEM's greater accuracy. The half-hourly dynamic simulation requires more granular input data to produce its more detailed results. This means assessors need to collect information earlier in the design process and work more closely with design teams.
ECaaS: The New Calculation Platform
ECaaS (Energy Calculation as a Service) is the cloud-based API run by MHCLG that delivers the official HEM implementation. It fundamentally changes how energy calculations are delivered to the industry:
- Single calculation engine: Every assessment uses the identical, government-maintained HEM engine, eliminating the inconsistencies between different SAP software providers
- API-based delivery: Software providers build their user interfaces on top of the ECaaS API. Your assessment software will change, but the underlying calculation is standardised
- Automatic updates: When the HEM engine is updated, all software providers receive the update simultaneously through the API
- Rust implementation: The ECaaS engine is built in Rust for performance, based on MHCLG's open-source implementation
Key Dates: New-Build (SAP) Path
The transition from SAP to HEM is tied to the Future Homes Standard implementation. During the transitional period, both SAP 10.3 and HEM can be used to demonstrate compliance:
| Milestone | Expected Date | What It Means for Assessors |
|---|---|---|
| Approved Documents published | 24 March 2026 | Final regulations and transitional arrangements confirmed |
| FHS comes into force (SAP 10.3 only) | 24 March 2027 | New compliance requirements begin; SAP 10.3 sole methodology |
| HEM approved; dual running begins | Delayed 8 Jun 2026 to the coming months | Both SAP 10.3 and HEM accepted for FHS compliance |
| Transitional period ends | 24 March 2028 | All new homes must comply with FHS |
The Future Homes Standard dates are confirmed; the HEM approval date slipped on 8 June 2026 and is now expected in the coming months. See our Timeline & Status page for the latest position.
Frequently Asked Questions
I assess existing dwellings (RdSAP) — does this apply to me?
Partly. HEM replaces both SAP (new-build, OCDEAs) and RdSAP (existing dwellings, DEAs), but on different timelines. This page focuses on the new-build SAP path. If you do existing-dwelling work, your route is a reduced-data form of HEM under EPC reform: new EPCs from H2 2027, compulsory 1 October 2029. See the RdSAP to HEM transition guide.
How long does a HEM assessment take compared to SAP?
Testing shows a standard house type takes approximately 1 hour 40 minutes to assess in HEM, compared to around 20 minutes in SAP (excluding geometry and U-value calculations). This increase is driven by the substantially greater data requirements. HEM needs detailed specifications for every heating, ventilation, and hot water component.
Will SAP assessors need new qualifications for HEM?
The government estimates SAP assessors will need approximately 44 hours of training to familiarise themselves with HEM and the new FHS requirements. Assessors will access ECaaS through their existing accreditation schemes, which will issue secure credentials. The significantly increased data requirements mean training must cover the new methodology, data collection processes, and the ECaaS platform. Certification bodies are expected to update their programmes ahead of the FHS coming into force in March 2027.
What is ECaaS and how does it affect assessors?
ECaaS (Energy Calculation as a Service) is the government's cloud-based calculator that provides the official HEM implementation. It replaces the current model where multiple software providers build their own SAP engines. All assessors will use the identical calculation engine, accessed via an API. Software providers will build user interfaces on top of it.
Can I still use SAP software during the transition?
Yes. During the dual running period, compliance can be demonstrated using either HEM (via ECaaS) or SAP 10.3. SAP 10.3 has a guaranteed minimum lifetime of 24 months from when the statutory instrument is laid, with six months' notice before withdrawal. Your software must be updated to the SAP 10.3 specification. After the dual running period ends, HEM via ECaaS will be the only compliance route.
What happens if data is missing from a HEM assessment?
Missing data triggers punitive default values that are far more severe than in SAP. Where SAP might assume a reasonable mid-range value, HEM applies worst-case defaults designed to discourage incomplete assessments. This means incomplete data produces significantly worse compliance results. Project teams should set up a data repository from the start to ensure all required information is collected.
Related Pages
SAP to HEM Transition Guide
New data requirements, ECaaS workflow, training, and a preparation checklist.
RdSAP to HEM Transition
The existing-dwelling (DEA) path: reduced-data HEM, the EPC-reform timeline, and what changes on site.
Compliance Pathways
HEM vs SAP 10.3 routes, notional building approach, and how to demonstrate compliance.
For Architects
How HEM changes design decisions for form factor, orientation, and glazing.
EPCs & HEM
How Energy Performance Certificates are changing under the Home Energy Model.