What is The Difference Between an SAP & EPC?
An EPC (Energy Performance Certificate) is the final statutory certificate, whereas SAP (Standard Assessment Procedure) is the government desktop calculation methodology used to produce it. While an EPC rates the energy efficiency of a building on a scale from A (most efficient) to G (least efficient), you cannot obtain one for a new build or conversion without completing a full SAP calculation first. Under Approved Document L Volume 1 (2021 edition incorporating 2023 amendments), the SAP 10.2 desktop model evaluates fabric heat loss, ventilation, and heating efficiency to produce your compliant as-built SAP EPC for final Building Control sign-off.
Understanding the core difference requires separating SAP from RdSAP (Reduced data SAP). Higher energy efficiency delivers lower fuel bills and reduced carbon emissions, but the assessment route depends entirely on whether a building is existing or newly constructed. Existing homes marketed for sale or let rely on RdSAP, an on-site physical survey where an assessor records visible fabric and heating arrangements. Conversely, a new build SAP EPC is a rigorous desktop assessment derived from architectural drawings, structural sections, and product data sheets long before practical completion.
Comparison: SAP vs RdSAP vs New Build EPC
The assessment methodology, technical inputs, and statutory lodgement routes vary depending on whether you are constructing a new dwelling or marketing an existing property:
| Feature | SAP Calculation (New Builds & Conversions) | RdSAP Assessment (Existing Dwellings) | New Build EPC (Final Certificate) |
|---|---|---|---|
| Primary Purpose | Demonstrates Building Regulations Part L compliance at design and as-built stages. | Measures energy efficiency of existing homes for sale or rent. | Statutory proof of energy rating (A–G) and Environmental Impact score required for Building Control completion sign-off and legal handover. |
| Input Data Source | Architectural plans, construction specifications, U-value calculations, HVAC data sheets, and verified airtightness test results. | Non-invasive visual inspection on site, relying on age-related default conventions where insulation is unverified. | Derived directly from the final verified as-built SAP calculation model. |
| Survey Requirement | Purely desktop calculation. Site inspections are replaced by contractor compliance evidence, signed BREL report, and photographic audits. | Mandatory on-site visual survey by a qualified Domestic Energy Assessor (DEA). | Desktop lodgement following site commissioning and air testing. |
| Statutory Target | Must beat target metrics: DER ≤ TER, DPER ≤ TPER, and DFEE ≤ TFEE under Approved Document L 2021. | Informational rating only; Minimum Energy Efficiency Standards (MEES) require Band E or above for domestic lets. | Reflects calculated SAP scores; provides an Energy Efficiency Rating (1–100) and Environmental Impact score. |
| Governing / Lodgement Body | Modelled by an accredited On-Construction Domestic Energy Assessor (OCDEA) via Elmhurst Energy or equivalent scheme. | Conducted by an accredited Domestic Energy Assessor (DEA). | Lodged onto the central government register; reviewed by the Building Control Body for completion sign-off. |
EPCs have been mandatory across England and Wales from 2007, establishing a legal obligation whenever a building is sold, let or built. Once obtained, an EPC remains valid for 10 years. On construction sites, unexpected material substitutions - such as swapping thermal blockwork or altering insulation specs - will derail your compliance metrics. If design figures do not match site reality, Building Control will withhold sign-off and freeze your final SAP EPC. Backed by 15+ years of compliance consultancy (est. 2008) and over 12,000 clients, ATSPACE stops design discrepancies from turning into handover delays by reviewing design-stage and as-built models within 5 to 10 working days.
Approved Document L Targets and The Notional Recipe
For new builds, the assessment methodology is far from simple. An energy assessor models the entire thermal envelope based on the construction build-up - including plasterboard linings, floor slab U-values, and airtightness membranes - alongside MEP systems. The calculation measures primary energy, target fabric energy efficiency (TFEE), space heating, hot water storage, cooling, ventilation, and renewable PV generation. Locking in these details early ensures your as-built SAP EPC aligns with site testing, avoiding eleventh-hour mastic panics around unsealed soil stacks or loft hatches. Under Section 1 of Approved Document L Volume 1, every new home in England must satisfy three statutory metrics concurrently:
- Dwelling Emission Rate (DER): The calculated carbon emissions (kgCO2/(m2.year)) must not exceed the Target Emission Rate (TER).
- Dwelling Primary Energy Rate (DPER): The calculated primary energy use (kWh_PE/(m2.year)) must not exceed the Target Primary Energy Rate (TPER).
- Dwelling Fabric Energy Efficiency (DFEE): The space heating and cooling demand (kWh/(m2.year)) must not exceed the Target Fabric Energy Efficiency (TFEE).
To pass these metrics, design teams cannot merely build to Table 4.1 limiting backstop values (external walls at 0.26 W/m2K, roofs at 0.16 W/m2K, floors at 0.18 W/m2K, and windows at 1.6 W/m2K). The calculation compares the design against Table 1.1 Notional Dwelling targets, requiring far tighter specifications such as external walls at 0.18 W/m2K, roofs at 0.11 W/m2K, ground floors at 0.13 W/m2K, windows at 1.2 W/m2K, fully filled party walls at 0.00 W/m2K, and an air permeability design target of 5.0 m3/(h.m2) at 50 Pa. Under the 2021 regulations, air testing is mandatory on every newly built dwelling; sample testing is no longer permitted.
The BREL Report and Photographic Evidence Audit
Under Appendix B of Approved Document L 2021, the transition from design SAP to final EPC requires an audited Building Regulations England Part L (BREL) report. The lead contractor and assessor must sign the as-built BREL confirming all building components match the approved model. In addition, the contractor must capture geo-located, timestamped photographs before concealment. These cover foundation edge insulation, external wall cavity closers, roof insulation continuity at eaves and loft hatches, window and door positioning relative to cavity closures, airtightness seals around pipe penetrations, and plant data plates on heating systems. Without these verified photographs and matching commissioning certificates, the energy assessor cannot sign off the as-built SAP, and Building Control will withhold the final completion certificate.
When construction concludes, the assessor incorporates the air pressure test result, heating commissioning certificates, and photo evidence into the model. Once all information is received, the assessor lodges the final calculation onto the central government register to officially create an EPC document. To avoid commercial disruption, ATSPACE offers same-day quotes (8am–4pm Mon–Fri) and dedicated account managers with design calculations reviewed within 5 to 10 working days, and compliance certification issued following site testing. Call our technical compliance team directly on 0345 6465 454 or email [email protected].
