SAP vs EPC Explained: What's the Difference?
SAP and EPC are closely related but serve different purposes. This guide explains when you need each and how they work together.
Quick Summary
SAP (Standard Assessment Procedure)
The calculation methodology used to assess energy performance of dwellings.
- • Required for new builds
- • Done from plans/specifications
- • Produces SAP rating (1-100+)
- • Used for Part L compliance
EPC (Energy Performance Certificate)
The certificate that communicates energy performance to owners, buyers and tenants.
- • Required for sales/lettings
- • Based on SAP or RdSAP
- • Shows A-G rating
- • Valid for 10 years
SAP Calculation Explained
SAP (Standard Assessment Procedure) is the UK Government's methodology for calculating the energy performance of dwellings. It's been the national calculation method since 1995 and is regularly updated (current version is SAP 10).
SAP calculates energy use based on the building's physical characteristics:
- Building fabric (walls, floors, roof, windows)
- Heating and hot water systems
- Ventilation strategy
- Lighting
- Renewable energy systems
The calculation uses standardised assumptions about occupancy, climate and usage patterns, making it possible to compare different dwellings fairly.
When Do You Need a SAP Calculation?
SAP calculations are required for:
- New build dwellings - to demonstrate Part L Building Regulations compliance
- Conversions to dwellings - e.g., barn conversions, office-to-residential
- Extensions - in some cases where Building Control requires it
- Material change of use - creating new dwellings from non-residential
RdSAP: The Simplified Version
RdSAP (Reduced Data SAP) is a simplified version of SAP used for existing properties. When you get an EPC for an existing home, the assessor uses RdSAP methodology.
The key differences are:
- RdSAP uses survey data and default assumptions where information isn't available
- Full SAP uses exact specifications from plans and construction details
The Relationship Between SAP and EPC
How They Connect:
- 1. New Build: SAP calculation → Generates EPC (from SAP data)
- 2. Existing Property: RdSAP survey → Generates EPC (from RdSAP data)
The EPC is essentially the "output" that presents the SAP/RdSAP results in a standardised, user-friendly format.
Cost Comparison
| Service | Typical Cost | When Needed |
|---|---|---|
| EPC (existing home) | £60-£120 | Selling or letting |
| SAP Calculation (new build) | £100-£300 | Design stage + completion |
| Commercial EPC | £180+ | Selling or letting commercial |