Does RdSAP measure my actual energy bills?

No. RdSAP uses standardised assumptions about occupancy and usage so properties can be compared fairly. Your real bills depend on how you actually live in and heat the home.

How often does the RdSAP methodology change?

The methodology and conventions are updated periodically as standards evolve. Always confirm the current version and rules on GOV.UK, as changes can affect how a property is assessed.

Can AI-EPC run an RdSAP assessment for me?

No. AI-EPC is AI-assisted and helps you understand RdSAP inputs and model how evidence might change your result. Any official EPC must be assessed and lodged by an accredited Domestic Energy Assessor; the AI supports interpretation and planning and never replaces the accredited assessor.

RdSAP

What is RdSAP? The EPC assessment method explained

Name: AI-EPC
Address: GB
Price range: ££

Updated June 2026

Quick answer

RdSAP stands for Reduced data Standard Assessment Procedure — the standardised method used to produce domestic EPC ratings for existing homes. An accredited assessor records data on walls, roof, heating, glazing and controls, then the software calculates a SAP score and band. Verified evidence replaces default assumptions and can improve the result.

This guide is general information, not legal advice. EPC assessment conventions and the RdSAP methodology are periodically updated. Always confirm the current rules on GOV.UK and have an accredited Domestic Energy Assessor assess your property — only an accredited assessor can produce an official EPC.

Almost every domestic EPC for an existing home is produced using RdSAP. Knowing what it is and how it works helps you understand why your band came out as it did — and why providing evidence to the assessor matters.

What does RdSAP stand for?

RdSAP stands for Reduced data Standard Assessment Procedure. It is a simplified version of SAP (the Standard Assessment Procedure) designed for assessing existing dwellings rather than new builds.

Because full design data is rarely available for an older home, RdSAP uses standard conventions and defaults where evidence cannot be gathered on site.

Source: Energy Performance Certificates, GOV.UK

How does RdSAP produce a domestic EPC rating?

The assessor enters the recorded property data into approved RdSAP software. The software models the home's energy use and produces a SAP score from 1 to 100+.

That score is mapped to a band from A to G and the certificate is then lodged on the national EPC register.

What data does the assessor collect on site?

The assessor surveys the building's key energy-affecting features and records measurements, construction types and the heating system.

RdSAP data areas — what the assessor records and why
Data area	What the assessor records	Why it matters
Walls	Construction type, thickness and any insulation	Walls are a major source of heat loss
Roof	Loft insulation depth or flat-roof construction	Roof insulation strongly affects efficiency
Heating	Heating system type, fuel and age	The main driver of running costs and band
Glazing	Single, double or secondary glazing and extent	Windows affect heat loss and comfort
Controls	Thermostats, timers and zoning	Good controls improve efficiency cheaply

What is the difference between assumed and verified inputs?

Where the assessor cannot see evidence, RdSAP applies a cautious default (an 'assumed' value) based on the property's age and type. These defaults are often pessimistic.

If you can show evidence — for example proof of cavity wall insulation or a newer boiler — the assessor can record a 'verified' value instead, which can raise the score and band.

Keep installation certificates for insulation, boilers and glazing.
Provide guarantees or invoices for upgrade works.
Point out loft hatches and access so insulation can be inspected.
Share any previous improvement records before the survey.

Why does providing evidence matter?

Without evidence, the assessor must apply defaults, which can understate how efficient your home really is and produce a lower band.

Good evidence lets the assessor record verified inputs, giving a fairer, more accurate result that may save you from unnecessary improvement works.

Frequently asked questions

Does RdSAP measure my actual energy bills?: No. RdSAP uses standardised assumptions about occupancy and usage so properties can be compared fairly. Your real bills depend on how you actually live in and heat the home.
How often does the RdSAP methodology change?: The methodology and conventions are updated periodically as standards evolve. Always confirm the current version and rules on GOV.UK, as changes can affect how a property is assessed.
Can AI-EPC run an RdSAP assessment for me?: No. AI-EPC is AI-assisted and helps you understand RdSAP inputs and model how evidence might change your result. Any official EPC must be assessed and lodged by an accredited Domestic Energy Assessor; the AI supports interpretation and planning and never replaces the accredited assessor.

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