Is RdSAP less accurate than full SAP?

It is less granular by design, since it relies on standardised assumptions where direct evidence is unavailable, but it is calibrated against the same underlying physics and is considered fit for purpose for the existing housing stock it covers. Full SAP is used for new builds because complete specifications exist from the design stage.

Can I see the RdSAP inputs used for my EPC?

Yes. The full technical assessment, including the property's construction assumptions and measurements, is available from your assessor and underpins the recommendations section on the certificate itself.

Does RdSAP account for how I actually use energy in my home?

No. It assesses the building's fabric and fixed systems under standardised occupancy and heating patterns, not your personal usage, which is why your actual bills can differ from the certificate's cost estimate.

Will an extension without building control sign-off affect my RdSAP assessment?

It can still be included, but without documentation the assessor is more likely to apply a conservative default for its construction, which may understate its actual performance.

How often does the RdSAP methodology change?

It is updated periodically by government to reflect new technology and data; the current version is RdSAP 10, and assessors are required to use whichever version is in force at the time of assessment.

Does having solar panels always improve my RdSAP rating?

Generally yes, since generation is credited against the property's energy use, but the effect on the final band depends on the rest of the fabric and heating performance, so it does not guarantee a specific band on its own.

Guide · guides

RdSAP explained: how your EPC rating is actually calculated

RdSAP is not a full building simulation — it is a standardised methodology built on age-based assumptions, on-site measurement and photographic evidence. Here is what actually goes into your number.

12 min readBy Nabhan Islam Reviewed by Awais Sarwar, MSc Updated 6 Sept 2026

RdSAP explained: how your EPC rating is actually calculated

The short answer

RdSAP (Reduced Data Standard Assessment Procedure) is the government-approved methodology used to produce EPCs for existing homes in England, Wales and Northern Ireland. Rather than modelling every material in a property, it combines measurements and evidence an assessor collects on site — dimensions, glazing, heating system, insulation evidence — with standardised age-band assumptions for anything that cannot be directly observed, such as wall construction defaults by build period. The current version is RdSAP 10. The output is a SAP score converted to an A-G band, plus estimated running costs and a list of improvement measures ranked by cost-effectiveness.

020 3092 8551

Current methodology

RdSAP 10

Used for all existing dwellings

Data source

On-site survey + evidence

Photos, invoices, building records

Applies to

Existing dwellings

New builds use full SAP instead

Output

SAP score, A–G band

Plus a tailored improvement list

What RdSAP actually is, and why it is not full SAP

RdSAP was developed because a full, first-principles energy model of every existing home in the country would need intrusive surveys and construction records that most owners simply do not have. Instead, RdSAP works from a smaller set of measurable inputs — room dimensions, window area, heating controls, visible insulation — and fills the remaining gaps with standardised default values drawn from the age and type of the building. Full SAP, by contrast, is used where a complete specification exists, which is why it is the methodology for new dwellings rather than existing ones. The trade-off is speed and consistency: RdSAP assessments can be completed in under an hour on most properties, and every assessor working from the same inputs reaches broadly the same output.

RdSAP relies on a Reduced Data set collected on site rather than full construction drawings.
Age bands (for example, 1950–1966, 1983–1990) drive default assumptions about wall and floor construction.
Full SAP is reserved for new dwellings where design specifications are known in detail.
RdSAP is periodically updated — the current version is RdSAP 10 — to reflect new technologies such as heat pumps and improved data on modern insulation products.

What an assessor measures on site

The site visit is where the reduced data set is actually gathered, and it is more thorough than most homeowners expect. An assessor works room by room, recording dimensions, wall and floor construction, glazing type and count, and every heating and hot water component, then photographs the evidence for each. This is also the point at which you can hand over documentation that changes the default assumption the software would otherwise apply.

Room-by-room dimensions to calculate total floor area and heat loss perimeter.
Wall construction type — cavity, solid, timber frame — checked visually and against age-band defaults.
Glazing type and count, including secondary glazing and door glazing.
Loft and roof insulation depth, measured directly where the loft hatch is accessible.
Heating system make, model and controls, and hot water cylinder insulation where fitted.
Any low-energy lighting, renewable technology or secondary heating present.

Evidence worth having ready before your assessment

FENSA or CERTASS certificates for replacement windowsConfirms glazing installation date and standard
Cavity wall or loft insulation installation invoicesOverrides an age-band default with confirmed depth or type
Boiler installation certificate or Gas Safe recordConfirms efficiency rating rather than an estimated one
Planning or building control documents for extensionsClarifies construction date and type for newer parts of the home
Solar PV or heat pump commissioning documentsEnsures renewable generation is correctly credited

The buildings behind this advice

Where default assumptions come from

When something cannot be verified on the day — most commonly the construction of a wall that cannot be inspected without opening it up — RdSAP applies a default drawn from national data on typical construction methods for that age band and property type. These defaults are not arbitrary; they are derived from large-scale housing stock surveys and are deliberately conservative where evidence is absent. This is the single biggest source of confusion for owners who feel their EPC does not reflect improvements they know they have made, because an unevidenced improvement is, by design, treated the same as if it had not happened.

Age bands are based on the likely predominant construction method for homes built in that period in that country.
Where a property has been altered from its original type, the assessor records the alteration date separately if evidence exists.
Extensions are usually assessed against their own likely construction date rather than the original dwelling's age band.
Defaults err toward the more common construction type for the band, which can occasionally mismatch an atypical property.

Why two similar-looking homes can score differently

Owners are often surprised that a near-identical neighbouring property has a different rating. The explanation is almost always in the detail rather than the headline size or age. A conservatory extension assessed under a different construction default, an unrecorded loft conversion, a boiler replaced without documentation, or simply a different assessor's photographic evidence of insulation depth can each move the score by several points.

Heating system efficiency and controls, which typically has the largest single impact on the score of any factor.
Loft and cavity wall insulation depth and presence, especially where one property has documented top-up and the other does not.
Glazing type, count and age, including whether double glazing has FENSA evidence or must be estimated.
Extensions or conversions assessed under a different construction-date default to the main dwelling.
Low-energy lighting proportion throughout the property.

From SAP score to A-G band and running costs

Once the reduced data set is compiled, the RdSAP software calculates a SAP score between 1 and over 100 — technically uncapped, though it is rare to exceed 100 outside highly efficient new-build-standard properties — reflecting energy cost per square metre of floor area. That score is then converted to the familiar A-G banding used on the certificate, alongside estimated annual heating, hot water and lighting costs, current carbon dioxide emissions, and a ranked set of recommended improvements with indicative costs and potential rating gains.

Band	SAP score range	Typical characteristics
A	92–100	New-build standard, renewables, excellent fabric
B	81–91	Modern construction, good insulation and controls
C	69–80	Reasonably efficient, often post-2010 or well-improved
D	55–68	Common for older housing stock without major upgrades
E	39–54	Limited insulation, older heating systems
F	21–38	Poor fabric performance, minimal insulation
G	1–20	Very poor performance, typically pre-1930s unimproved

Approximate SAP score bands used for the A–G rating shown on an EPC.

RdSAP 10: what changed from earlier versions

RdSAP has been revised several times as building technology and available data have evolved, and the current version reflects better treatment of heat pumps, smart heating controls, and modern insulation products that earlier versions handled with cruder defaults. Assessors working under an older methodology version could produce a different rating for the same property to one assessed under the current version, which is one reason a certificate is a snapshot rather than a permanent scientific measurement.

Improved modelling of air source and ground source heat pumps, previously assessed with more conservative default efficiencies.
Better recognition of modern glazing specifications and U-values.
Refined treatment of thermal bridging and party wall assumptions in terraced and semi-detached properties.
Updated fuel cost and carbon factor data to reflect current energy pricing and grid carbon intensity.

Challenging or correcting an RdSAP result

If you believe your EPC does not reflect your property accurately, the first step is checking whether the assessor had the evidence they needed at the time, since most inaccuracies trace back to missing documentation rather than assessor error. Where genuine evidence exists that was not available or not provided during the original visit, a new assessment carried out with that evidence to hand will usually produce a more favourable result without needing any formal dispute process.

Gather documentary evidence for anything you believe was defaulted incorrectly, such as insulation invoices or glazing certificates.
Contact the original assessor first, since minor recording errors can sometimes be corrected without a full re-visit.
Commission a fresh assessment with the additional evidence to hand if the original visit predates the improvement or documentation.
Escalate to the assessor's accreditation scheme if you believe the methodology itself was misapplied, rather than a data gap.

Get an RdSAP assessment carried out properly the first time

Our assessors ask for your evidence upfront so defaults are only applied where genuinely needed. EPCs from £35, lodged same or next working day.

Written by

Nabhan Islam

Head of Marketing & Commercial Lead

Reviewed by

Awais Sarwar, MSc

Accredited Energy Assessor — Accredited domestic and commercial energy assessor, listed on the EPC Register

12 min readLast reviewed 6 September 2026Facts verified 6 September 2026 Checked quarterly

Sources

The Energy Performance of Buildings (England and Wales) Regulations 2012 — legislation.gov.uk
Find an energy certificate — GOV.UK
SAP and RdSAP — Standard Assessment Procedure documentation — BRE Group
Elmhurst Energy — accreditation and technical guidance — Elmhurst Energy

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