RdSAP 10: What Changed in 2025, and the August 2026 Conventions Update

RdSAP 10 is the updated Reduced data Standard Assessment Procedure. It replaced RdSAP 9.94 on 15 June 2025 for assessments of existing homes; new-build assessments use full SAP. The changes include more detailed window, ventilation and lighting data, more options for building elements, and recognition of newer technologies.

The method has not stood still since. The conventions assessors work to were revised again on 6 August 2026, in version 12.2. That update is covered further down.

What is RdSAP and why does it matter?

SAP (Standard Assessment Procedure) is the UK government's method for calculating the energy performance of homes. Full SAP is used for new buildings. The Reduced data SAP (RdSAP) is used for existing homes: "reduced" because some inputs are assumed from the construction type and age rather than measured. RdSAP 10 forms part of SAP 10.2.

RdSAP produces your EPC rating. It sets what the assessor records, what defaults apply when something cannot be confirmed, and how the inputs feed the final score. A change to RdSAP can change a rating even if nothing about the property has changed.

The main changes in RdSAP 10

1. Energy costs and carbon are different measures

The headline Energy Efficiency Rating is based on modelled energy costs. Carbon and primary energy figures describe other aspects of performance. A change in the electricity carbon factor does not, by itself, tell you whether a property's headline band goes up or down.

2. More detailed building information

RdSAP 10 records window measurements and more detail about ventilation and room-in-roof construction. It also offers more options for insulation and building elements. Supporting evidence the assessor can accept can affect which inputs are recorded.

3. Heating and newer technologies

The calculation uses the heating system and its product data where available. RdSAP 10 also recognises technologies including PV battery storage and diverters. No heating technology guarantees a particular band or a fixed improvement over an older assessment.

For a practitioner's summary, see Elmhurst's RdSAP 10 overview. The property's recorded data and the approved calculation decide its result.

4. A more detailed lighting count

Lighting is counted lamp by lamp, not fitting by fitting. The rules assessors follow are specific:

• LED lamps are recorded as LED. Only LEDs.
• Fluorescent tubes and compact fluorescent lamps (CFLs) are recorded as CFL. They are counted separately from LEDs.
• Tungsten and halogen lamps are recorded as incandescent. These are not low energy, and that includes low-voltage halogen downlights, which people often assume are efficient because they are small.
• LED strip lighting counts as one lamp per running metre, rounded to the nearest metre.
• Every lamp in a permanent fitting counts, including fixed under-cupboard kitchen lights. Table lamps, standard lamps and anything plugged into a socket do not.
• If a lamp is missing or its type cannot be identified, that lamp is recorded as CFL.
• If more than half of the lamps that are not incandescent are missing or cannot be identified, all of them are recorded as low-energy lighting rather than counted one by one.

Effect: the lighting section records what is installed and identifiable on the day of the visit, and it separates LED from CFL. So if halogen or incandescent lamps have already been replaced with LEDs, that work counts. Lamps sitting in a box out of their fittings do not, because a missing lamp is recorded as CFL. Halogen and incandescent lamps are recorded as not low energy, whatever their wattage.

5. How conservatories are treated

The RdSAP 10 specification separates two kinds of conservatory:

• Separated from the home: it is left out of the assessment, whether or not it has fixed heaters.
• Not separated: its floor area and volume are added to the home's, and its walls and roof are treated as fully glazed.

Effect: whether a conservatory is separated from the home can change the result. The August 2026 conventions, below, also define what counts as a conservatory in the first place.

6. Updated primary energy factors

Primary energy factors, which convert delivered energy into the energy used to extract, process and distribute it, have been revised. They feed the primary energy figure on the certificate. The headline rating is based on modelled energy costs instead.

What changed again in August 2026

RdSAP 10 is the method. Sitting on top of it is a separate document, the RdSAP conventions, which tells assessors how to handle situations the method alone does not settle. It is updated more often than the method, and where a convention conflicts with the published specification, the convention wins.

The current edition is version 12.2, issued 13 July 2026 and in force from 6 August 2026. Four conventions in it are new rather than reworded. The two most likely to affect a homeowner are the conservatory definition and heat as a service.

Conservatories now have a hard definition

For the first time the conventions state exactly what counts as a conservatory: at least 50% of the external heat-loss walls glazed, and at least 75% of the roof glazed.

That matters because of what follows. If a conservatory no longer meets the roof test, because the glazed roof has been replaced with a solid one or a false ceiling has been fitted, it is no longer assessed as a conservatory. It is recorded as an extension, part of the home.

Why this catches people out: replacing a conservatory roof is a common improvement, usually sold on comfort rather than efficiency. It changes how the room is assessed, and whether that helps or hurts the rating depends on the property and on what went in above the ceiling. With visual or documentary evidence of insulation, the assessor records the insulation that is there. Without evidence, a roof pitched at under 10 degrees is recorded as flat with unknown insulation, and a roof pitched above 10 degrees with no access to the void is recorded as having no insulation.

What to do: if you have had a conservatory roof replaced, find the paperwork before the assessment: an installer's specification, a building control notice, or a guarantee that names the insulation. It lets the assessor record what is in place rather than fall back on a default. It is evidence of the real construction, and it does not decide the outcome.

Heat as a service is treated as community heating

Where a provider owns or operates the heat generator and sells you heat as a service, it is now assessed as community heating, whether or not the equipment sits inside your property. Systems powered by electricity are treated as a community heat pump, and everything else as a community boiler. Specific documentary evidence is required to confirm the arrangement, so keep the contract.

Electric combi boilers have their own instruction

Electric combi boilers are now recorded in a defined way rather than left to interpretation. Water heating from one is recorded as instantaneous at the point of use.

Two ventilation systems in one home

Where a property has two mechanical ventilation systems, the conventions now set out which one is entered, in a defined order of precedence, rather than leaving it to the assessor.

Smaller changes worth knowing

• Solar batteries: a single battery larger than 5 kW is now entered as two, a first entry of 5 kW and the rest as a second battery. If you have battery storage, have the manufacturer, model and usable capacity to hand; otherwise a default figure is used.
• Air pressure tests: if material changes have been made to the property since the original test, the test certificate can no longer be used in the assessment.
• Secondary glazing: unusual arrangements, such as plastic glazing units fitted over single glass, are now explicitly treated as secondary glazing.

What comes next: new-style EPCs

The government plans a new kind of EPC for homes. Its partial response of 21 January 2026 says new-style domestic EPCs will replace today's single cost-based rating with four headline measures: energy cost, fabric performance, heating system and smart readiness. The current rating would stay on the certificate as well, so new and old EPCs can be compared.

They will not arrive in October 2026. On 9 March 2026 the government said it had decided to move the launch to the second half of 2027, and that it would agree a new launch date with industry by the summer. When we checked its page on 29 October 2026, no date had been added.

The same response sets out other changes it intends to make, all subject to parliamentary approval:

• Validity stays at 10 years. Existing EPCs keep their 10 years, and the new-style ones will last 10 years too.
• An EPC before marketing. It intends to require a valid EPC when a home is marketed, instead of allowing up to 28 days after marketing starts.
• Rooms in an HMO. It intends to require an EPC for the whole house in multiple occupation when a single room is let. Today a single room does not need one.
• Short-term lets and heritage homes. It intends to bring short-term lets into scope whoever pays the bills, and to remove the exemptions for heritage properties.

Whether a rented home will need a new EPC when its old one expires during a tenancy is still being decided.

Should you get a new EPC now or wait?

There is nothing to wait for. Today's EPCs are still made with RdSAP 10, and the government intends existing certificates to keep their 10 years. Get a new one when you need it to sell or let, or when you have made improvements you want recorded.

What hasn't changed

Method changes worry landlords with recently issued EPCs, so here is what stays the same under RdSAP 10:

• The A to G bands. 69 to 80 is still C, 55 to 68 is still D, and 39 to 54 is still E.
• What the assessor looks at. Wall type, roof, glazing, heating, hot water and lighting are still the categories recorded. Some are now recorded in more detail, and the conventions updates add to that detail rather than replacing it.
• The 10-year validity period. EPCs issued under RdSAP 9.94 remain valid for their full 10 years unless a newer EPC is lodged. The method change is not a reason to renew.
• The minimum standard for rented homes. The current E minimum still applies to rentals within scope unless exempt. The confirmed higher standard for 1 October 2030 still needs legislation; it is not current law.
• Where certificates go. They are still lodged on the government EPC register.
• The accreditation schemes. Elmhurst, Quidos and the other approved schemes carry on as before.

Do you need to re-assess your property?

No, if your existing EPC was issued under RdSAP 9.94 and is within its 10 years. The old certificate remains valid for selling or letting.

Yes, if:

• your certificate is more than 10 years old and you are selling or letting;
• you have made improvements since the last assessment and want them recorded.

A score close to a band boundary does not tell you whether a new assessment will move it up or down. Think about why you need an updated certificate, and gather the property evidence, before you book another assessment.

Practical implications for London landlords and sellers

The current minimum remains E for rentals within scope unless exempt. The confirmed 2030 policy uses reformed metrics, so it should not be described as applying today's SAP 69 threshold to every tenancy.

For a portfolio, check current compliance and the future transitional rules separately. A new assessment establishes a result; it does not guarantee a C.

For sellers it is similar. If your agent or solicitor is querying a borderline rating, a fresh assessment under RdSAP 10 records the property with today's data and evidence.

We carry out all assessments under RdSAP 10 across all London boroughs, including Tower Hamlets, Hackney, Ealing, Bromley and 30 others. Book an assessment or see our pricing page. For what the visit involves, see our what happens during an EPC assessment guide.