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Understand when SAP calculations are needed, how domestic Part L compliance works, and what SAP 10.2 and SAP 10.3 mean for projects in England.
Why Part L SAP Compliance?
Part L of the UK Building Regulations sets out mandatory energy efficiency standards for new residential properties. This legal requirement ensures that new homes are built to conserve fuel and power while reducing carbon emissions. Meeting Part L requirements is essential for:
- Regulatory Approval: Ensure your project meets the latest Building Regulations and avoids compliance issues
- Energy Efficiency: Reduce energy consumption through better insulation, efficient heating systems, and sustainable design
- Lower Running Costs: Minimising heating, cooling, and energy expenses for homeowners
- Future-Proofing: Enhancing property value by aligning with evolving energy standards and sustainability goals
As Part L regulations continue to evolve, our team provides expert guidance to ensure your project meets the latest compliance requirements with ease.
When are SAP calculations needed?
A SAP calculation is required for a new dwelling in England, both before construction (Design stage) and when the dwelling is complete (As built). It models the proposed home under standardised conditions and compares its performance with regulatory targets.
Building control uses the design-stage and as-built outputs as part of the evidence for compliance with Part L of the Building Regulations. SAP also provides the methodology used to produce an Energy Performance Certificate (EPC) for a new home, although an EPC and a Part L compliance assessment are not the same deliverable. (SAP) (approved methodologies)
SAP can also become relevant when an existing building is converted to create dwellings, when a project changes a building’s energy status, or when an extension has more openings than the usual elemental guidance allows. The exact route depends on what is being built, the applicable edition of Part L, the project’s notice or full-plans date, its commencement date and, where relevant, its higher-risk-building status. An extension does not automatically require a full new-dwelling SAP assessment, and exceeding the familiar “25% glazing rule” does not automatically mean failure.
This guide is specific to England. Wales, Scotland and Northern Ireland have different regulations, guidance and approved methodologies. It reflects the verified position on 4 September 2026. The regulatory position and approved software should be checked again before publication and for every live project.
What a SAP calculation does and what it does not do
The Standard Assessment Procedure, or SAP, is the Government’s methodology for assessing the energy performance of a dwelling. Approved software applies the SAP rules to the geometry, construction, thermal bridges, airtightness, ventilation, heating, hot water, lighting, renewable technologies and other relevant features of the home. (SAP) (SAP10)
SAP is an asset assessment. It uses standardised occupancy, climate and operating assumptions so that one dwelling can be compared consistently with another. It is not a prediction of a particular household’s bills. Actual consumption depends on weather, energy tariffs, occupancy, thermostat settings, hot-water use, appliance loads, maintenance and how occupants operate the building.
For a new dwelling, the Part L calculation creates two models. One represents the actual dwelling as designed or built. The other is a notional dwelling of the same size and shape, populated with reference specifications from the approved methodology. The outputs from the actual model must be no worse than the relevant target outputs generated by the notional model. This is why there is no single national DER, DFEE or DPER number that every home must achieve.(SAP10)
The notional specification is a target-setting device, not a universal shopping list. If the notional dwelling uses an air-source heat pump, photovoltaic panels, wastewater heat recovery or a particular ventilation arrangement, that does not by itself make each item legally mandatory in the actual dwelling. A different, coordinated specification may comply if it satisfies every applicable target and minimum standard.
SAP is for dwellings. Most non-domestic buildings use the National Calculation Methodology and SBEM, not domestic SAP. Readers working on offices, retail, industrial or other non-domestic space should be directed to the proposed Part L SBEM Calculations pillar page. Operational energy analysis, such as CIBSE TM54, is also distinct from a regulatory SAP calculation.
Which compliance route applies to your project?
A useful starting point is to classify the work correctly. “Residential project” is not specific enough: a newly built dwelling, a barn conversion, a rear extension and a thermally separated conservatory can follow different routes.
| Project route | Typical Part L approach in England | Where SAP may fit | Key point for the client |
| New dwelling | Design-stage and as-built assessment against dwelling-specific targets, plus minimum standards and evidence | Core compliance methodology | Freeze the right inputs early and update the assessment with final construction data |
| Conversion or material change of use creating a dwelling | Existing-dwelling provisions for upgraded fabric, services and openings; project scope and regulation 6 determine requirements | May support a permitted whole-building or multi-dwelling route where the guidance allows | Do not assume the new-build target route applies unchanged |
| Ordinary dwelling extension | Elemental standards normally apply to new fabric, services and openings | A SAP comparison can provide an alternative route for a highly glazed design | The 25% benchmark concerns total openings in the extension, not glass alone |
| Glazed extension, conservatory or porch | Depends on size, thermal separation, heating and other exemption conditions | May be needed if the proposal is treated as a normal extension or uses a compensating route | “Under 30 m²” is not, by itself, a complete exemption test |
New dwellings
For a new home, SAP should start before the design is submitted to building control. The assessor models the proposed construction and reports the target and actual performance rates. Under the established England framework, the three comparisons are primary energy, carbon emissions and fabric energy efficiency. All three must pass; success against one does not offset failure against another.
The design assessment informs the Building Regulations England Part L report, commonly called the BREL report. At completion, the model is updated to reflect what was actually built. Relevant changes might include insulation products, thermal-bridge details, windows, heating controls, ventilation, photovoltaics and the measured air-permeability result. The as-built report is therefore not an administrative copy of the design assessment. It is a final calculation based on evidenced construction information.
On multi-dwelling developments, each building and dwelling must be set up correctly. Approved guidance allows a defined floor-area-weighted averaging route for flats within one building, but not across separate buildings on a site or across a row of terraced houses.4 The project team should agree the assessment structure before relying on an assumed “site average”.
Conversions and material change of use
Creating a dwelling within an existing building can trigger the energy-efficiency requirements associated with a material change of use under regulation 6. A loft, garage, office, agricultural or mixed-use conversion needs to be classified from the actual proposal; the word “conversion” does not determine the route by itself.
For a newly created dwelling under the current Approved Document L Volume 1, the benchmark area for windows, roof windows and doors is 25% of that dwelling’s total floor area. This is different from the extension calculation described below. The guidance’s stated SAP alternative for this opening provision is limited to buildings containing more than one dwelling.
Existing fabric, retained thermal elements, consequential work, services and practical constraints can all affect the specification. Listed-building or conservation status is not a blanket exemption. Where strict application would unacceptably alter character or appearance, the approach requires project-specific consideration and early discussion with the relevant conservation and building-control teams.
Extensions and the 25% glazing threshold
For an ordinary dwelling extension, the current England guidance sets a benchmark maximum area of windows, roof windows and doors equal to:
25% of the extension’s internal floor area, plus the area of any existing windows or doors that will no longer exist or be exposed because the extension is built.
For example, a 20 m² extension starts with an allowance of 5 m². If the extension covers an existing 2 m² patio door that becomes an internal opening, the benchmark allowance becomes 7 m². The assessment concerns the total area of relevant openings, including external doors, roof windows and rooflights. It is not 25% of the whole existing house, 25% of the elevation or 25% transparent glass.
Going above this benchmark is not an automatic prohibition or automatic failure. It means the simple opening-area route no longer demonstrates the intended level of heat-loss control on its own. A project may use a recognised compensating approach, often involving a SAP comparison between the proposed extension and a compliant notional extension, subject to the applicable guidance and building-control acceptance. Better insulation elsewhere cannot be claimed casually; the calculation must demonstrate the compensation.
This is most effective during concept design. The assessor can test whether the architectural priority is best preserved by adjusting the amount or performance of glazing, improving extension fabric, upgrading retained elements where allowed, or revising another part of the energy strategy. The final details must remain buildable and consistent with ventilation, condensation and overheating requirements.
Our SAP calculation and Part L compliance process
A reliable service is more than entering drawings into software. It is a controlled process that links design assumptions to procurement, construction evidence and the final assessment.
1. Establish the regulatory basis
We first identify the site jurisdiction, project type, building-control route, notice or application date, commencement date, applicable transition and whether the work concerns a higher-risk building. This step matters particularly in 2026–28 because approval of SAP 10.3, commencement of the 2026 Part L changes and project transition dates are not the same event.
The output should be a clear statement of the assessment basis: the Part L edition, SAP methodology and project route being used, with any transition assumption made explicit.
2. Collect and review design information
A design-stage assessment normally needs dimensioned plans, elevations and sections; orientation and site information; construction build-ups; insulation conductivities; window and door sizes and performance; thermal-bridge details; proposed airtightness; ventilation; heating and hot-water systems; controls; lighting; renewable systems; and accredited product evidence where applicable.
Information quality matters. A generic wall U-value does not resolve junction heat loss. A heat-pump label does not define the full system, emitter temperatures, controls or hot-water arrangement. An airtightness aspiration does not replace a buildable air-barrier strategy.
3. Build the model and diagnose compliance
The assessor constructs the dwelling geometry, zones and envelope, then enters the proposed systems and evidence. The first calculation is a diagnosis, not the end of the service. If a target is missed, the useful question is why.
A DFEE failure points toward the fabric model: U-values, thermal bridging, exposed area, glazing and airtightness. A DER or DPER failure may require coordinated changes to fabric, system efficiency, fuel, controls, renewables or distribution losses. The recommended response should be practical, proportionate and compatible with the architecture and construction sequence.
4. Issue design-stage outputs
Once the design works, the assessor issues the required design-stage calculations and BREL output, together with the assumptions that the project team must preserve. The architect, energy assessor, mechanical designer and contractor should resolve discrepancies before procurement. Drawings, specifications and schedules should describe the same design.
5. Manage changes and evidence during construction
Product substitutions and site changes should be checked before installation where possible. Evidence can include product data, invoices, installation records, commissioning documents, photographs of concealed work, thermal-bridge information and airtightness documentation. The 2026 Approved Document guidance explicitly strengthens the role of as-built and photographic evidence, but evidence control is already essential to any defensible completion calculation.
6. Complete the as-built SAP assessment
At completion, the assessor replaces design assumptions with verified as-built inputs, including the measured air-permeability result for a newly erected dwelling. The final SAP and BREL outputs record the completed performance. The EPC is then generated through the appropriate approved process. A dwelling that passed at design stage can still fail as built if fabric, services or test results have changed.
SAP 10.2 explained in plain English
SAP 10.2 was the approved domestic calculation method used with the 2021 Part L uplift in England from 15 June 2022, subject to transition arrangements. It remains important because many live projects were registered or commenced under an earlier regime and may qualify to continue on that basis.
The methodology works through a standardised energy balance. It estimates heat losses through walls, roofs, floors, openings, thermal bridges and air leakage. It accounts for useful gains from occupants, lighting, appliances and the sun. It then calculates the energy needed for space heating, hot water, pumps, fans and lighting, together with contributions from technologies such as photovoltaics. Fuel-dependent factors convert delivered energy into carbon-emissions and primary-energy results.
SAP 10.2 represented a major practical change from SAP 2012 because electricity carbon factors were updated and varied by month, reflecting a cleaner and more variable electricity system. The methodology also dealt more fully with technologies and controls relevant to modern homes. However, no headline about grid decarbonisation tells a project team whether a particular design passes. Geometry, fabric, systems, fuel and evidence still interact in the actual calculation.
A SAP 10.2 model is therefore best understood as a consistent regulatory model, not a digital twin. It answers, “How does this dwelling perform under the approved standard assumptions?” It does not answer, “What will this household pay next January?”
SAP 10.3 status and implications
As at 4 September 2026, SAP 10.3 is published and Government-approved for the relevant new-dwelling energy-performance calculations in England. The formal approval notice took effect on 24 March 2026, and GOV.UK describes SAP 10.3 as the current, sole approved methodology while introduction of the Home Energy Model (HEM) remains pending.
This does not mean every project in England must immediately abandon SAP 10.2, nor does it mean the 2026 Future Homes Standard performance requirements were already generally in force in September 2026. These are separate issues:
- Methodology approval: the 24 March 2026 notice approved SAP 10.3 for specified calculations under regulations 24 and 25.
- New standards commencing: the 2026 Part L changes generally take effect on 24 March 2027 for ordinary building work and 24 September 2027 for specified higher-risk-building work.
- Transition: qualifying ordinary work notified before 24 March 2027 and commenced before 24 March 2028 may remain under the earlier regime. Higher-risk buildings have separate Gateway 2 and saved-transition provisions.
The correct methodology must therefore be established from the individual project facts. A broad statement such as “all projects now use SAP 10.3” is not safe professional advice.
BRE’s published version history identifies limited, specific changes from SAP 10.2: Appendix R reference values, Tables 12, 12e and 12f, and the fabric-energy-efficiency calculation. Confirmed changes include revised notional-dwelling assumptions and the use of non-monthly average electricity carbon and primary-energy factors. The SAP 10.3 notional dwelling shifts from a gas-boiler and radiator reference toward an electric air-source heat-pump reference, with revised assumptions for ventilation, air permeability, lighting, photovoltaics and wastewater heat recovery.
These are reference assumptions used to generate targets. They do not make the same technologies mandatory in the actual dwelling. They also do not support a universal claim about cost, EPC band, energy bills or the percentage by which a design will improve or worsen. Each scheme must be recalculated.
HEM is intended to succeed SAP, but Government delayed its launch. The live GOV.UK status says it will be introduced alongside SAP 10.3 “in the coming months” and does not give a fixed launch date. HEM consultation material should not be presented as a current compliance requirement. Before this page is published or advice is given on a live project the methodology, approved software list and HEM status should be rechecked.
From SAP 2012 to SAP 10.3
SAP’s history helps explain why old calculations cannot simply be relabelled in new software. Methodology updates and changes to the Part L targets are related but distinct.
| Milestone | Date and status | Why it matters |
| SAP 2012 | Approved in England from 6 April 2014 | Supported new-dwelling CO₂ and fabric-efficiency comparisons and new-home EPCs under the 2013 Part L regime |
| SAP 10.0 | Published 24 July 2018; not for official purposes | Signalled technical development, but SAP 2012 remained the approved route |
| SAP 10.1 | Published 8 November 2019 as a Future Homes Standard consultation reference | Useful development context, not evidence that the version became the legal compliance method |
| SAP 10.2 | Published 20 August 2021 and subsequently amended; approved with the England 2021 uplift from 15 June 2022, subject to transition | Added the primary-energy target to the established emissions and fabric comparisons |
| SAP 10.3 | Edition dated 13 January 2026; published and approved 24 March 2026; correction dated 12 June 2026 | Current approved methodology at the research date, with project-specific transition and future Part L commencement still to be checked |
Under SAP 2012, Part L already included the target emissions rate and target fabric energy-efficiency rate. The 2021 England uplift added the target primary-energy rate, so the current framework is not just a carbon check. This three-part structure is central to design decisions.
The three Part L performance tests: DER, DFEE and DPER
A new-dwelling result should not be reduced to “the SAP passed”. The actual dwelling must satisfy three distinct comparisons against targets generated from its notional counterpart.
| Actual-dwelling metric | Plain-English definition | Target comparison | Main influences |
| DER: Dwelling Emission Rate | Calculated greenhouse-gas emissions per square metre of floor area per year | DER must not exceed TER, the Target Emission Rate | Fabric heat loss, system efficiency, fuel carbon factors, controls and low-carbon generation |
| DFEE: Dwelling Fabric Energy Efficiency | Normalised space-heating and cooling demand per square metre of floor area per year | DFEE must not exceed TFEE, the Target Fabric Energy Efficiency rate | U-values, thermal bridges, glazing, exposed area and airtightness; it is the fabric-focused test |
| DPER: Dwelling Primary Energy Rate | Calculated primary energy per square metre of floor area per year, accounting for energy used and upstream conversion and supply | DPER must not exceed TPER, the Target Primary Energy Rate | Fabric demand, system efficiency, fuel primary-energy factors, distribution and generation |
The words “must not exceed” are important: lower actual rates are better for these comparisons. TER, TFEE and TPER are calculated for the notional dwelling. They are not fixed national limits that can be copied from another plot.
All three tests are cumulative. Photovoltaics or a low-carbon heating system may help DER and DPER, but cannot repair a DFEE failure because DFEE isolates the fabric-related heating and cooling requirement. Conversely, excellent fabric does not guarantee that inefficient services will satisfy the primary-energy and emissions tests.
These calculated targets also sit alongside minimum standards and procedural requirements. A passing headline rate does not authorise a project to ignore limiting U-values, controls, commissioning, insulation continuity, system sizing or evidence. Approved Document L provides a recognised guidance route, but it is not the legislation itself and does not remove the need for a project-specific building-control decision.
Approved Document L Volume 1 and air-tightness testing
Approved Document L Volume 1 explains a recognised way to meet the conservation-of-fuel-and-power requirements for dwellings in England. At the research date, the 2021 edition incorporating 2023 amendments remained the practical current guidance for ordinary work. The 2026 edition had been published, but its new requirements generally commence on 24 March 2027, subject to transition.
For current new-dwelling work under the 2021/23 guidance, the limiting air-permeability standard is 8.0 m³/(h·m²) at 50 Pa, and every newly erected dwelling should be pressure tested. The measured result must be inserted into the as-built DPER, DER and DFEE calculations. Failures and retests must be reported to building control.
The 8.0 figure is a limiting value in the Approved Document route, not a sensible default design target. The project’s design value may need to be lower to satisfy its dwelling-specific targets. Equally, the SAP 10.3 notional reference of 4 m³/(h·m²) at 50 Pa is a target-setting assumption, not a universal instruction that every actual dwelling must test at exactly 4.
Airtightness has to be designed, detailed and inspected. Continuity at floors, roofs, party elements, windows, service penetrations and structural junctions matters more than a note reading “seal all gaps”. The ventilation strategy must be coordinated with the intended permeability. Under Approved Document F, design permeability below 5 and as-built permeability below 3 m³/(h·m²) at 50 Pa identify a highly airtight dwelling for ventilation guidance; they are not alternative Part L maximum values, and an as-built result below 3 does not automatically mandate mechanical ventilation with heat recovery.
Building Regulations regulation 43 governs pressure testing and notice of results. Current guidance refers to approved test procedures and permits building control to accept certificates from testers registered through ATTMA or Elmhurst.
Part O Overheating: related, but not part of a SAP pass
Part O requires reasonable provision to limit unwanted summer solar gains and provide a means to remove excess heat in new residential buildings in England. The current Approved Document O offers a simplified route and a dynamic thermal modelling route based on CIBSE TM59.
Part O matters alongside SAP 10 because the same design decisions can pull in different directions. Larger windows may improve daylight and winter solar gains but increase summer risk. Very low heat loss does not prevent solar overheating. Window opening, external shading, glazing g-values, orientation, security, noise, fall protection and ventilation should therefore be resolved early.
A SAP calculation does not produce a Part O pass, and Part O modelling does not replace SAP. Where fixed mechanical cooling is installed, it must be represented in SAP, but SAP does not calculate the Part O cooling load from dynamic modelling. Part O applies to new residential buildings, not later extensions, later conservatories or change-of-use work under the current England guidance.
Coordinate SAP with the wider energy strategy
SAP is one part of a compliant and coherent design. Planning authorities may require an energy statement that applies local policy, an energy hierarchy or carbon-reduction evidence beyond Building Regulations. Link to Energy Statements.
Mixed-use projects also need clean scope boundaries. Dwellings generally use SAP, while non-domestic areas normally use SBEM/NCM. The completion and certification outputs differ. Link to Part L SBEM Calculations and Non-domestic EPCs.
Simplifying your Part L SAP Journey
Navigating Part L compliance might seem challenging, but with our expert advice and support, we’ll help you through the process with ease. Whether you need assistance at the planning stage or a final confirmation at the end of construction, we are here to guide you through each step.
- Pre-Assessment: Get early feedback to optimise your building’s energy efficiency and ensure compliance with Part L
- Design Stage: Once your design is finalised, we will perform detailed SAP calculations to ensure it meets all Part L criteria, including U-values, ventilation, heating systems, and renewable energy solutions
- Construction Stage: We can advise on material and specification changes throughout the build to ensure when complete Part L requirements are still met
- Photographic Evidence: We offer a photographic evidence service to ensure the necessary photographic evidence are recorded throughout the build
- As-Built Stage: After construction, we will carry out the final checks and provide compliance reports confirming that your building meets Part L standards
FAQs
Is a SAP calculation required before planning permission?
Not usually as a universal national planning requirement, but planning policy or an energy statement may require early modelling. For Part L, waiting until after planning can be risky if the approved massing, glazing or external appearance leaves little room to improve performance. Early SAP work can test the design without confusing planning evidence with Building Regulations evidence.
Does every extension need a SAP calculation?
No. Many extensions follow the elemental standards in Approved Document L without a full SAP comparison. SAP is often used where the opening area exceeds the normal allowance or where a compensating design route needs to be demonstrated. The applicable route should be agreed from the actual extension design and current guidance.
What does the 25% glazing rule actually mean?
For an ordinary extension, the benchmark is the total area of windows, roof windows and doors: 25% of the extension floor area, plus the area of existing openings that cease to exist or be exposed because of the extension. It is not a glass-only percentage. For a newly created dwelling following material change of use, the guidance uses 25% of that dwelling’s total floor area, which is a different calculation.
Can an extension have more than 25% openings?
Potentially, yes. Exceeding the benchmark does not itself ban the design. It means a recognised alternative or compensating calculation is needed, often a SAP comparison with a notional compliant extension. The result depends on the complete design and building-control acceptance.
What is the difference between a SAP calculation and an EPC?
SAP is the methodology used for new-dwelling Part L calculations and new-home EPC generation. The Part L assessment compares actual and target rates and records compliance evidence. The EPC communicates an energy rating for the dwelling. An EPC alone does not prove that all Part L requirements have been met
Does a good EPC rating guarantee Part L compliance?
No. Part L requires the relevant DER/TER, DFEE/TFEE and DPER/TPER comparisons, as well as minimum standards, commissioning and evidence. An EPC rating is a different output. It should not be substituted for the BREL compliance documentation.
Is SAP 10.3 mandatory for my project?
SAP 10.3 is the current Government-approved methodology at the research date, but project transition can preserve an earlier route. The answer depends on notice or application dates, commencement, the applicable Part L edition and any higher-risk-building provisions. The assessor should document the basis rather than infer it from today’s date alone.
Has the Home Energy Model replaced SAP?
No, not as at 4 September 2026. Government says HEM’s launch was delayed and that it will be introduced alongside SAP 10.3 in the coming months, without a fixed published date. The live status must be checked before publication and project advice.
Does passing SAP predict low energy bills?
No. SAP uses standardised assumptions to compare the asset consistently. Bills depend on occupants, weather, tariffs, controls, appliance use and maintenance. Operational-energy analysis, including approaches such as CIBSE TM54 where appropriate, addresses a different question.
Do I need both SAP and Part O assessments?
A new residential building in England may need both. SAP supports Part L energy compliance; Part O addresses overheating through its own simplified or dynamic-modelling route. Neither assessment substitutes for the other.
What information should I send for a SAP calculation?
Send the latest dimensioned drawings, site plan and orientation, construction build-ups, insulation data, window and door schedules, thermal-bridge strategy, heating and hot-water design, ventilation, controls, lighting, renewable systems and proposed airtightness. Also provide the building-control application or notice date, commencement information and any earlier assessments. Missing or provisional information should be identified explicitly rather than treated as a confirmed as-built specification.
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