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What Are SAP Calculations?

‘SAP’ stands for Standard Assessment Procedure and essentially is the government’s method for calculating the energy performance of dwellings. The following applies to Regulations in England. Although the SAP methodology is used in Scotland Northern Ireland and Wales, different building regulations are applied to the results.

SAP calculations provide a comprehensive framework for calculating the energy performance of individual flats and self-contained dwellings that looks at:

  • Energy consumption per unit floor area
  • Energy cost rating adjusted for floor area (the SAP rating) and expressed on a scale of 1 to 100, where a lower rating indicates higher running costs
  • CO2 emissions adjusted for floor area (Environmental Impact rating or EI rating) and expressed on a scale of 1 to 100, where a lower rating indicates higher greenhouse gas emissions.
  • The Dwelling Emission Rate (DER) and the Target Emission Rate (TER) where CO2 emissions and target are expressed in kg/m²/year
  • The Dwelling Fabric Energy Efficiency (DFEE) and the Target Fabric Energy Efficiency (TFEE) – where the fabric energy efficiency is expressed in  units of kilowatt-hours per square metre of floor area per year (kWh/(m2.year)

These calculations are based on:

  • Construction materials and thermal insulation of the building fabric
  • Solar gains
  • Choice of fuel for space and water heating, ventilation and lighting
  • Air leakage and ventilation equipment
  • Space cooling
  • Efficiency and control of heating systems
  • Renewable energy technologies

The target setting calculations that generate the TER and TFEE are hybrid SAP calculations using some of the dwelling parameters, some reference values and some additional rules in order to create individual targets for each dwelling calculation.

SAP calculation methodology are a set of calculations and tools that are used to assess

  • The energy performance of dwellings for official statistics
  • To ensure that building work is compliant with Part L of the building regulations in England (Energy efficiency) or their equivalent in Northern Ireland Scotland and Wales
  • Create an EPC - required for new builds and material change of use only.

In England Part L regulation for dwellings come in two formats.

  • L1A for new builds 
  • L1B for conversions, renovations and extensions

How are SAP Calculations Completed?

In order to undertake calculations in the most efficient and accurate way possible, the customer should provide as much information as possible. This includes, but is not necessarily limited to;

  • Elevations
  • Site plans
  • Sections
  • Floor plans
  • Specifications of heating systems
  • U-value of openings
  • Insulation thickness/type
  • Renewable technologies
  • Percentage of low energy lighting
  • Hot water generation
  • Accredited construction details
  • Specification of ventilation system

SAP Conventions

As well as the calculation method there is a series of conventions and rules that re-enforce a consistent platform for energy assessment. These Rules and conventions are regularly reviewed and updated to improve the method and to keep pace with new technology.  

Although the basic SAP methodology is the same,  the way it is applied varies depending upon the type of project, an extension, conversion or a new build.

L1A SAP Calculation Information and Guidance

For new builds, your SAP calculations will be divided into two reports; ‘design stage’ and ‘as built’.

Design Stage

Before any construction begins, SAP is applied  to assess and approve the construction details to ensure that the planned construction is compliant with part L Data is entered the software in order to produce the various hybrid SAP calculations

  • DER /TER - Dwelling CO2 Emissions and associated target   
  • DFEE/TFEE Dwelling fabric emissions and associated target

The performance ratings at this stage are predicted ratings because the dwelling is not complete. These may be required to sell unfinished properties where an EPC cannot be issued.

  • Predicted SAP Energy SAP rating 
  • Predicted Environmental Rating  

Also, the Assessor can generate a range of reports to inform the client or building control

  • Compliance to L1A
  • Construction details required
  • U values
  • Linear Junction details
  • Air leakage targets
  • Heating and ventilation details
  • Opening and window details
  • A draft of the sign off report

Although L1a provides for some limiting values and efficiencies for individual elements and services, these are not normally the main constraint. There are two rules involving calculations that are very important;

  • The DER should be less than or Equal to TER
  • The DFEE must be less than or equal to the TFEE

The main difficulty is to maintain the DER and DFEE below their targets without significantly altering the build.

The DFEE is only influenced by the building fabric such as external floors, walls roofs, junctions and air leakage. If the DFEE is too high, these parameters should be improved first. Improving the DFEE will probably also lower the DER

The DER is influenced by the above as in the DFEE but also the building services;

  • Heating systems and controls
  • Ventilation systems and controls
  • Renewable technologies such as Solar PV

A principal role of the Assessor is to balance the requirements of the builder against the limitations of these two targets.  Is the insulation adequate? Is the heating optimised? How can these be changed to improve the compliance? What is the most effective way of ensuring compliance? It is here that the experience of the assessor comes into play.

The design specification provides a blueprint to produce a compliant house, if during the construction there are proposed changes then it is advisable to get these checked out first. Changes often includes the final specification of the make and model of boiler. Installing a less efficient model than that assumed at design may well upset the final compliance. 

The specific details and efficiencies of most common makes and models of heating systems and other services are available in an approved database, and these details and efficiencies are utilised by SAP. The Product Characteristics database can be found here…

As Built

Once the building or construction is complete, the SAP input is re-assessed, and any final details are checked and if necessary, amended. The developer normally signs of the construction details to provide an audit trail to the assessor. An air leakage test is normally required. Additional evidence may be required for example window U values. A finalised report run generates  the Energy Performance Certificate and compliance reports for the developer to send to building control.

L1B SAP Calculation Information

L1b regulations are applied when

  • A dwelling is created from a material change of for example, a flat or barn conversion or a building that was formerly commercial
  • An extensions to existing dwellings
  • Renovation of the building fabric.

Material Change of use 

The SAP calculations are basically the same as is L1A except that the DER and TER are not required, and compliance is determined using the guidance and tables in L1b.  The resulting SAP calculation model is used to generate the EPC on completion.


Part L1B applies to extensions but compliance can often be obtained by providing details meet the elemental standards contained in Part L1b - using SAP methodology but without resorting to a full SAP calculation. However U values must be correctly calculated and checked against the regulations along with any other service details. Part L1b is not easy for an untrained person to understand. 


In many circumstances changes to the building fabric and services are also covered by L1B. Renovation of surfaces may trigger a requirement to improve the insulation details. Changes to approved services such as boilers are included in the regulations. Building control are not always routinely involved but sometimes they are notified by approved trades such as gas fitters and window installers.

Alternative compliance methods in L1b

Part L1b is flexible to enough to apply to various circumstances. If an element or service is unable to comply with the elemental regulations in L1b, there is the provision of alternative methods of compliance. One must demonstrate that the dwelling as proposed performs as well as (or better than) the same construction as built to the limiting standards of elemental construction in Part L1b.

Two SAP calculations are required, one is for the proposed dwelling, and another is prepared containing notional details that are fully compliant to L1B. Compliance is obtained if the proposed dwelling performs better than the notional one.


If an element or service does not fully comply, then it is possible with some limitations to offset - that is to improve another element or service to compensate so that the overall SAP assessment is not compromised. Again, both Notional and Proposal SAP calculation are required.

Highly Glazed extensions

Offsetting is often used where more glass is required than that allowed in the regulations – a highly-glazed extension. The notional calculations reduces the area of glazing to the elemental maximum allowed. The proposed construction may contain some offsetting if required to improve performance.

Other exceptions to L1b elemental compliance

L1b also contains notes where the regulations can be disregarded to some extent including... 

  • An unreasonable reduction in internal space (e.g., head height)
  • Planning restraints and listed buildings
  • Where the insulation is not economic (15-year payback method)

These normally require expert interpretation or evidence.

What Happens If My Building Fails the SAP Calculations?

If the dwelling originally passed at design stage, then changes must have been made to the specification or the dwelling has failed to attain the air leakage target. Legally, If the dwelling fails part L, then changes should be made to upgrade it. In practice it is building control that makes the final decision. The scope to improve the SAP more expensive and difficult later in the construction project, as the scope of changes available becomes more limited. An EPC is generally issued immediately if the dwelling passes but if it fails Part L then the picture is unclear. The EPC is a final document and is difficult to cancel and amend.

It is against the law to sell or rent a property without an Energy Performance Certificate (EPC). And it is difficult to sell on an incomplete dwelling, not signed off by building control. Therefore, it is crucial that a dwelling passes its SAP/ Part L assessment. The best way to do so is to start with a compliant design ensure that the SAP is updated as changes are planned or implemented throughout the course of the build.

Design calculations are legally required prior to commencement. 

There is a legal requirement to have valid SAP calculations in place before any building or groundwork commences. SAP calculations should be done at the end of the planning stage before any construction starts – draft plan and construction details should be sent to the SAP assessor. Changes can then be made if required, based upon the results obtained and advice and guidance provided by the assessor.

New dwellings completed without a Design Assessment?

Despite the legal requirement it appears that some dwellings are unfortunately constructed without design calculations - in this case building control will still ask for final SAP calculations, an Air leakage test and an EPC. The developer will not know if the dwelling passes or not until an assessment is completed. If it fails, then building control can demand changes - not a comfortable position to be in but an experienced SAP assessor will be able to advise of any changes that are viable to gain compliance.

Imminent changes to Part L and SAP

The regulations are changing, a new set of regulations is due to come into force Part L 2021. The government is committed to reduce building emissions of greenhouse gasses. However, it has dithered and cancelled previous proposed changes so that a considerable tightening of the regulations is required if the government is to meet its climate change obligations. Consultation is finished: draft new regulations amendments to SAP have been published.

Transitional Arrangements to Part L 2021

For transitional arrangements to apply to an individual building, developers will need to both: 

  • Submit a building / initial notice or deposited plans by June 2022; and 
  • Commence work on each individual building by June 2023

Where notices or plans are submitted after June 2022, transitional arrangements will not apply, and homes must be built in line with Part L 2021 standards.

Where notices or plans are submitted before June 2022 but work on any individual building does not commence by June 2023, the uncommenced buildings must build in line with Part L 2021 standards.

Commencement defined as:

  • Excavation for strip or trench foundations or for pad footings.
  • Digging out and preparation of ground for raft foundations.
  • Vibroflotation (stone columns) piling, boring for piles or pile driving. 

Drainage work specific to the building(s) concerned.