Glossary

Every Part L term we use, without the jargon

46 definitions covering Part L, SAP, SBEM and the compliance evidence, in the language a building control body expects. Each one is linked automatically the first time it appears in a guide on this site.

46 of 46 terms

accreditation scheme

An approved body, such as Elmhurst Energy or ECMK, that qualifies, audits and maintains standards for the assessors permitted to produce and lodge calculations.

air permeability

A measure of uncontrolled air leakage through the building fabric, tested on completion and fed into the as-built calculation.

air pressure test

The on-site test that measures how much air leaks through a completed building's fabric, carried out by a competent tester and reported as part of the compliance evidence.

Approved Document L1

The volume of Approved Document L that applies to dwellings, covering both new dwellings and work to existing homes.

Approved Document L2

The volume of Approved Document L that applies to buildings other than dwellings, covering new non-domestic buildings and work to existing ones.

Approved Document L3

The volume introduced with Part L 2026 dealing with on-site renewable electricity generation for new dwellings and buildings containing dwellings.

as-built SAP

The SAP assessment reworked at completion using what was actually built and installed, including the measured air tightness result and any specification changes.

BER

The carbon emission rate calculated for a non-domestic building, which must not exceed the target emission rate for that building.

BPER

The primary energy rate calculated for a non-domestic building, compared against its target primary energy rate.

BREL report

The standard output produced from a SAP assessment for new dwellings, submitted with photographic and commissioning evidence to demonstrate Part L compliance.

BRUKL report

The compliance output produced by SBEM or an approved dynamic simulation model for non-domestic buildings, showing the building's rates against the targets.

building control body

The local authority or registered building control approver that checks a project against the Building Regulations and receives the Part L compliance documents.

commissioning

Setting up and testing building services on completion so they operate as designed, evidenced by records submitted with the compliance documents.

consequential improvements

Additional energy efficiency measures that may be required when certain work is carried out to larger existing buildings.

DER

The carbon dioxide emission rate calculated for the dwelling being assessed, which must not exceed the target emission rate.

design stage SAP

The SAP assessment produced from drawings and specifications before work starts, used to demonstrate that the design as proposed can satisfy Part L.

DFEE

The space heating and cooling demand of the dwelling as designed, a fabric-only measure independent of the heating system.

DPER

The primary energy demand calculated for the dwelling being assessed, which must not exceed the target primary energy rate.

dynamic simulation modelling

An hourly thermal model of a building used in place of SBEM where the design is complex, and required for some servicing strategies and overheating work.

fabric performance

The thermal performance of the walls, roof, floors, windows and doors that form the building envelope, before heating systems are considered.

g-value

The proportion of solar energy passing through a glazing unit. Higher g-values bring more useful winter heat but increase overheating risk in summer.

glazing ratio

The relationship between glazed area and floor area used in extension work, where Part L allows glazing equal to 25% of the new floor area plus the area of any openings removed.

heat pump

A system that moves heat from outside air, ground or water into a building, typically delivering low carbon heating at high seasonal efficiency.

higher-risk building

A building falling within the higher-risk category under the building safety regime. Higher-risk buildings have their own, later commencement date under the Part L 2026 arrangements.

material change of use

Work that changes what a building is used for, such as an office becoming flats. It brings specific Part L requirements for the elements being altered or created.

notional dwelling

A reference building of the same size and shape as the actual one, built to the specification set out in Part L, from which the targets are calculated.

on-construction EPC

The Energy Performance Certificate produced for a newly built dwelling from the as-built calculation, and lodged before the building is occupied, sold or let.

overheating assessment

An assessment of the risk of high indoor temperatures in new residential buildings, carried out under the overheating requirements of the Building Regulations.

Part L

The part of the Building Regulations covering conservation of fuel and power. It sets the energy and carbon standards new and altered buildings must meet, and is supported by Approved Documents L1 and L2.

Part L 2021

The edition of Approved Document L that came into force on 15 June 2022, introducing tighter fabric and primary energy standards and the BREL report for new dwellings.

Part L 2026

The edition of Approved Document L published on 24 March 2026, which raises the standards for new buildings and adds requirements on on-site renewable electricity generation.

photographic evidence

Dated site photographs recording insulation, junctions and services as they were installed, required as part of the compliance evidence for new dwellings.

renewable technologies

On-site generation and low carbon systems such as solar photovoltaics and heat pumps, which reduce the emission and primary energy rates in a Part L calculation.

SAP

The Government methodology for assessing the energy performance of dwellings against Part L, carried out from design information rather than a site survey.

SBEM

The calculation tool most commonly used to assess non-domestic buildings against Part L, modelling the building zone by zone against a notional equivalent.

solar PV

Panels that generate electricity from sunlight. Under Part L 2026 on-site renewable electricity generation is dealt with in its own Approved Document.

specification change

A product or system swapped after the design stage assessment. Every substitution has to be re-checked, because a like-for-like swap on paper can still fail the calculation.

TER

The carbon emission rate a dwelling must beat, derived from a notional dwelling of the same size and shape built to the standards in Part L.

TFEE

The fabric energy efficiency limit a dwelling must meet, so a good heating system cannot be used to excuse weak insulation and glazing.

thermal bridging

Heat loss concentrated at junctions and penetrations in the building fabric, accounted for in calculations either through accredited details or calculated psi values.

TPER

The primary energy limit a dwelling must meet, derived from the notional dwelling used in the SAP calculation.

transitional arrangements

The rules that decide whether a project is assessed under the outgoing or incoming edition of Part L, based on when the application was made and when work started on that individual building.

U-value

A measure of how quickly heat passes through a building element such as a wall, roof, floor or window. Lower U-values mean better performance.

ventilation strategy

The way fresh air is supplied and stale air removed, from trickle ventilators and extract fans to mechanical ventilation with heat recovery. It must work alongside the airtightness assumed in the calculation.

y-value

The overall heat loss allowance from thermal bridging used in a SAP calculation, either a default figure or one built up from individual junction psi values.

zoning

Dividing a non-domestic building into thermal and activity zones so each area is modelled with its own use, servicing and occupancy pattern.

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