What Part L actually asks of a development
Part L sets a performance target for each building rather than a fixed specification. For dwellings the calculation compares your design against a notional building on four measures: the Dwelling Emission Rate against its target, the Dwelling Primary Energy Rate against its target, the Dwelling Fabric Energy Efficiency against its target, and a set of limiting standards that no element may fall below. All four have to pass, which is why a scheme can meet its carbon target and still fail on fabric. For buildings other than dwellings the test compares the Building Emission Rate and Building Primary Energy Rate against their targets, and the output is a BRUKL report.
How the two stages fit together
The design-stage calculation is submitted to the building control body before work starts on the building. It proves the design as drawn and specified will comply, and it fixes the assumptions your site has to build to: U-values, air permeability, heating and ventilation specification, thermal bridging details. The as-built calculation at completion updates that model with what was actually built and installed, and is supported by the air test result, commissioning records and photographs of the key construction details. The on-construction EPC for each dwelling comes out of the as-built calculation.
- Design stage: calculation plus BREL report, before work starts
- Construction: build to the modelled specification and record the evidence
- Completion: as-built calculation, air test, commissioning records, photographs
- Handover: on-construction EPC for each new dwelling
What we need from you to start
For most schemes a design-stage model can begin with the drawings and an outline specification. The more of the specification that is fixed, the fewer iterations are needed later, but we would rather start early with assumptions clearly stated than model a scheme that has already been priced and ordered.
- Floor plans, elevations and sections, to scale, with a site plan showing orientation
- Unit schedule: how many of each house or flat type
- Outline fabric specification, or a note that it is still open
- Intended heating, hot water and ventilation strategy
- Any planning conditions affecting energy or carbon
- The building control body, if it has been appointed
Where developments most often lose money
The expensive failures are rarely the calculation itself. They are the sequencing. A design-stage model produced after the fabric has been ordered leaves no room to adjust anything cheap, so the route back to compliance becomes renewables rather than insulation. A single plot with an unusual orientation or a large glazed gable is modelled with the standard type and fails on its own. Air permeability is specified at a figure the site has never achieved, then the as-built model has to absorb a worse result. Products are substituted for equivalents that are not thermally equivalent. Each of these is straightforward to avoid if the model exists early and someone is watching it through construction.
Working with your building control body
Building work is signed off either by local authority building control or by a registered building control approver, and higher-risk buildings sit within the Building Safety Regulator's gateway regime, where Part L evidence forms part of the submission. We issue reports in the form these bodies expect and respond directly to their queries on the calculation rather than passing them back to you. We do not promise timescales or outcomes on your behalf: building control decides, and pretending otherwise would not help you plan.
