Every new non-domestic building in England must show compliance with Part L of the Building Regulations, which covers the conservation of fuel and power. Compliance is demonstrated with approved calculation software: SBEM for simpler buildings, or Dynamic Simulation Modelling such as DesignBuilder for more complex designs.
Used early, DesignBuilder becomes a design tool, not just a compliance check.
What Part L requires
Part L sets targets for primary energy and CO2 emissions, compared against a notional building of the same size and shape. It also sets minimum standards for fabric, building services efficiency and controls, and requires checks on solar gains and overheating risk.
Two calculations are normally produced: a design-stage calculation before work starts, and an as-built calculation on completion, which also generates the building's first EPC.
Why use DSM rather than SBEM?
DSM simulates the building hour by hour, so it can represent features SBEM handles poorly: atriums, natural and mixed-mode ventilation, complex shading, heat recovery strategies and advanced controls.
It also gives design teams useful extra outputs, such as comfort and overheating analysis, daylighting and plant sizing, from the same model.
Model early, avoid redesign
The most expensive Part L problems are found late. If glazing ratios, fabric or plant choices are fixed before a compliance model is run, failing the targets can mean redesign or costly late additions such as extra PV.
An early DesignBuilder model lets the team test options while they are still cheap to change, and builds the evidence trail needed at as-built stage.
Design stage to as-built
Changes during construction (different glazing, plant substitutions, value engineering) must be reflected in the as-built model. Keeping the model current through the project avoids a last-minute failure at completion.
Air-tightness testing and commissioning results feed into the final calculation.
How we help
We provide design-stage and as-built Part L calculations, option testing and EPC lodgement for non-domestic projects. Get in touch to discuss your scheme.
This article is general guidance for England. Wales, Scotland and Northern Ireland have their own energy regulations.
Frequently asked questions
- What is Part L?
- Part L of the Building Regulations covers the conservation of fuel and power, setting energy and emissions targets plus minimum fabric and services standards.
- When do I need a Part L calculation?
- For new non-domestic buildings, a design-stage calculation before work starts and an as-built calculation on completion. Some extensions and refurbishments also have Part L requirements.
- Can DesignBuilder be used for Part L?
- Yes, DesignBuilder is a widely used approved DSM tool for non-domestic Part L calculations, alongside SBEM for simpler buildings.
- What happens if a design fails Part L?
- The design has to change, for example better fabric, more efficient plant or renewables, until it meets the targets. Early modelling makes this far cheaper.