Part L Building Regulations: Lighting Requirements Explained

What is Part L?

Part L is the section of the Building Regulations for England that deals with conservation of fuel and power. It applies to new buildings, extensions, and certain refurbishment works including lighting system replacements.

The 2021 edition (effective June 2022) introduced significantly tighter carbon targets — a 27% reduction in CO₂ emissions for non-domestic buildings compared to the 2013 edition. Lighting is one of the main regulated energy uses and is therefore a key lever for compliance.

Scotland (Section 6) and Wales (Part L Wales) have equivalent but separate regulations with similar requirements.

Lighting Requirements: Efficacy

Part L 2021 sets minimum lamp and luminaire efficacy requirements:

• General lighting in offices, retail, industrial and educational spaces must achieve a minimum luminaire efficacy of 95 luminaire-lumens per circuit watt (lm/W)

• Display lighting must achieve a minimum of 80 lm/W

• Emergency lighting is exempt from efficacy requirements but must comply with BS 5266

In practice, modern LED luminaires comfortably exceed these thresholds — most EOS products achieve 130–160 lm/W. However, specifiers must verify efficacy from TM66 or photometric test data, not from LED chip claims which do not account for driver, optical and thermal losses.

LENI — Lighting Energy Numeric Indicator

LENI is a metric expressed in kWh/m²/year that quantifies the total annual lighting energy consumption of a building, including parasitic power (emergency lighting, controls, drivers on standby).

Part L uses the LENI approach from BS EN 15193-1 to assess lighting energy performance within the National Calculation Methodology (NCM). The calculated LENI feeds into SBEM (Simplified Building Energy Model) or approved dynamic simulation software.

Key factors that reduce LENI:

• Lower installed power density (W/m²) through high-efficacy luminaires • Automatic absence detection — reducing operating hours in intermittently occupied areas • Daylight-linked dimming — reducing output near windows when daylight is sufficient • Task-ambient design — lower ambient levels supplemented by efficient task lighting • Effective commissioning — ensuring controls are programmed correctly and sensors are positioned optimally

Lighting Controls Under Part L

Part L 2021 mandates automatic lighting controls in most non-domestic spaces:

• Presence/absence detection is required in spaces that are intermittently occupied (toilets, corridors, meeting rooms, store rooms)

• Daylight-linked dimming is required in spaces with significant daylight contribution (within 6m of a window wall or below a rooflight)

• Time scheduling must be provided for all lighting circuits to ensure lights switch off outside occupied hours

• Scene-setting or dimming capability should be available in spaces where activities vary

DALI-2 addressable systems satisfy all of these requirements through a single wired infrastructure, with the added benefit of energy monitoring, remote diagnostics, and future adaptability.

How to Comply with Part L for Lighting

A practical compliance checklist:

1. Specify LED luminaires with ≥ 95 lm/W circuit efficacy (ideally ≥ 130 lm/W to build in margin)

2. Design to target installed power densities below the NCM benchmarks — e.g., ≤ 8 W/m² for cellular offices, ≤ 10 W/m² for open-plan

3. Include DALI presence/absence detection in all intermittently occupied spaces

4. Provide daylight dimming on all perimeter zones within 6m of glazing

5. Include time scheduling for all circuits

6. Commission controls properly — incorrect sensor placement or programming undermines the design intent

7. Provide a Building Log Book (as required by Part L) documenting the lighting control strategy and maintenance requirements

EOS provides full Part L compliance support including LENI calculations, lighting designs and controls specifications.