Approved Document Part L (Conservation of fuel and power) directly affects almost every calculation an MEP engineer runs. Whether you're sizing ventilation, specifying insulation, or selecting fan equipment, Part L sets the benchmarks. Here's what matters.
Which Part L?
Part L was updated in 2021, with the new edition taking effect in June 2022. It comes in two volumes: Volume 1 covers dwellings, and Volume 2 covers buildings other than dwellings. Most MEP engineers working on commercial, education, or healthcare projects will reference Volume 2. Always verify you're using the 2021 edition — the 2013 version is superseded and should not be used for new projects.
SFP limits (Specific Fan Power)
This is the section engineers search for most. SFP limits determine whether your ventilation system is compliant.
| System type | SFP limit W/(L/s) |
|---|---|
| Central mechanical ventilation (specific) | 1.6 |
| Central mechanical ventilation with cooling | 2.2 |
| Local mechanical ventilation (supply or extract) | 0.5 |
| Fan coil units | 0.8 |
| Terminal units | 1.2 |
Source: Approved Document L, Volume 2: Buildings other than dwellings (2021 edition)
SFP is calculated as total fan power (W) divided by design flow rate (L/s). This includes the motor, drive, and any inverter losses. If your system exceeds these limits, you need to justify it or redesign.
Quick check: Use our free SFP Calculator to verify your fan selection meets Part L limits before you submit.
U-value targets
Key U-value limits for non-domestic buildings (limiting fabric parameters):
| Element | Limiting U-value W/(m²·K) |
|---|---|
| Wall | 0.26 |
| Floor | 0.18 |
| Roof | 0.16 |
| Windows (area-weighted) | 1.6 |
| Curtain walling | 1.6 |
These are the “limiting” values — actual Part L compliance is demonstrated via the notional building method (SBEM or DSM), not just by meeting these U-values. However, no individual element should exceed these limits.
How Part L affects MEP design
Fan selection is directly constrained by SFP limits. You can't just pick the cheapest fan — the combination of fan efficiency, motor efficiency, and system resistance determines SFP. Good duct design with low pressure drop is the most effective way to keep SFP within limits, because it reduces the total pressure the fan needs to overcome.
Part L's fabric performance targets directly affect heating and cooling load calculations. Better fabric (lower U-values, better airtightness) means lower peak heating loads, which means smaller plant. The difference between internal design temperature and external design temperature drives heat loss, so fabric improvements by the architect can reduce the size and cost of your mechanical systems.
Lighting is also covered. Part L sets maximum lighting energy densities (in W/m²) for different space types, which affects electrical load calculations and, indirectly, cooling loads. Higher-efficiency luminaires reduce both the electrical demand and the internal heat gain that the cooling system needs to handle.
Finally, Part L expects certain HVAC control strategies to be in place. Weather compensation for heating systems, optimum start/stop control, and zone-level time and temperature control are all referenced. These aren't optional extras — they're expected as part of a compliant design.
Related free tools
MEP Desk tracks Part L alongside 62 other UK building services standards, and includes calculators that help you design compliant systems from the start.
Check SFP compliance → Try MEP Desk free →