Building Regulations Part L 2021: A Tradesman's Guide

What Is Part L and Why Did It Change in 2021?

Approved Document L of the Building Regulations covers the conservation of fuel and power in buildings across England. The 2021 edition came into force on 15 June 2022, with a transition period running to 15 June 2023 for projects that were already under way. It was a significant step toward the UK's net-zero targets.

The headline change for new homes was a 31% reduction in carbon emissions compared to the 2013 standard, achieved primarily through tighter fabric requirements, improved heating system efficiency, and mandatory heating controls. For tradesmen working on existing dwellings or existing non-domestic buildings, the changes are equally important — particularly when you're replacing a boiler, upgrading lighting, or altering hot water systems.

The 2021 edition also restructured the guidance. The previous four documents (L1A, L1B, L2A and L2B) were replaced by two volumes, each covering both new and existing buildings:

• Volume 1: Dwellings — new and existing homes
• Volume 2: Buildings other than dwellings — new and existing non-domestic buildings

Most day-to-day trade work on homes falls under Volume 1, so that's where we'll focus, with notes on Volume 2 where the requirements differ.

What It Means for Gas Engineers

Boiler Efficiency Standards

When replacing a boiler in an existing dwelling under Part L (Volume 1), the replacement unit must achieve a minimum seasonal efficiency of 92% (ErP) for gas boilers. In practice, every modern condensing combi or system boiler sold today meets this threshold, but it's still your responsibility to verify the ErP rating before specifying.

Oil-fired boilers have a separate pathway and must also be condensing unless a specific exemption applies — for example, where there is no suitable drain for the condensate, or where the boiler serves a listed building.

Heating Controls — This Is Where Engineers Get Caught Out

Part L (Volume 1) requires that whenever a boiler is replaced, you must also bring the controls up to current standards unless the existing controls already comply. The minimum required controls for a wet central heating system are:

• A programmer or time switch (separate timing for heating and hot water where applicable)
• A room thermostat (or equivalent — smart thermostat, programmable room thermostat)
• Thermostatic radiator valves (TRVs) on all radiators except the one in the room with the room thermostat
• Boiler interlock — the boiler must not fire when there is no heat demand

These requirements are set out in the Domestic Building Services Compliance Guide (DBSCG), which Approved Document L references and which carries equal weight. For combi boilers, the Boiler Plus standard also requires one additional energy-saving measure — a smart thermostat with automation and optimisation, weather compensation, load compensation, or flue gas heat recovery. If you're replacing a boiler and the existing system only has a simple on/off timer and no TRVs, you need to fit the full control package — or document a valid reason why you can't.

Hot Water Cylinder Insulation

If you're installing or replacing an unvented or vented hot water cylinder, Part L (Volume 1) sets a minimum insulation standard via the DBSCG: factory-insulated cylinders must meet a maximum standby heat-loss limit that scales with cylinder volume. In practice, any current cylinder marked as compliant from a reputable manufacturer will meet it, but you should retain the manufacturer's data sheet as evidence.

What It Means for Electricians

Lighting Efficiency

Under Part L (Volume 1), when you're carrying out new or replacement fixed lighting in a dwelling, at least 75% of fixed light fittings must use lamps with a luminous efficacy of at least 75 lumens per circuit-watt — raised from 45 lm/W in the 2013 edition. LED fittings comfortably exceed this, and for most domestic jobs this requirement is straightforward to meet.

For non-domestic buildings (Volume 2), the requirements are more demanding and lighting controls — including daylight sensing, occupancy detection and local switching — may be required depending on the scale of the work.

When Does Part L Apply to Electrical Work?

Part L triggers for electrical work when you are creating a new dwelling, carrying out an extension, or making a material change of use. For like-for-like replacements (a new socket, a new circuit for an existing purpose), Part L generally doesn't impose additional obligations. However, if you're installing new fixed building services — such as mechanical ventilation with heat recovery (MVHR), underfloor heating, or a heat pump — Part L applies to those systems and their controls.

Heat Pumps and Low-Carbon Systems

With the push toward heat pumps under both Part L 2021 and the forthcoming Future Homes Standard, electricians are increasingly commissioning heat pump systems. The Domestic Building Services Compliance Guide sets out minimum efficiency requirements for heat pumps installed in existing dwellings, measured to EN 14825, and installation to the MCS standard is the usual route to demonstrating compliance.

What It Means for Plumbers

Pipe Insulation

One of the practical requirements most often overlooked on the tools is pipe insulation. Approved Document L (Volume 1) and the DBSCG both require that primary pipework — the flow and return pipes between the boiler and the hot water cylinder — is insulated wherever it passes through an unheated space. The insulation thickness required depends on pipe diameter and whether the pipe is in a heated or unheated area, but as a practical rule:

• 22mm pipe in an unheated space: around 25mm of insulation
• Pipes in a heated space still require insulation if they're primary circulation pipes
• Cold water supply pipes in roof voids must also be insulated against frost risk

The relevant standard for pipe insulation is BS 5422, with practical thicknesses summarised in the TIMSA guidance the DBSCG points to. Using compliant pre-formed foam lagging in the correct thickness is the simplest way to meet this requirement.

Hot Water Temperature and Legionella

Part L also interacts with the requirement to store hot water at 60°C to control Legionella risk (under HSE L8 guidance). Some energy efficiency measures — such as lowering cylinder temperatures to reduce heat loss — conflict with this. Always store hot water at 60°C and distribute at no less than 55°C to comply with both L8 and maintain Part L compliance through system efficiency rather than lower temperatures.

Documentation and Certification

What You Need to Leave Behind

Part L (Volume 1) requires that on completion of any notifiable work, the building owner receives a Building Regulations Compliance Certificate (issued via Building Control or a Competent Person Scheme) along with documentation confirming:

• The make, model and ErP rating of any boiler or heat generator installed
• Details of the controls installed and their configuration
• A copy of the system commissioning record
• Manufacturer data sheets or product certificates for energy-relevant equipment

For Gas Safe registered engineers, the boiler installation is self-certified through Gas Safe, which satisfies the Building Regulations notification requirement. However, the controls and any associated electrical work may require separate notification through a Competent Person Scheme such as NAPIT or NICEIC if the work is notifiable under Part P.

Commissioning Records

The DBSCG specifies that all new or replacement heating systems must be commissioned in accordance with the manufacturer's instructions and the relevant industry standard. For wet heating systems, this means following BS 7593:2019 (treatment of water in domestic hot water central heating systems) — including system flushing, inhibitor dosing, and a record of the inhibitor type and concentration left in the system.

This commissioning record must be left with the homeowner and is increasingly being requested by letting agents, home warranty providers, and energy assessors. Keeping a digital copy on file protects you professionally.

Common Compliance Mistakes

• Forgetting controls on a boiler swap — the most common Part L failure. Replacing a boiler is not just about the boiler; the controls package must comply too.
• No pipe insulation in the loft — particularly when a cylinder has been moved or primary pipework has been extended. Don't leave uninsulated primaries in a cold roof void.
• Assuming the customer's existing cylinder is compliant — older cylinders (pre-2006) often don't meet current insulation standards. If you're doing a full system replacement, specify a new cylinder.
• Not documenting exemptions — if there is a genuine reason why you can't fully comply (listed building, existing pipework constraints), document it. An undocumented non-compliance looks like negligence; a documented one with reasons looks like professional judgement.

Looking Ahead: The Future Homes Standard

Part L 2021 is a stepping stone, not the final destination. The Future Homes Standard, expected in 2025 but delayed and now being introduced from 2026, will require new homes to produce 75–80% fewer carbon emissions than a 2013 baseline, effectively ending gas boiler installations in new builds. Our Future Homes Standard guide covers what's changing in more detail. Getting comfortable with the current Part L requirements now puts you in a strong position to adapt as the standards tighten further.

For existing dwellings, an Existing Homes Standard has also been consulted on and will likely bring stricter retrofit requirements — making the fabric-first, controls-compliant approach outlined in the current Part L (Volume 1) increasingly important for your everyday work.

Published 2026-06-23. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.