Part L For Water Heating In New Build Properties

The minimum energy efficiency requirements for non-domestic buildings are set by Part L (Conservation of fuel and power) of Schedule 1 and Part 6 of the Building Regulations. And with the Future Homes and Buildings Standards (FHBS), these regulations set even more ambitious requirements for energy efficiency and heating for buildings. The landscape for commercial water heating changed significantly in March 2026 with the publication of the Approved Document L, Volume 2 (2026 edition). This update, which supports the full implementation of the FHBS, marks the most aggressive shift yet toward zero-carbon-ready non-domestic buildings.

They intend that no further work will be needed for new buildings to produce zero carbon emissions as the electricity grid decarbonises. This brings building regulations in line with the government’s stated aim of meeting a 2050 net zero target in the UK. While much of the spotlight shines on building fabric and heating systems, the requirements for Domestic Hot Water (DHW) are rigorous, reflecting the fact that in many commercial settings – like hotels, gyms, schools and hospitals – water heating is a primary energy consumer.

For new builds, the regulation moves away from merely adequate efficiency and pushes toward a holistic performance model. In a new build, your water heating system isn’t judged in a vacuum. It contributes to the building’s overall performance against two mandatory targets: Target Emission Rate (TER), which is the maximum CO² emissions allowed per square meter per year, and the Target Primary Energy Rate (TPER). TPER is a metric that accounts for the energy used to produce the fuel (for example, the efficiency of the grid for electricity vs. the extraction of gas). Because the notional building (the theoretical benchmark) is now modelled with high-efficiency technology – often including heat pumps – installing an inefficient DHW system makes it nearly impossible to pass the overall building compliance.

The 2026 edition (effective for work starting from March 2027) moves beyond the incremental uplifts of 2021/2023. So what do these latest requirements mean for water heating in new build non-dwellings?

The Notional Buildings

The most critical change in the 2026 edition is the reconfiguration of the Notional Building. Compliance is determined by comparing your building to a theoretical ‘perfect ‘ version of itself. In previous versions, the notional building often used a high-efficiency gas boiler as a benchmark. The 2026 standards now assume low-carbon heat sources (typically air-to-water or water-to-water heat pumps) for domestic hot water (DHW).

While gas is not technically banned, the Target Emission Rate (TER) and Target Primary Energy Rate (TPER) are now so low that using a gas-fired water heater requires massive offsetting elsewhere, such as extreme levels of fabric insulation, solar thermal or massive solar PV arrays, making low-carbon solutions the only practical choice for most new builds.

Low-Carbon Mandates

The 2026 amendments to Part L further tighten the minimum performance for hot water generators.

As such, the regulations heavily favour heat pumps as part of the UK’s decarbonisation strategy. For DHW, heat pumps must meet a minimum seasonal coefficient of performance (SCOP), which has been raised to reflect technological maturity, typically requiring a system-wide efficiency of 2.5 to 2.8, depending on the application, to meet the TER.

Direct electric water heating is permitted but discouraged for large-scale use because electricity has a higher ‘primary energy’ factor than on-site renewables. It is generally reserved for point-of-use applications where the demand is very low, such as a single sink in a remote bathroom.

Connection to low-carbon heat networks is now prioritised. If a building is within a Heat Network Zone, the DHW system must be designed to be ‘connection-ready’ or utilise the network as the primary source.

For any remaining gas applications (such as specific industrial processes and special exceptions), non-condensing water heaters are essentially obsolete. Direct-fired condensing heaters must now exceed 92% Gross Thermal Efficiency. For indirect systems, if the DHW is provided by the main space-heating boiler, that boiler must meet the strict seasonal space heating efficiencies (93% for gas).

On-Site Generation, Storage & Distribution

A major addition in the Part L March 2026 update is the integration of Requirement L2 (On-Site Generation of Electricity) into the building services strategy.

New builds are now expected to include significantly more on-site renewable generation, with a focus on Solar PV integration (space heating) and particularly solar thermal for DHW.  The guidance encourages using DHW storage cylinders as ‘thermal batteries’ to soak up excess solar electricity generated on-site, using smart immersion controllers to pre-heat water when the sun is shining.

Generating heat efficiently is pointless if it leaks out before reaching the tap. To minimise ‘parasitic’ energy loss from pipes and tanks, the 2026 edition introduces stricter insulation requirements to limit standing losses. Part L already covered insulation of hot water cylinders to meet or exceed the loss rates specified in the document (often referencing BS 1566 or BS EN 12897), whilst all primary pipework (connecting the heat source to the cylinder) and secondary pipework (the circulation loop that keeps water hot at the tap) must be insulated. The latest update revises pipework insulation thickness requirements. For example, secondary circulation loops in large buildings must use insulation with lower thermal conductivity to prevent the plant room from overheating and the water from cooling. There is also renewed emphasis on insulating omitted items like valves and flanges in commercial plant rooms, which were historically left bare for maintenance but are now recognised as major heat-loss points. This is in addition to existing regulations on the length of uninsulated pipes leading from the main loop to the tap (dead legs) which must be kept to a minimum to prevent water and energy waste while waiting for hot water to arrive.

Commissioning and Documentation

A system is not considered compliant until it is proven to work as designed. The installer must therefore test the DHW system, balance the primary circuits, and ensure all controls are calibrated. A commissioning notice must be given to the Building Control Body (BCB).

For new builds, the Building Regulations England Part L (BREL) report is now a mandatory, strictly formatted document for non-dwellings, mirroring the rigour previously seen in housing. Geo-tagged, time-stamped photographs of the hot water plant, pipework insulation (at intersections and valves), and metering installations must be provided by assessors to prove that what was designed has been installed, before they are covered by finishes. A formal Certificate of Commissioning must also be provided, signed by a competent person, confirming that the DHW flow rates, storage temperatures, and controls operate exactly as specified in the SBEM (Simplified Building Energy Model) design.

Building Handover

The Part L March 2026 update places a heavy burden on usability. It is no longer enough to install an efficient system; the building owner must know how to use it. Clear instructions on how to operate the DHW system efficiently must include a ‘non-technical summary’ of the hot water system, explaining how the heat pump and controls interact.

But you can’t manage what you don’t control. New non-domestic water systems must be smart enough to shut down when not needed. DHW systems must be controlled independently of space heating. They should have dedicated timers and thermostats for temperature control. If the system uses a pump to circulate hot water around the building, that pump must be controlled by a time switch. This ensures systems don’t run, for example, at 3:00 AM in an empty office block.

While Part L focuses on energy, it acknowledges that DHW must still be stored at 60°C to prevent Legionella. Controls must balance this safety requirement with energy-saving ‘setback. modes during unoccupied hours.

For buildings with a total useful floor area over 500m², Approved Document L requires the installation of metering (enhanced sub-metering). This means the energy used specifically for water heating must be recorded separately. This data allows building managers to identify inefficiencies post-occupancy, which forms a key part of the ‘Performance Gap’ initiative.

For more complex buildings, there is a push (though not always mandatory) toward providing digital operation and maintenance manuals that include real-time performance tracking for the DHW system.

Part L essentially treats hot water as a critical infrastructure component, with the March 2026 amendments representing the final closing of the door on fossil-fuel dominance in new-build water heating, shifting the industry toward an ‘electric first’ infrastructure. Specifying the absolute minimum efficiency levels for hot water generators ensures that if you fall below these, the building fails, regardless of how good the building insulation is. By enforcing high generator efficiency, mandatory system insulation, and always-on monitoring, the regulations ensure that the hot water in commercial spaces doesn’t come at an undue cost to the planet.

Visit:

https://www.gov.uk/government/publications/approved-document-l-2026

For information on commercial heat pumps for DHW

For information on commercial solar thermal systems

For information on commercial hybrid electric water heating

For information on commercial-grade cylinders for water heating and storage