This article covers new non-domestic buildings in England under Approved Document L, Volume 2, 2021 edition (incorporating the 2023 amendments). The 2026 edition takes over from 24 March 2027; see the end of the article.
What the BRUKL report checks
The BRUKL (Building Regulations UK Part L) report is the standard output of SBEM or an approved dynamic simulation tool. It compares the actual building with a "notional building" of the same size and shape, with standard fabric and services set out in the National Calculation Methodology (NCM) Modelling Guide.
| Check | What has to be true | Where it's set |
|---|---|---|
| Carbon | Building emission rate (BER) no higher than the target emission rate (TER) | Approved Document L Vol 2, para 2.3 |
| Primary energy | Building primary energy rate (BPER) no higher than the target primary energy rate (TPER) | Para 2.3 |
| Limiting standards | Every element and system meets the minimum fabric and services standards | Section 4 (Table 4.1) and Section 6 |
| Summer solar gains | Each occupied or cooled space gains no more solar heat, April to September, than the reference glazing | Paras 4.17 and 4.18, Table 4.3 |
| As built | The targets are still met with the as-built specification and the measured air permeability | Paras 2.2, 2.5 and 7.6 |
The report is needed twice: a design-stage BRUKL before work starts, and an as-built BRUKL at completion (Appendix C). The as-built report is signed by the energy assessor and by the client.
A building can beat both targets and still fail, because of one rooflight or one air handling unit. All the checks matter.
Fail 1: emission rate or primary energy rate over target
This is the usual failure. The notional building is not built to the limiting values. Paragraph 2.9 says the targets "are not likely to be met" with fabric at the Section 4 minimums alone. A design that just meets every limiting U-value and has ordinary services will normally fail.
The electric heating trap
Part L 2021 uses the SAP 10.2 fuel factors (SAP 10.2, Table 12):
| Fuel | Carbon (kgCO₂e/kWh) | Primary energy factor |
|---|---|---|
| Mains gas | 0.210 | 1.130 |
| Grid electricity | 0.136 | 1.501 |
Electricity is now lower carbon than gas but has a higher primary energy factor. So a building heated by direct electric panels or electric water heaters often passes the TER and fails the TPER. Heat pumps reverse this. A heat pump delivering 3 kWh of heat for every 1 kWh of electricity beats gas on both counts.
Usual fixes, roughly in order of cost-effectiveness
| Fix | Why it works |
|---|---|
| Heat pumps for space heating and hot water | Lowers both BER and BPER. The biggest single lever in most buildings |
| LED lighting with good efficacy, plus daylight dimming and occupancy sensing | Lighting is a large share of the calculated energy in offices, schools and retail |
| Lower specific fan power and heat recovery on air handling units | Fan energy counts against both targets |
| Point-of-use or heat pump hot water instead of long circulated loops | Cuts distribution losses, especially in buildings with low hot water demand |
| Air permeability target better than 5 m³/(h·m²) at 50 Pa, where the contractor can deliver it | Cuts infiltration heat loss. Only claim what the air test will achieve |
| Solar PV | Offsets electricity use, so improves both rates |
| Better U-values and thermal bridging details | Helps, but on its own it rarely closes a large gap |
The cheapest moment to make these choices is at RIBA Stage 2 or 3, before the M&E design is fixed. A first-pass model with indicative inputs shows how far from target the building is and which lever matters most.
Fail 2: an element or system misses a limiting standard
The BRUKL lists every element and system against its limiting value. A single fail is a fail, whatever the targets say.
Limiting U-values and air permeability (Approved Document L Vol 2, Table 4.1)
| Element | Maximum U-value (W/m²K) |
|---|---|
| Flat roof | 0.18 |
| Pitched roof | 0.16 |
| Wall | 0.26 |
| Floor | 0.18 |
| Windows, roof windows, curtain walling | 1.6 |
| Rooflights | 2.2 |
| Pedestrian doors, including glazed doors | 1.6 |
| Vehicle access and similar large doors | 1.3 |
| High-usage entrance doors | 3.0 |
| Roof ventilators, including smoke vents | 3.0 |
| Air permeability, new buildings | 8.0 m³/(h·m²) at 50 Pa |
The fabric values are area-weighted averages for new buildings, so one poor panel can be balanced by better ones of the same type.
The services standards in Section 6 catch out as many buildings as the fabric does. Typical ones:
- General lighting: average luminaire efficacy of 95 luminaire lumens per circuit-watt, or the LENI method (paragraph 6.60).
- Specific fan power: for example, 1.9 W/(l·s) for a central balanced ventilation system with heating only, and 0.3 W/(l·s) for local extract fans such as toilet extract (Table 6.9).
- Heating, cooling and hot water plant: minimum seasonal efficiencies in Tables 6.1 to 6.8.
Common catches: curtain walling quoted at centre-pane rather than whole-system U-value, rooflights and smoke vents left at default values, older luminaires carried over from a previous scheme, and air handling units selected before the SFP is checked.
Fail 3: too much summer solar gain
For buildings other than residential ones, each space that is occupied or mechanically cooled must not gain more solar heat through its glazing, from April to September, than a reference glazing system would (paragraph 4.18). Residential buildings follow Approved Document O instead.
| Type of space | Reference glazing (Table 4.3) |
|---|---|
| Side-lit | East-facing, full width to 1,000 mm high, 10% frame, g-value 0.48 |
| Top-lit, zone up to 6 m high | Rooflights over 10% of the roof area, 25% frame, g-value 0.48 |
| Top-lit, zone over 6 m high | Rooflights over 10% of the roof area, 15% frame, g-value 0.42 |
Fully glazed south- and west-facing offices, receptions and classrooms are the usual fails. The fixes are a lower g-value, external shading such as fins, brise soleil or overhangs, or less glass on the worst elevations. Circulation spaces and toilets are excluded, so moving glazing to a stair core can also help.
A pass here doesn't mean the space won't overheat. If comfort matters, a TM52 assessment is the right test.
Fail 4: the as-built BRUKL
The as-built calculation uses what was actually installed and the measured air permeability (paragraph 2.2). Both targets must still be met (paragraph 7.6). The usual causes of an as-built fail:
- Air test worse than the design value. The building has to be improved and retested until it passes (paragraph 7.7), and every test result, including the failures, goes to building control (paragraph 7.8).
- Substituted plant. A cheaper heat pump with a lower seasonal efficiency, a different air handling unit or a value-engineered lighting scheme.
- Lost PV. Arrays reduced or dropped late in the programme.
The software compares the design and as-built inputs and lists every change (Appendix C, paragraph C3), so substitutions are visible to building control.
Buildings with less than 500 m² total useful floor area don't need an air test, provided the calculation uses 15 m³/(h·m²) at 50 Pa (paragraph 7.5). That penalty is large, so a test is often worth doing anyway.
Before the design-stage BRUKL: a checklist
- Agree SBEM or dynamic simulation early. Complex buildings, atria and mixed-mode ventilation often suit dynamic simulation.
- Confirm the heating and hot water strategy. Direct electric heating is likely to fail the primary energy target.
- Get whole-unit U-values for windows, curtain walling, rooflights and doors, not centre-pane figures.
- Get the lighting design's luminaire efficacies and controls.
- Get SFP figures for every fan system and check them against Table 6.9.
- Set an air permeability target the contractor has agreed to.
- Check glazed spaces facing south and west against the solar gain limit.
- Allow roof space for PV, even if it may not be needed.
- Run the model at Stage 2 or 3, not at tender.
Changes from 24 March 2027
The 2026 edition of Approved Document L, Volume 2 takes effect on 24 March 2027, and on 24 September 2027 for higher-risk buildings. Transitional arrangements are in Circular Letter 01/2026 on gov.uk. The BRUKL report stays, with a design-stage and an as-built version, but the targets and limiting values change. Check which edition your project falls under before the design-stage model is built.
More on the service: Part L2 BRUKL and EPC assessment and dynamic simulation modelling.
