Design Stage BRUKL Report

A design stage BRUKL report is the first of two compliance submissions required for every new non-domestic building under Part L of the Building Regulations. It must be provided to the building control body before building work begins, demonstrating that the proposed design meets both the Target Primary Energy Rate (TPER) and Target Emission Rate (TER) calculated from the notional building (AD L2 2026, Appendix C).

When is a design stage BRUKL required?

The design stage BRUKL report must be submitted alongside the building control application or initial notice. Building control bodies — whether local authority or approved inspectors — require it as evidence that the proposed design will comply with Part L before construction is authorised (AD L2 2026, Regulations 27 and 27C).

The requirement applies to:

• All new non-domestic buildings
• Large extensions treated as new buildings (both greater than 100 m² and greater than 25% of the existing building's total useful floor area)
• Change of use and major renovation work where Part L compliance must be demonstrated through the National Calculation Methodology

Without a satisfactory design stage BRUKL, the building control application cannot proceed. This makes early engagement with a qualified energy assessor essential — ideally at RIBA Stage 3 or earlier.

What must a design stage BRUKL contain?

The design stage report is produced by SBEM (via the iSBEM interface) or an approved Dynamic Simulation Model. It must include (AD L2 2026, Appendix C):

• Target Primary Energy Rate (TPER) and Building Primary Energy Rate (BPER) — expressed in kWh_PE/(m²/year), demonstrating the proposed design does not exceed the target
• Target Emission Rate (TER) and Building Emission Rate (BER) — expressed in kgCO₂/(m²/year)
• Building geometry — floor areas, zone definitions, envelope areas and orientations
• Fabric specification — U-values for walls, roof, floor, windows and doors, all meeting the limiting values in Table 3.1
• Design air permeability — the target value set at design stage, which must not exceed the limiting air permeability (AD L2 2026, Section 3)
• Thermal bridging assumptions — either calculated junction details (following BRE BR 497) or the generic uplift from BRE IP 1/06
• HVAC system specifications — heating, cooling, ventilation and hot water, including system efficiencies and fuel types
• Lighting specification — luminaire efficacy or LENI calculation
• Renewable energy contributions — including solar PV capacity, orientation and tilt

Level of design detail needed

A common misconception is that the design stage BRUKL can be based on outline specifications. In practice, the calculation requires a substantial level of detail:

• Fabric — specific U-values for each element type, not just “to be confirmed” placeholders. The assessor needs construction build-ups or manufacturer data
• HVAC — system types, fuel sources, efficiencies (COP/SCOP for heat pumps, SEER for cooling) and control strategies. Default values exist in SBEM but produce conservative results that may cause failure
• Lighting — average luminaire efficacy across each zone, or a full LENI calculation. Under the FBS, the notional building assumes 105 lm/W
• Renewables — PV panel capacity, array orientation, tilt angle and shading factors. The FBS notional building includes PV at 40% of foundation area for both side-lit and top-lit zones (consultation response, para 3.11, p18); no PV if 100% heat pump heating (NCM 2021, para 83)
• Air permeability — a realistic design target. Setting an ambitious target at design stage creates risk if the building cannot achieve it during pressure testing

The more accurate the design stage inputs, the less likely the project is to encounter compliance problems at the as-built stage. Assessors should push back on incomplete design information rather than relying on assumptions that may not hold.

Relationship to building control application

The design stage BRUKL forms part of the evidence package submitted to building control. Under Regulations 27 and 27C, a notice must be given to the building control body of the energy calculations before works begin (AD L2 2026). In practice, this means:

• The design stage BRUKL is submitted with the full plans application or initial notice
• Building control checks that both BPER ≤ TPER and BER ≤ TER
• The commissioning plan should also be provided at this stage, identifying which systems will be tested, the testing schedule and documentation requirements (AD L2 2026, para 7.5)
• If the BRUKL shows non-compliance, building control will require design changes before approving the application

Common reasons for failure

Design stage BRUKL submissions fail for predictable reasons. Understanding these helps design teams avoid delays:

• Fossil fuel heating — specifying gas boilers when the notional building assumes low-carbon heating creates a large emissions gap that is very difficult to bridge through other measures
• Insufficient PV — the FBS notional building includes PV at 40% of foundation area for both zone types (consultation response, para 3.11, p18). Omitting or undersizing PV makes it extremely hard to meet the TPER
• Poor fabric specification — while Table 3.1 sets limiting U-values (e.g. 0.26 W/m²K for walls), the notional building uses significantly better values (0.18 W/m²K for walls). Designing only to the limiting values will not achieve compliance
• Overreliance on SBEM defaults — default values for unspecified systems are deliberately conservative. Submitting a BRUKL with multiple defaulted inputs almost always fails
• Unrealistic air permeability targets — setting an ambitious design air permeability (e.g. 2.0 m³/(h·m²)) helps the design stage calculation but creates risk if the building cannot achieve it during pressure testing
• Lighting below 105 lm/W — the FBS notional building assumes this efficacy. Specifying less efficient luminaires adds to the compliance burden on other elements

What happens if the design changes after submission?

Design changes after the design stage BRUKL has been submitted are common. The key question is whether those changes affect Part L compliance:

• Minor changes that do not affect energy performance (e.g. internal partition layouts, finishes) do not require a revised BRUKL
• Material changes to the building envelope, HVAC systems, lighting or renewables require the assessor to re-run the calculation and produce an updated design stage BRUKL. This must be resubmitted to building control
• System substitutions — swapping a specified heat pump for a different model, or changing window specifications, may or may not affect compliance. The assessor should be consulted before substitutions are confirmed

All changes between the design stage and as-built submissions must be documented. The as-built BRUKL must reflect what was actually constructed, and building control will compare it against the design stage submission (AD L2 2026, Appendix C).

Practical workflow for design teams and assessors

An effective design stage BRUKL process follows this sequence:

1. Early engagement (RIBA Stage 2–3) — appoint the energy assessor and establish the energy strategy. Under the FBS, the choice of heating system fundamentally determines the compliance approach
2. Preliminary modelling — the assessor runs an initial SBEM or DSM calculation using the developing design to identify compliance risks early. This is not the formal submission but a design tool
3. Design refinement — iterate the building fabric, HVAC, lighting and PV specifications in response to the modelling results. This is where the design team and assessor collaborate to find cost-effective routes to compliance
4. Formal BRUKL production — once the design is substantially fixed, the assessor produces the formal design stage BRUKL using finalised specifications
5. Building control submission — the BRUKL is included with the full plans application or initial notice, along with the commissioning plan
6. Track changes — maintain a log of any design changes after submission that could affect the energy calculation, ready for the as-built stage

Design stage BRUKL under the FBS

The Future Buildings Standard raises the compliance bar significantly. The notional building now assumes low-carbon heating, improved fabric, 105 lm/W lighting, and solar PV at 40% of foundation area for all zone types. This means design stage BRUKL submissions under the FBS require:

• Heat pump or equivalent low-carbon heating as the baseline assumption — fossil fuel heating creates an immediate compliance deficit
• Better-than-limiting fabric performance — designing to the Table 3.1 maximum U-values alone will not achieve the targets set by the notional building
• PV as standard — most building types will need rooftop solar to meet the TPER
• Updated software — assessors must use versions of iSBEM or approved DSM tools that implement the 2026 notional building specification

Need a design stage BRUKL?

If you need BRUKL calculations for your non-domestic project, visit Easy EPC for professional SBEM assessment and compliance support from qualified Non-Domestic Energy Assessors.