The notional building is a theoretical reference building of the same size, shape, and use as the actual building, but with standardised construction specifications. Under the Future Buildings Standard, the Target Emission Rate (TER) and Target Primary Energy Rate (TPER) are calculated from this notional building – the actual building must perform at least as well (AD L2 2026).
Fabric specifications
The notional building fabric specifications set the baseline thermal performance. The actual building does not need to match these values exactly – it must achieve an overall BER ≤ TER and BPER ≤ TPER – but these are the starting point for the compliance calculation (AD L2 2026).
| Element | Side-lit / Unlit (2021) | Top-lit (2021) |
|---|---|---|
| Roof U-value | 0.15 W/m²K | 0.18 W/m²K |
| External wall U-value | 0.18 W/m²K | 0.26 W/m²K |
| Floor U-value (ground and exposed) | 0.15 W/m²K | 0.22 W/m²K |
| Windows | 1.4 W/m²K | n/a |
| Rooflights | n/a | 2.1 W/m²K |
| Vehicle access doors | 1.3 W/m²K | 1.3 W/m²K |
| Pedestrian and high usage doors | 1.9 W/m²K | 1.9 W/m²K |
| Air permeability | 3 m³/(h·m²) at 50 Pa | 5 m³/(h·m²) at 50 Pa |
Source: NCM Modelling Guide 2021 Edition, Table 1 and Table 3.
Heating systems
The notional building assumes a low-carbon heating system, such as an air-source heat pump with a suitable coefficient of performance (COP). Buildings specifying fossil fuel heating (gas boilers) will find it very difficult to achieve a BER ≤ TER, as the notional building's emissions are based on electric heating with a low carbon factor (AD L2 2026).
Solar PV
The FBS policy requires solar PV covering 40% of the building's foundation area for both side-lit and top-lit zones (consultation response, para 3.11, p18). The current NCM 2021 implements this using a weighted formula (Equation 9): 20% for side-lit/unlit zones and 40% for top-lit zones, proportioned by floor area. The 2026 NCM (not yet published) will implement the flat 40% rule. If 100% of heating is from heat pumps, the notional building has no PV at all (NCM 2021, para 83). Higher-risk buildings are also exempt. See the dedicated solar PV requirements page for full details, exemptions, and calculation guidance.
Lighting efficacy
The notional building specifies general lighting with an efficacy of at least 105 luminaire lumens per circuit watt (lm/W). This is a significant step up from previous standards and effectively mandates LED lighting throughout (AD L2 2026).
Ventilation and heat recovery
The Part L 2021 notional building assumes mechanical ventilation with heat recovery (MVHR) where applicable, with a 76% sensible seasonal heat recovery efficiency (NCM 2021, para 49) and SFP of 1.5 W/(l/s). The FBS impact assessment raises the target to 80% for the preferred option specification (FBS Impact Assessment, Tables 7–8, p48). The heat recovery is bypassed in cooling mode. Natural ventilation strategies must demonstrate equivalent performance through the SBEM calculation.
BER, TER, BPER, and TPER
Compliance requires the actual building to meet two independent targets (AD L2 2026):
- BER ≤ TER: the Building Emission Rate must not exceed the Target Emission Rate (CO₂ emissions)
- BPER ≤ TPER: the Building Primary Energy Rate must not exceed the Target Primary Energy Rate (primary energy consumption)
Both targets are derived from the notional building calculation. Meeting one but not the other is insufficient – both must be satisfied. The compliance output is documented in a BRUKL report.
Frequently Asked Questions
What is the notional building?
The notional building is a theoretical reference building of the same size, shape, and use as the actual building, but with standardised construction specifications defined in Approved Document L. The target emission rate (TER) and target primary energy rate (TPER) are derived from this notional building.
Does my building need to match the notional building specifications exactly?
No. The notional building sets the performance target, not the design prescription. Your building must achieve a BER ≤ TER and BPER ≤ TPER overall, but individual elements can exceed the notional specification if compensated elsewhere – this is the principle of trade-off within the SBEM calculation.
Related Pages
Solar PV Requirements
The flat 40% PV rule for all zone types, exemptions, and calculation guidance.
What is SBEM?
The calculation methodology that uses the notional building to set targets.
What is BRUKL?
The compliance report that documents whether the building meets targets.
BRUKL under FBS
How the new notional building raises the compliance bar for BRUKL.