NCM Modelling Guide

The National Calculation Methodology (NCM) is the government-approved framework for calculating the energy performance of non-domestic buildings in England. It defines everything from the notional building specification to fuel emission factors, ensuring that every compliance assessment – whether run through SBEM or an approved Dynamic Simulation Model - uses identical rules and data (NCM 2021, paras 22–27).

What the NCM calculates

At its core, the NCM produces two pairs of compliance metrics for every non-domestic building (NCM 2021, para 22):

• Building Emission Rate (BER) and Target Emission Rate (TER) – measured in kgCO₂/m²/year
• Building Primary Energy Rate (BPER) and Target Primary Energy Rate (TPER) – measured in kWh_PE/m²/year

Compliance requires BER ≤ TER and BPER ≤ TPER. Passing on emissions alone is not enough – the dual metric prevents gaming through low-carbon but energy-intensive strategies (NCM 2021, paras 85–87).

The eight pillars of the NCM

The NCM Modelling Guide establishes eight components that together define how a non-domestic energy calculation works:

1. Notional building specification – the reference building used to derive TER and TPER
2. Standard activity databases – fixed schedules of occupancy, heating, lighting, equipment, and ventilation for each zone type
3. Fuel and emission factors – standardised CO₂ emission factors and primary energy factors for all fuel types
4. HVAC system classification – a taxonomy of heating, ventilation, and air-conditioning system types with default efficiencies
5. Zoning rules – how to subdivide a building into calculation zones
6. Weather data – standard Test Reference Year (TRY) files for 14 UK locations
7. Solar PV methodology – how on-site generation is calculated and credited
8. The compliance process – how BER/TER and BPER/TPER are derived and compared

Zoning: dividing the building into calculation zones

Correct zoning is fundamental to an accurate NCM calculation. The building must be divided into calculation zones – contiguous areas of floor space with uniform conditions. A new zone boundary is mandatory wherever any of the following changes (NCM 2021, paras 200–206):

• Activity type – different NCM activity types (office space vs meeting room vs circulation) require separate zones
• HVAC system – areas served by different heating, cooling, or ventilation systems must be separate zones
• Lighting type or controls – areas with significantly different lighting installations or control strategies
• Orientation – areas with different primary facade orientations, where orientation significantly affects solar gains
• Ceiling height – areas with significantly different floor-to-ceiling heights
• Adjacency conditions – areas adjacent to different external or internal conditions (ground floor vs upper floor, perimeter vs core)

The perimeter zone rule for daylighting

In buildings with mixed facade orientations, the NCM recommends splitting large open-plan spaces into perimeter and core zones (NCM 2021, paras 203–204):

• Perimeter zones extend approximately 6 metres from each external facade with significant glazing (SBEM Technical Manual v6.1.e, Section 16.1)
• Core zones are the remaining interior area, beyond the daylight zone from any facade
• Perimeter zones on different orientations (north, east, south, west) should be modelled separately because they experience different solar gains and heat losses

This rule is particularly important for large office floor plates and retail spaces. Ground-floor zones are always modelled separately from upper floors (different heat loss characteristics), and top-floor zones separately from intermediate floors (roof heat loss).

Activity databases: standard operating conditions

The NCM activity databases define fixed operating conditions for every building and zone type. These cannot be changed by the assessor – both the notional and the actual building use identical activity data, ensuring a like-for-like comparison (NCM 2021, para 24).

For each zone type, the database specifies:

• Occupancy density and schedule (persons/m² and hours of operation)
• Heating and cooling set points (maintained temperatures during occupied and unoccupied hours)
• Ventilation rate (fresh air supply in l/s per person or per m²)
• Hot water demand (litres/day per m² or per person)
• Equipment gains (internal heat gains from appliances in W/m²)
• Maintained illuminance (lux level for lighting calculations)

Example: general office conditions

A general office zone uses the following standard conditions from the NCM Activity Database (NCM 2021, para 193):

The NCM classifies buildings into top-level types (A1 Retail, B1 Office, B8 Storage/Distribution, C1 Hotels, D1 Institutions, D2 Assembly/Leisure, and others) with multiple zone activity types within each. An office building, for example, will typically contain zones for general office, reception, meeting rooms, server room, WCs, kitchen, storage, circulation, and plant rooms – each with its own standard conditions (NCM 2021, para 190–194).

HVAC system categorisation

The NCM classifies HVAC systems into defined types, each with associated default performance parameters. The classification determines how the energy calculation handles system efficiency, distribution losses, auxiliary energy (pumps and fans), and control effectiveness (NCM 2021, paras 43–70).

Heating system categories

• Central heating – wet system: boilers, heat pumps (ASHP, GSHP, WSHP), biomass boilers, CHP with LTHW distribution
• Central heating – warm air: gas-fired or electric warm air heaters with ducted distribution
• Local heating – radiant: gas radiant tube heaters, electric radiant panels, infrared heaters
• Local heating – convective: fan convectors, storage heaters, electric panel radiators
• Heat pump systems: ASHP, GSHP, WSHP, VRF/VRV systems
• District/community heating: heat networks with various heat sources

Cooling system categories

• Central chilled water: air-cooled and water-cooled chillers, absorption chillers
• Split/multi-split DX: wall-mounted splits, ceiling cassettes, ducted splits
• VRF/VRV: variable refrigerant flow/volume multi-indoor-unit systems
• Fan coil units: two-pipe and four-pipe fan coils with central chilled water
• Chilled beams/ceilings: passive or active chilled beams and radiant ceilings

Ventilation system categories

• Natural ventilation: no fan energy, no heat recovery credit, infiltration-based fresh air
• Mechanical extract only: extract fans with natural intake, fan energy calculated from SFP
• Mechanical supply and extract (MVHR): balanced system with heat recovery credited at declared efficiency
• Mixed-mode: combination of natural and mechanical, modelled according to operational strategy

Notional building derivation

The notional building is a hypothetical version of the proposed building with the same size, shape, orientation, and activity type, but with a fixed, defined specification. The TER and TPER are the results of running the NCM calculation on this notional building – the actual building must then achieve rates no worse than these targets (NCM 2021, paras 23–27, 85).

Critically, the notional building specification is published exclusively in the NCM Modelling Guide – not in AD L2 itself. This makes the NCM Modelling Guide the authoritative source for notional building parameters.

Notional building fabric

The top-lit values are more relaxed because top-lit buildings (warehouses, industrial sheds) typically have metal-clad construction with inherently higher U-values. These notional values are tighter than the limiting (backstop) U-values in AD L2 Table 3.1. The limiting values are the absolute worst permitted for any individual element; the notional values define the benchmark against which targets are set (NCM 2021, Table 1, para 34).

The notional building also assumes air permeability of 3 m³/(h.m²) at 50 Pa for side-lit/unlit zones (5 for top-lit zones per NCM 2021, Table 3). Non-repeating thermal bridges are handled using psi-values from NCM 2021 Table 2 (or a 10% addition to notional U-values as an alternative per para 34). Glazing g-values are 0.29 for side-lit and 0.40 for top-lit zones (NCM 2021, Table 4).

Notional building services (Part L 2021)

The values below are from the Part L 2021 NCM Modelling Guide. The 2026 NCM, which will define the FBS notional building, has not yet been published. Services specification depends on the actual building's fuel choice and zone type (NCM 2021, paras 43–84):

The FBS consultation response confirmed a flat 40% for both zone types (consultation response, para 3.10 - 3.11, p18). The current NCM 2021 still uses a weighted formula (20% side-lit / 40% top-lit, Equation 9), but the 2026 NCM will align with the flat 40% policy.

BER/TER and BPER/TPER calculation flow

The NCM compliance process follows a clear four-step sequence (NCM 2021, paras 85–87 and 125–126):

Step 1: Define the actual building geometry

Input the building's geometry – floor areas, zone boundaries, wall and glazing areas, roof areas, orientations, and heights. Assign NCM activity types to each zone from the standard database.

Step 2: Calculate the notional building

The software automatically constructs the notional building with the same geometry and activity types, but using the standard specification (notional U-values, air permeability, heating system, ventilation, lighting, and PV). Standard activity conditions and weather data are applied. The result is annual energy consumption by fuel type, which is then converted:

• Applying fuel emission factors gives the TER (kgCO₂/m²/year)
• Applying primary energy factors gives the TPER (kWh_PE/m²/year)

Step 3: Calculate the actual building

The same geometry and activity types, but with the actual proposed specification – real U-values, tested or design-stage air permeability, declared HVAC efficiencies, actual lighting, and actual PV. The same standard activity conditions and weather data are applied (not actual usage patterns). The result:

• Applying fuel emission factors gives the BER (kgCO₂/m²/year)
• Applying primary energy factors gives the BPER (kWh_PE/m²/year)

Step 4: Compliance check

Both conditions must be met for the building to pass:

• BER ≤ TER – pass on CO₂ emissions
• BPER ≤ TPER – pass on primary energy

Additionally, the limiting fabric standards (backstop U-values), minimum system efficiencies, and fuel restrictions must also be met – regardless of how well the BER and BPER perform (AD L2 2026, Section 3).

What counts as regulated energy

The BER, TER, BPER, and TPER include only regulated energy - energy consumed by fixed building services subject to Building Regulations:

Standard weather data

The NCM uses standardised Test Reference Year (TRY) weather data for 14 UK locations – from Belfast and Edinburgh to Plymouth and London. The same weather file is used for both the notional and actual building calculations, ensuring the comparison is purely about building specification rather than climate assumptions (NCM 2021, paras 198–199).

Each TRY file contains hourly data for dry-bulb temperature, solar irradiance, wind speed and direction, humidity, and cloud cover. SBEM uses monthly summary data derived from these hourly files, while Dynamic Simulation Models typically use the full hourly data. The building's postcode determines which weather file is assigned.

Solar PV calculation methodology

On-site solar PV generation reduces both BER and BPER by offsetting grid electricity consumption. The NCM calculates monthly PV generation from declared array specifications (peak power, orientation, tilt, overshading) and standard weather data (NCM 2021, para 81–84).

Generation is first applied to offset on-site regulated electricity demand each month. Surplus generation beyond on-site demand receives a reduced export credit, reflecting that exported electricity displaces grid generation rather than on-site consumption. Under the FBS, the notional building PV area is 40% of foundation area for both side-lit and top-lit zones (consultation response, para 3.11, p18), oriented due south at 30° tilt. The current NCM 2021 uses a weighted formula - 20% side-lit / 40% top-lit (Equation 9) - which the 2026 NCM will update. If 100% of heating is from heat pumps, the notional building has no PV (NCM 2021, para 83). Higher-risk buildings are also exempt.

NCM consistency across software tools

Both SBEM and approved Dynamic Simulation Models must use identical NCM inputs (NCM 2021, Appendix C, paras 207–214):

• The same notional building specification
• The same activity databases and standard conditions
• The same fuel and emission factors
• The same weather data files
• The same output format (BRUKL report)
• The same compliance test (BER ≤ TER and BPER ≤ TPER)

The only difference is in how building physics are modelled - SBEM uses a monthly quasi-steady-state method based on EN ISO 13790, while DSMs use hourly or sub-hourly dynamic thermal simulation. This means BER/BPER values can differ between tools for the same building. A building that fails in SBEM may pass in a DSM (or vice versa), depending on how its design features interact with the calculation method (NCM 2021, Appendix C, para 209).