Non-Domestic Overheating Considerations Under the FBS

Part O (Approved Document O) does not apply to non-domestic buildings. Its scope is limited to dwellings, residential institutions, and rooms for residential purposes. For offices, retail, schools, warehouses, and other non-domestic building types, there is no standalone overheating regulation. Instead, overheating risk is managed indirectly through SBEM cooling load calculations and solar gain limits in AD L2 2026. This page explains what applies, what does not, and what designers should do in practice.

Part O: residential only

Approved Document O (2021) applies exclusively to new residential buildings: dwellinghouses, flats, residential institutions (care homes, schools with sleeping accommodation), halls of residence, and shared communal spaces within buildings containing residential units. Hotels are explicitly excluded. General non-domestic buildings – offices, retail, warehouses, schools (non-residential portions), hospitals (non-ward areas), and leisure facilities – are outside Part O's scope entirely (AD O, O1 of Schedule 1).

In mixed-use buildings, Part O applies only to the residential portions. The non-domestic portions follow the standard Part L regime: solar gain limits under AD L2 Table 3.3 (if mechanically cooled), SBEM calculation for BPER and BER, and no prescriptive glazing limits or cross-ventilation requirements (AD L2 2026, Section 3).

SBEM cooling loads and the energy calculation

For non-domestic buildings, overheating risk is managed through the National Calculation Methodology (NCM), implemented in SBEM or approved Dynamic Simulation Models. Cooling energy feeds directly into the Building Primary Energy Rate (BPER) and Building Emission Rate (BER): higher cooling demand increases both metrics, making it harder to meet the Target Primary Energy Rate (BTPER) and Target Emission Rate (BTER) (AD L2 2026, Section 2).

This creates an indirect incentive to reduce solar gains and minimise cooling loads through passive design. However, SBEM does not fail a building for overheating risk as such – it penalises excessive cooling energy. A naturally ventilated office that overheats but has no cooling system will not be flagged by SBEM, because no cooling energy is consumed.

Solar gain limits in AD L2

AD L2 2026 (paragraphs 3.16–3.17) addresses solar gains for non-domestic buildings, but only for occupied spaces that are mechanically cooled. For each such space, solar gains through glazing (April to September inclusive) must be no greater than through the relevant reference glazing systems in Table 3.3 (AD L2 2026, Table 3.3).

The reference glazing benchmarks are:

• Side-lit spaces: full-width east-facing glazing of 1,000 mm height, 10% framing factor, g-value of 0.48
• Top-lit spaces (≤ 6 m zone height): 10% of roof area, 25% framing factor, g-value of 0.48
• Top-lit spaces (> 6 m zone height): 10% of roof area, 15% framing factor, g-value of 0.42

Critically, naturally ventilated non-domestic spaces without mechanical cooling have no equivalent solar gain limit under Part L. The test applies only to cooled spaces and limits solar gain to control cooling energy demand, not thermal comfort (AD L2 2026, para 3.17).

Unlike residential buildings under Part O, non-domestic buildings have no prescriptive glazing area limit. The constraint is economic: more glazing increases solar gains, which increases cooling demand, which increases BPER, which makes compliance harder.

CIBSE TM52 and TM59

CIBSE TM52 (The Limits of Thermal Comfort: Avoiding Overheating in European Buildings) is the primary industry standard for assessing overheating risk in non-domestic buildings. It is not referenced in the Building Regulations but is widely used in practice and often required by planning conditions, BREEAM assessments, and client briefs.

TM52 defines overheating during occupied hours (May to September) using three criteria. A space fails if it fails any two:

• Criterion 1 – hours of exceedance: operative temperature exceeds the upper limit of the adaptive comfort range by 1K or more for no more than 3% of occupied hours
• Criterion 2 – daily weighted exceedance: on any one day, the severity of overheating (weighted by how far temperature exceeds the upper limit) must not exceed 6 degree-hours
• Criterion 3 – upper limit temperature: operative temperature must not exceed the upper limit by 4K or more at any time

CIBSE TM59 is the residential counterpart – it is referenced in AD O for the dynamic thermal modelling compliance route and has regulatory standing for dwellings. TM52 has no direct regulatory standing but is the accepted methodology for non-domestic overheating assessment.

Although not mandated, TM52 assessments are commonly required by local planning authorities (particularly for schools, hospitals, and offices), BREEAM Health and Wellbeing credits (Hea 04), and sector-specific guidance including BB101 for schools and HTM/HBN for healthcare buildings.

Practical design considerations

In the absence of a prescriptive overheating regulation, designers of non-domestic buildings should adopt a passive-first approach to managing overheating risk:

Orientation and solar shading

• Orient the building to minimise east and west-facing glazing – these orientations receive low-angle sun that is hardest to shade
• Provide external shading devices (brise-soleil, overhangs, fins) sized using solar geometry for the building's latitude – external shading is significantly more effective than internal blinds
• Specify glazing g-values appropriate to orientation: lower g-values on south, east, and west facades; higher g-values on north facades to maximise useful daylight

Ventilation and thermal mass

• Design for natural or mixed-mode ventilation where site conditions permit
• Night purge ventilation (pre-cooling the building mass overnight) can significantly reduce peak daytime temperatures in heavyweight buildings – AD L2 2026 acknowledges this as a means of reducing the building's primary energy rate
• Expose internal thermal mass (concrete soffits, masonry walls) to absorb daytime heat gains and release them during cooler night hours – thermal mass is only effective when combined with night ventilation
• Ensure heat recovery units include a summer bypass to prevent recovering unwanted heat during warm weather

Internal gains and cooling efficiency

• Specify energy-efficient lighting and equipment to reduce internal heat gains – lighting energy is both a direct Part L compliance factor and a contributor to cooling load
• Where comfort cooling is provided, AD L2 Table 5.7 sets minimum SEER values (e.g. 5.0 for split systems and VRF) and controls must prevent simultaneous heating and cooling in the same space (AD L2 2026, para 5.40)
• Cooling systems should not be sized for more than 120% of the design cooling load (AD L2 2026, para 5.39)

Future non-domestic overheating requirements

The FBS consultation response (2026) confirmed that Part O currently applies to new residential buildings only and that for non-domestic buildings, overheating is addressed through the SBEM/DSM calculation methodology. The Government will proceed with a full technical review of AD O (consultation response, p10), but there is no current proposal to extend Part O or create an equivalent standard for non-domestic buildings.

Several developments suggest that non-domestic overheating regulation may evolve: rising summer temperatures increase overheating risk in all building types; schools and healthcare already face scrutiny through sector-specific guidance; and the SBEM limitation of only penalising cooling energy (not assessing thermal comfort) is widely recognised. Designers should consider conducting voluntary TM52 assessments as a precaution against future regulatory tightening.