What does Building Regulations Part O require for overheating in new homes?

Summary

Part O is one of the newest additions to the Building Regulations suite, introduced as part of the Future Homes Standard transition. It exists because UK homes — particularly those built with high levels of thermal insulation but limited external shading — have been overheating with increasing frequency as average temperatures rise. Mechanical air conditioning is considered a last resort, not a solution: installing cooling plant without addressing the root cause simply shifts the energy burden while allowing speculative development to offload the consequences onto residents.

The core principle of Part O is "passive first": design should use passive measures — limited solar glazing area, external shading, orientation, exposed thermal mass — to keep peak indoor temperatures below the overheating criteria. Only where passive measures are demonstrably insufficient should active cooling (mechanical ventilation with cooling) be considered, and even then the energy and carbon cost must be addressed.

For builders and developers, the practical impact is primarily on window and glazing specifications, particularly on south-facing and west-facing facades. A south-facing extension with large glazed sliding doors or a conservatory-style roof may struggle to comply without shading. Part O also has a disproportionate impact on high-rise residential development in London and other urban areas, where urban heat island effects and limited opportunity for cross-ventilation make compliance challenging.

Part O applies in England only. Wales, Scotland, and Northern Ireland have separate overheating guidance under their respective building standards.

Key Facts

• Came into force: 15 June 2022 for all new residential buildings in England
• Scope: New dwellings, including new houses, flats, extensions creating new habitable rooms, and material change of use creating dwellings
• Does NOT apply: Existing dwellings, extensions to existing homes (unless creating a new self-contained dwelling)
• Two compliance routes: (1) Simplified method — prescriptive limits on glazing area; (2) Dynamic thermal modelling — CIBSE TM59 methodology
• Simplified method — glazing limits: Total glazing area (windows + rooflights) must not exceed a set proportion of floor area, with adjustments for orientation; rooflights limited to 20% of floor area of any room they serve
• Orientation impact: South-facing (120°–240° from North) and west-facing glazing (240°–300°) is most restricted due to high solar gain
• Shading credit: External shading (fixed overhangs, fins, brise-soleil) reduces effective glazing area in the simplified calculation
• Free cooling requirement: Dwellings must have the ability to purge heat through ventilation — opening windows must have a sufficient "free area" to provide purge ventilation at 4 air changes per hour minimum
• Mechanical cooling: Cannot be used as the primary compliance strategy — must demonstrate passive measures are inadequate first
• CIBSE TM59: The technical methodology for dynamic thermal modelling — requires specialist software and a qualified assessor
• High-risk locations: Greater London, the South East, and urban heat island locations are subject to more stringent dynamic modelling requirements
• Part O vs Part F interaction: Ventilation requirements under Part F (for IAQ and moisture) and Part O (for cooling) must both be met simultaneously
• SAP/BREDEM: Current SAP 10.2 assessment does not fully model overheating — Part O is assessed separately

Quick Reference Table

Detailed Guidance

The Simplified Compliance Method

The simplified method is set out in Approved Document O and is designed to be accessible without specialist software. It works by comparing the actual glazing area against a maximum permitted area that varies with orientation.

Step 1: Identify the floor area of each habitable room.

Step 2: Calculate total glazing area (windows + rooflights) for each room.

Step 3: Compare against the maximum glazing fraction for the room's predominant orientation (the orientation of the largest glazed area). The limits are specified in Approved Document O tables.

Step 4: If glazing exceeds the limit, apply shading credits. External fixed shading devices (overhangs, fins) reduce the effective solar gain area in the calculation.

Step 5: Verify that opening windows provide sufficient free area for purge ventilation (4 ACH in each habitable room).

Key limitation of the simplified method: it is only valid for houses and individual dwellings on straightforward sites. Flats in Greater London, dwellings with complex form factors, and urban sites with significant heat island effects typically need the full TM59 dynamic modelling route.

CIBSE TM59 Dynamic Thermal Modelling

TM59 is the CIBSE technical methodology for assessing overheating risk in dwellings. It requires:

1. Dynamic thermal modelling software (e.g., IES-VE, DesignBuilder, EnergyPlus) to model the thermal performance of the building hour by hour over a representative hot summer year.
2. Standard internal heat gains and occupancy assumptions from TM59.
3. Weather data: Current standard is CIBSE Design Summer Year (DSY) data, typically the 2020 version which incorporates climate change allowances.
4. Pass criteria:
   • Criterion 1 (bedrooms): Hours above 26°C should not exceed 1% of occupied hours (approximately 32 hours per year for a typical bedroom occupancy pattern)
   • Criterion 2 (living spaces): Operative temperature should not exceed 28°C for more than 3% of occupied hours
5. Free cooling must be modelled: The model must demonstrate that natural ventilation through opening windows is available and provides sufficient cooling before mechanical cooling is considered.

TM59 modelling must be undertaken by a competent assessor — typically a building services engineer or specialist energy consultant. The result is a report demonstrating compliance or identifying measures needed.

Ventilation for Free Cooling

Part O requires that opening windows be provided and sized to deliver purge ventilation of at least 4 air changes per hour in each habitable room. This must be achievable without occupants needing to open windows in a way that creates a security or noise nuisance.

Practical implications:

• High-level opening lights on south-facing glazing can provide effective stack-driven ventilation
• Cross-ventilation (openings on opposite sides of the building) is significantly more effective than single-sided ventilation — plan layout to enable it
• Secure ventilation (night latch positions, restricted openings with retention clips) must still provide adequate free area
• Trickle ventilators do not count towards purge ventilation — they address background ventilation (Part F)

Interaction with Other Regulations

• Part F (Ventilation): Part F requires background ventilation (trickle ventilators) and extraction for kitchens and bathrooms. Part O requires purge ventilation for cooling. Both must be provided — they serve different functions.
• Part L (Energy Efficiency): High levels of insulation and airtightness reduce winter heat demand but can worsen summer overheating. Part L and Part O must be balanced — a very airtight, heavily glazed south-facing building may satisfy Part L but fail Part O.
• Part B (Fire Safety): In flats, opening windows for ventilation may interact with compartmentation requirements — automatic opening vents (AOVs) for smoke control may need to be coordinated with natural ventilation strategy.
• Permitted Development: Extensions under permitted development still need to comply with Building Regulations including Part O if they create a new dwelling. The most common residential extensions (adding a room to an existing house) do not create a new dwelling, so Part O does not apply.

Practical Guidance for Builders

The most common Part O-related situations for builders:

1. New house or flat: Designer must confirm compliance route at design stage. Check the specification includes appropriate glazing and shading before ordering. Large areas of unshaded south or west-facing glazing are likely to need assessment.

2. Garage conversion to habitable room: If creating a new self-contained dwelling, Part O applies. If extending the existing dwelling, it does not.

3. Loft conversion with rooflights: If creating a new dwelling unit, assess rooflight area against the 20% of floor area limit. Loft rooms in existing dwellings are not subject to Part O.

4. South-facing extension: Not covered by Part O (existing dwelling), but worth raising overheating risk with the client regardless — a south-facing conservatory extension with no operable glazing will overheat regardless of regulatory compliance.

Frequently Asked Questions

Does Part O apply to my extension?

No — not unless the extension creates a new self-contained dwelling (e.g., a separate annexe or flat). Extensions to existing dwellings are exempt from Part O. However, the principles of overheating risk management are worth applying in design regardless: a south-west-facing garden room with glazed roof will become uncomfortably hot in summer even if regulations don't require you to prevent it.

Can I specify air conditioning to comply with Part O?

Not as a primary strategy. Approved Document O requires passive measures to be exhausted first. If a dynamic thermal model demonstrates that even with maximised solar shading, appropriate glazing areas, and optimised natural ventilation, the TM59 criteria cannot be met, then mechanical cooling may be considered — but the system must still be energy efficient and carbon-assessed.

Where is the simplified method not available?

The simplified method cannot be used for flats in Greater London (all London Boroughs). These must demonstrate compliance through dynamic thermal modelling (TM59). The rationale is that urban heat island effects and the higher density of surrounding buildings create conditions that the simplified method's generic assumptions cannot capture accurately.

What is the design summer year (DSY) used for TM59?

The DSY is a climate file representing a hot summer year derived from historic weather data with climate change projections applied. CIBSE released updated DSY files in 2020 that incorporate projections to 2080 under different climate scenarios. For regulatory compliance, the CIBSE 2020 DSY for the relevant location should be used. The choice of climate scenario (e.g., 50th percentile or 90th percentile) may be specified by the local planning authority.

How does Part O interact with planning requirements for glazing?

Some planning authorities and their design guides have their own overheating requirements that may be more stringent than Part O. Greater London Authority guidance (the London Plan) requires assessment against its overheating SPG, which references the CIBSE TM59 methodology. Always check local planning conditions — Part O compliance does not automatically satisfy planning requirements.

Regulations & Standards

• Building Regulations 2010, Requirement O1 (SI 2010/2214 as amended) — The statutory overheating requirement

• Approved Document O (2021 edition, in force from June 2022) — Statutory guidance for compliance

• CIBSE TM59:2017 — Design methodology for the assessment of overheating risk in homes

• CIBSE TM52 — The limits of thermal comfort: avoiding overheating in European buildings (background methodology)

• CIBSE Design Summer Years 2020 — Climate files for dynamic thermal modelling

• Approved Document O (GOV.UK) — Official statutory guidance

• CIBSE TM59 — Overheating in dwellings — Dynamic modelling methodology

• Greater London Authority — Cooling hierarchy — London Plan guidance on overheating

• part l energy — Energy efficiency requirements that interact with overheating risk

• part f ventilation — Ventilation requirements that overlap with Part O purge ventilation

• building regs overview — Overview of all Building Regulations parts