Part O - Overheating

Part O Overheating: England Assessments, Design and Evidence

Part O is England’s Building Regulations requirement for limiting unwanted summer solar gains and providing an adequate means to remove heat in covered new residential accommodation. It is not a SAP result, a Part L pass or a promise that a home will always feel comfortable. The legal test is whether the work makes reasonable provision; the strategy must also take account of occupant safety and reasonable enjoyment. Mechanical cooling is permitted only where insufficient heat can be removed without it.

For most new homes, the important decision is made early: either design to the Approved Document O (ADO) simplified method, or use dynamic thermal modelling (DTM) to CIBSE TM59 under ADO Section 2. That decision affects façade design, glazing, shading, openable areas, ventilation, security, noise mitigation, guarding and the evidence submitted to building control. A late window, balcony, shading or ventilation change can therefore create a compliance issue even where the original concept was sound.

Energy Report helps teams turn that requirement into a coordinated, auditable process. Our role is to clarify scope and route, test the proposed strategy against the applicable guidance, coordinate the Part O assumptions with the wider energy and ventilation design, and assemble clear evidence and handover information. We support the project team; the relevant building control body remains responsible for its own acceptance of the evidence.

Scope: what this page covers and what it does not

Part O is not a general retrofit-overheating rule. The starting point is whether the building work falls within Requirement O1’s application and whether the accommodation is within ADO’s scope. The table below is a practical scoping aid, not a substitute for a project-specific reading of the regulations and guidance.

Selecting the route: simplified method or TM59 dynamic thermal modelling

A route-selection discussion should happen before the façade, floor plans and ventilation approach are fixed. Both routes are capable of supporting a Part O submission, but they answer the problem differently.

The DTM/TM59 route: project-specific analysis with Part O rules applied

DTM is not a generic summertime-comfort simulation and it is not automatically a more permissive calculation. Under ADO Section 2, the model should use CIBSE TM59 while applying the ADO-specific limits and operating assumptions. The output must demonstrate a TM59 pass in the form expected by ADO, and building control should receive the report and the relevant Appendix B Part 2b checklist information.

This route is often appropriate where a building’s response cannot be represented fairly by the simplified method. ADO identifies, for example, very high insulation and airtightness, unusual site circumstances such as Manchester city centre, and substantial permanent shading as reasons DTM may be suitable. The model should record the geometry, orientation, glazing and solar properties, fixed shading, permanent adjacent shading, ventilation design, heat sources, weather data, room use and controls that underpin the result. These inputs are not administrative detail: an unrecorded façade or controls change can invalidate the evidence.

For DTM, ADO constrains how operable openings are represented. In occupied daytime rooms, the guidance models windows beginning to open above 22°C, fully open above 26°C and closing as temperature falls. At night, openings are modelled fully open only if they are first floor or above, not easily accessible, and the internal temperature exceeds 23°C at 23:00. Ground-floor and easily accessible openings have tighter assumptions. When unoccupied, they may be open during the day only if secure, while windows, patio doors and balcony doors are modelled closed at night; an entrance door is modelled closed at all times.

The practical conclusion is simple: a TM59 result is defensible only if the specified windows, secure ventilation elements, restrictors, louvres, shading and controls can deliver the modelled operating pattern. The design team should retain the model files, assumptions, schedules, weather files, TM59 outputs and mitigation specifications in a controlled project record.

Heat removal, opening areas and the reality of use

After limiting gains, the design needs a credible means of removing indoor heat. ADO identifies windows, external-wall ventilation louvres, mechanical ventilation and mechanical cooling as possible means. The Part O route determines the precise evidence required, but the physical question is consistent: how does heat leave the rooms when residents can safely and reasonably use the strategy?

For the simplified route, free area and equivalent area are technical quantities. They are not the same as the overall glazed area or the nominal opening size on an elevation. The actual window type, hinge arrangement, opening angle, restrictor, insect screen, guard, louvre geometry and any secure-ventilation device can affect the usable airflow area. ADO Appendix D provides the equivalent-area approach, including measurement to BS EN 13141-1.

For DTM, the same physical constraints must be represented through credible window-control assumptions. Do not model unrestricted night opening and then specify an opening limiter, security restriction or acoustic arrangement that prevents it. Do not count the clear area behind a Juliet-balcony or guard if the protection system changes the effective airflow. Openings wider than 100 mm that are relied on by the strategy engage ADO’s guarding guidance, and guarding or Juliet-balcony arrangements can reduce effective airflow area.

Part F and Part O must also be kept distinct. Part F addresses ventilation for indoor air quality. Part O addresses summer solar gains and heat removal. Airtightness does not replace designed background, extract or purge ventilation, and the purge-ventilation provision suitable for Part F is not necessarily sufficient for Part O. Where secure, acoustic or pollution-constrained openings are part of the solution, the ventilation and overheating strategies must use the same installed design.

Mechanical cooling: last resort, then fully coordinated

Mechanical cooling is not prohibited, but it is deliberately not the first response. ADO requires passive means to be used as far as reasonably practicable before mechanical cooling and requires evidence to building control that practicable passive measures have been used. For ADO, “passive means” excludes mechanical cooling and can include openable windows or mechanical-ventilation fans. Cooling is therefore considered after the façade, shading, openings and ventilation options have been properly tested, not before them.

Where cooling remains necessary, it creates further coordination work. The system type, zones, controls, plant space, maintenance needs and resident instructions should be captured in the design and handover. Installed cooling must also be reflected in SAP, but SAP does not calculate the cooling loads calculated by the DTM method. A cooling entry in SAP is consequently not a Part O pass, and a TM59 pass does not remove the need to account for cooling in SAP.

SAP 10.3 and Part L 2026: current position versus future regime

These dates need careful wording. As checked on 5 September 2026, GOV.UK identifies SAP 10.3 as the sole approved methodology, with the Home Energy Model launch delayed. This tells a project which domestic energy methodology is currently approved; it does not turn SAP 10.3 into a Part O methodology.

The earlier-standard Part L guidance remains relevant to projects under the prior regulatory regime. The Future Homes and Buildings Standards regulations and the 2026 Part L guidance are stated to come into force on 24 March 2027, with specified higher-risk-building provisions on 24 September 2027, subject to transition provisions. Project teams must confirm the application or notice date, work commencement date, higher-risk status and the precise transitional rules before stating which Part L version applies. Do not describe the 2026 Part L position as already current in September 2026, and do not assume it creates a new Part O edition.

A practical Part O service workflow

Our workflow is designed to avoid a compliance report being produced after decisions have become expensive to change.

• Scope and risk review. We confirm that the work is within England Part O scope, identify the accommodation and relevant residential corridors, flag mixed-use boundaries, and establish the building-control and planning context.
• Route and evidence plan. We test whether the simplified method is suitable or whether DTM/TM59 is the robust route. The plan records the information required from the architect, services engineer, acoustic consultant, contractor and client.
• Coordinated design review. We review glazing, solar control, permanent shading, room layout, cross-ventilation, actual opening areas, noise, pollution, security, guarding, ventilation and communal-service heat gains together.
• Assessment and mitigation. We complete the selected assessment route, identify proportionate changes where needed and maintain a transparent record of the strategy and assumptions.
• Part L, Part F and planning interfaces. We cross-check shared assumptions with SAP or SBEM, ventilation and airtightness work, and any planning energy statement. Separate deliverables remain separate, but contradictions are resolved before submission.
• Building-control package and change control. We prepare the relevant report and ADO checklist information, then support the team in reviewing material design changes against the agreed strategy.
• Completion and resident information. We help translate the final installed measures into owner-facing operating and maintenance information for the regulation 40B handover.

Effortless Part O Compliance

At Energy Report, there are two methodologies available for meeting overheating regulations: the Simplified Method and Dynamic Thermal Modelling (TM59). Our expert assessors guide you through the best approach for your project, ensuring full compliance every step of the way:

Simplified Method

Best suited for simpler building designs where glazing and floor area limits are met. Key factors like building location, aspect and orientation, and window size and area are considered. This method is ideal for less complex buildings, with specific glazing and orientation limitations.

TM59 Dynamic Thermal Modelling

For more complex or larger projects, this flexible approach provides a detailed analysis to mitigate overheating risks, ensuring compliance with CIBSE’s TM59 standards. Using advanced thermal simulation software, the building’s internal environment is assessed to predict overheating risks based on factors likes building design, materials, climate data, and orientation.