Part O Overheating Assessments

An overheating assessment under Part O of the Building Regulations evaluates the risk of a new residential building becoming dangerously warm during summer months. UK Building Compliance provides both simplified method assessments and full CIBSE TM59 dynamic thermal modelling to demonstrate Part O compliance. As registered SAP assessors with Elmhurst Energy, we help you design out overheating risk at the earliest stage of your project.

What Is an Overheating Assessment?

An overheating assessment analyses the likelihood of a building experiencing excessively high indoor temperatures during warm weather. Part O of the 2021 Building Regulations was introduced specifically to tackle overheating in residential buildings — an issue that has become increasingly significant as homes are built to higher insulation and airtightness standards.

The assessment evaluates factors including glazing orientation, glazing area, solar shading, ventilation provision, and thermal mass to determine whether a dwelling can maintain safe indoor temperatures without reliance on mechanical cooling. The aim is not to guarantee occupant comfort, but to protect the health and welfare of residents by reducing the occurrence of dangerously high indoor temperatures.

There are two methods for demonstrating Part O compliance: the simplified method, which uses a prescriptive checklist approach, and dynamic thermal modelling using CIBSE TM59 calculations, which provides a more detailed, hour-by-hour simulation of the building's thermal performance throughout a typical summer.

Why Do You Need an Overheating Assessment?

Part O applies to all new residential buildings in England and Wales, including dwellings, institutional buildings where people sleep (such as care homes and student accommodation), and multi-use residential buildings. Shared communal rooms, common spaces, and live/work units also fall within the scope of Part O.

Overheating in buildings is a serious public health concern. The UK experienced record temperatures in recent years, and poorly designed buildings can trap heat to dangerous levels — particularly upper-floor flats with large areas of south- or west-facing glazing. Without a compliant overheating assessment, building control cannot sign off your project.

Commissioning your overheating assessment at design stage allows you to identify and resolve potential issues early. Design changes such as reducing glazing areas, adding external shading, or specifying opening windows are far more practical and cost-effective during the design phase than after construction has begun. This assessment should be coordinated with your SAP calculations and thermal bridging calculations for a comprehensive approach to building performance.

Our Overheating Assessment Process

UK Building Compliance offers both compliance routes under Part O, guiding you to the most appropriate and cost-effective method for your project.

1. Initial review — We assess your architectural drawings, glazing schedule, orientation, and ventilation strategy to determine whether the simplified method is likely to achieve compliance or whether dynamic thermal modelling will be required.
2. Simplified method assessment — Where appropriate, we carry out the Part O simplified method, evaluating glazing ratios, solar shading provisions, and ventilation opening areas against the prescriptive criteria in Approved Document O.
3. CIBSE TM59 dynamic thermal modelling — If the simplified method cannot demonstrate compliance (or if your project requires a more detailed analysis), we carry out full dynamic thermal modelling using CIBSE TM59 methodology. This simulates the building's thermal performance hour-by-hour across a design summer year.
4. Design recommendations — If the assessment identifies overheating risk, we provide practical recommendations including adjustments to glazing areas, shading solutions, ventilation strategies, and thermal mass improvements.
5. Compliance report — We produce a comprehensive overheating assessment report suitable for building control submission, demonstrating that your design meets Part O requirements.

What's Included

• Part O overheating assessment using the simplified method or CIBSE TM59 dynamic thermal modelling
• Analysis of glazing orientation, area, and solar gain potential
• Assessment of ventilation provision and purge ventilation capacity
• Evaluation of external shading, internal blinds, and other mitigation measures
• Design recommendations to mitigate overheating risk
• Comprehensive compliance report for building control
• Coordination with SAP calculations and energy statements

Who Needs an Overheating Assessment?

Part O overheating assessments are required for a wide range of residential construction projects:

• New-build dwellings — All new houses and flats in England and Wales require a Part O assessment.
• Purpose-built student accommodation — Residential buildings where people sleep on the premises fall within Part O scope.
• Care homes and institutional buildings — Establishments where vulnerable people sleep require particular attention to overheating risk.
• Live/work units — Mixed-use units containing both living and commercial space are within scope.
• Conversions creating new dwellings — Where a material change of use creates new residential units, Part O compliance must be demonstrated.

Developers, housebuilders, architects, and housing associations all need to address overheating risk as part of their building regulations compliance strategy.

Why Choose UK Building Compliance?

UK Building Compliance has extensive experience delivering overheating assessments for projects of all sizes, from single dwellings to large multi-unit developments. Our assessors are registered with Elmhurst Energy and hold accreditations with ATTMA, Stroma, ABBE, BPEC, and UKAS.

We offer both the simplified method and full CIBSE TM59 dynamic thermal modelling, advising you on the most appropriate route for your specific project. Our team works closely with architects and designers to resolve overheating issues at the earliest stage, avoiding costly redesigns later in the project.

As a full-service building compliance consultancy, your overheating assessment can be combined with SAP calculations, air tightness testing, water efficiency calculations, and EPC production — providing a coordinated, cost-effective compliance package. Based in Hinckley, Leicestershire, we serve clients across the entire United Kingdom. Call 01455 242297 for a free quote.