What is a TM59 overheating assessment?

A TM59 overheating assessment is a CIBSE-approved methodology that uses dynamic thermal modelling to predict whether a residential building will overheat during summer months. It simulates indoor temperatures hour-by-hour using Design Summer Year (DSY) weather data and assesses compliance against thermal comfort criteria. TM59 assessments are required for planning applications in many UK local authorities and for demonstrating Part O compliance via the dynamic method.

What is Part O of Building Regulations?

Part O is a UK Building Regulation introduced in June 2022 that requires new residential buildings to limit unwanted solar gains and provide adequate means of removing excess heat. It applies to new homes, flats, care homes, and student accommodation. Compliance can be demonstrated through either the Simplified Method (glazing limits and ventilation openings) or Dynamic Thermal Modelling using CIBSE TM59 methodology.

How much does a TM59 assessment cost?

TM59 overheating assessment costs typically range from £225 to £2,000 depending on the project complexity. Factors affecting price include the number of dwelling types, building complexity, ventilation strategy, window configurations, and turnaround time required. Simple single-dwelling assessments start from around £225, while complex multi-unit developments with varied typologies may cost £1,500-£2,000.

When is a TM59 assessment required?

A TM59 assessment is required for: 1) Part O compliance when using the dynamic method instead of the simplified approach, 2) London Plan Policy SI4 for major residential developments, 3) Planning applications where local authorities request overheating analysis, 4) BREEAM HEA 04 credits for thermal comfort, and 5) Complex buildings where the Part O simplified method cannot be satisfied.

What is the difference between Part O Simplified Method and Dynamic Thermal Modelling?

The Simplified Method is a calculation-based approach using glazing ratios and ventilation opening sizes to demonstrate compliance - suitable for straightforward designs. Dynamic Thermal Modelling (TM59) uses simulation software to model hour-by-hour temperatures throughout summer, accounting for thermal mass, orientation, shading, and ventilation patterns. Dynamic modelling is more accurate and required for complex buildings, high-risk locations, or when the simplified method fails.

What are common overheating mitigation strategies?

Common overheating mitigation strategies include: external shading devices (brise soleil, overhangs, shutters), reduced glazing ratios, high-performance solar control glass (low g-value), cross-ventilation design, night purge ventilation, increased thermal mass, mechanical ventilation with heat recovery (MVHR), and optimised building orientation. Early-stage design changes are most cost-effective, which is why TM59 assessments should be conducted at RIBA Stage 2-3.

TM59 Experts

TM59 Overheating Assessment UK | Part O Compliance | CIBSE TM59 Reports

TM59 Overheating Reports for Architects, Developers & Homeowners

Part O Building Regulations and London Plan requirements now demand rigorous overheating analysis for new residential buildings. Our Elmhurst accredited assessors deliver CIBSE TM59 compliant reports using dynamic thermal modelling – ensuring your project achieves planning approval and regulatory compliance.

Our Specialist Overheating Assessment Services

Part O Compliance

Part O Building Regulations

Full compliance via Simplified Method or Dynamic Thermal Modelling.

Mandatory•New Build

Planning Policy

Planning Applications

Overheating assessments to support planning applications and meet local council requirements.

UK Nationwide•Planning

CIBSE Standard

TM59 Assessment (Dynamic)

The industry standard for predicting overheating risk in homes using dynamic simulation.

Industry Standard•Accurate

Compliance

Part O Compliance

Comprehensive simplified and dynamic assessments to meet Building Regulations Part O.

Regulatory•Essential

Get quick overheating risk estimates without complex simulations. Our web-based calculator delivers actionable TM59 compliance insights in minutes.

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Why Early TM59 Assessment Matters

Climate change is reshaping building regulations. Get ahead of overheating risks with the right approach from day one.

60%+ London homes at risk 1,000+ projects completed Elmhurst accredited

1

Assess Early

We evaluate your design at RIBA Stage 2-3 when changes are simple and cost-effective.

2

Advise Strategically

Targeted mitigation: glazing, shading, ventilation, and passive cooling strategies.

3

Achieve Compliance

TM59 & Part O compliant homes that are comfortable, healthy, and planning-ready.

TM59 Overheating Knowledge Base

TM59 overheating assessments and Part O compliance made simple. Visual guides and expert mitigation strategies for your project.

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Part O Compliance • TM59 Criteria • Mitigation Strategies

Case Studies & Impact

Real projects, real results – demonstrating how we solve complex overheating challenges with innovative thermal engineering solutions.

Multi-Unit Residential

Newbury Racecourse Apartments

Newbury, Berkshire

TM59 overheating assessment for a 4-storey L-shaped apartment block with 1 and 2-bedroom flats. The acoustic environment created unique constraints, requiring innovative ventilation strategies to achieve Part O compliance while maintaining indoor noise criteria.

The Challenge

Noise-affected facades limiting night-time window opening (30 dB LAeq,8hr limit)
South & West exposures subject to high solar radiation
Ground floor security concerns restricting external ventilation
Balconies providing partial shading but insufficient for compliance

Our Solution

Mechanical Extract Ventilation at 50 l/s for failing bedrooms
Acoustic-constrained modelling with windows closed at night
Strategic apartment sampling to optimize simulation time
Facade-specific ventilation strategies per exposure

4+3 Floors 1 & 2 Bed Flats DesignBuilder Analysis Swindon DSY1 Weather

Part O & TM59 Compliance Achieved

Contemporary Design

Sawtons, Cambridge

Cambridge, Cambridgeshire

Part O TM59 assessment for a striking contemporary residence featuring double-height spaces and extensive glazed façades. The high passive solar-gain potential combined with restricted natural ventilation opportunities required a comprehensive, multi-layered mitigation strategy.

The Challenge