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 Overheating Assessment UK | Part O Compliance

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

TM59 Risk Estimator
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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

Assess Early

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

Advise Strategically

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

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 Storeys 1 & 2 Bed Flats Swindon DSY1

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

Double-height spaces with extensive south-facing glazing
High passive solar-gain potential from floor-to-ceiling windows
London Gatwick weather file intensifying overheating risk
No surrounding buildings for external shading; trees excluded per ADO

Our Solution

Low g-value glazing (0.4) to minimise solar heat gains
600mm - 1000mm overhangs above windows & French doors
Water-cooled air conditioning in high-gain kitchen/living area
Bedroom fans for targeted thermal comfort enhancement

1 Residence Detached Dwelling London Gatwick DSY1

MMC Innovation

Tutbury Discovery Plots

Tutbury, Staffordshire

Part O TM59 assessment for five bespoke Future Homes demonstration plots pioneering Modern Methods of Construction (MMC). Each plot tested unique combinations of closed timber frame systems, air source heat pumps, and advanced fabric specifications – requiring extensive Design Team Meetings with suppliers to establish accurate thermal properties.

The Challenge

Five unique specifications with different MMC systems
Ultra-low airtightness (down to 0.5 m³/h/m²) limiting passive ventilation
Limited thermal data for novel closed timber frame panels
Validating cooker hood MVHR integration and reversible heat pumps

Our Solution

Extensive DTM collaboration with suppliers for accurate thermal properties
Triple glazing (g-factor 0.6) throughout all plots
MVHR strategies including cooker hood integration
Individual models for each bespoke specification

5 Demo Plots MMC Demonstration Homes Nottingham DSY1

Monks Farm Phase 2B - Large residential development with apartment blocks

Large-Scale Residential

Monks Farm Phase 2B

Grove, Oxfordshire

Part O TM59 assessment for approximately 108 plots comprising residential dwellings and three-storey apartment blocks. Significant noise constraints across the site required restricted window opening angles in bedrooms, with mechanical ventilation strategies proposed to achieve compliance while maintaining acceptable internal noise levels.

The Challenge

Significant noise issues across the entire site
Restricted window opening angles in all bedrooms
Swindon weather file intensifying overheating risk
Ground floor security concerns for apartment blocks

Our Solution

Mechanical Extract Ventilation at 50–60 l/s for failing bedrooms
Acoustic-constrained modelling with limited window angles
Beneficial overshadowing from surrounding buildings
Strategic dwelling sampling for efficient site coverage

108 Plots Bungalows & Apartments Swindon DSY1

Talbot Place Whitchurch - Large residential development near A41 bypass

Phased Assessment

Talbot Place, Whitchurch

Whitchurch, Shropshire

Comprehensive Part O TM59 assessment for a 283-plot residential development near the A41 bypass. The project required careful coordination across multiple phases, with evolving acoustic conditions and design changes mid-project. Updated noise assessments reduced mitigation requirements, while roof modifications provided additional compliance pathways.

The Challenge

283 plots delivered across multiple phases
Noise from A41 bypass affecting southern plots
Facade-specific window constraints per plot location
Evolving acoustic conditions requiring mid-project updates

Our Solution

Phased assessment: 88 initial + 195 follow-on plots
32 plots updated with reduced mitigation after revised acoustics
Roof modifications (GRP pitched roofs) aiding compliance
Systematic tracking to avoid duplicate work across phases

283 Plots Residential Phase Nottingham DSY1

Darwin Green BDW4 - Large residential development in Cambridgeshire

Large-Scale Residential

Darwin Green BDW4

Darwin Green, Cambridgeshire

A Part O TM59 assessment for approximately 351 plots comprising residential dwellings and apartment blocks. Noise constraints resulted in restricted window openings at night-time. To achieve Part O compliance, Mechanical Extract Ventilation was tested via DMEVs, with MVHR and active cooling proposed where extract rates were too high.

The Challenge