Overheating Assessments for Planning Applications

Overheating Assessments for Planning Applications: What You Need to Know

As UK summers grow warmer and homes become more airtight, overheating is now a key issue in residential design. Planning departments are increasingly demanding Overheating Assessments as part of applications for new homes—especially in dense urban areas, flats, and developments with extensive glazing.

In this article, we explain what an Overheating Assessment is, when it’s needed for planning, and how to pass, so your project avoids delays, meets policy, and creates comfortable, healthy homes.

🔥 What Is an Overheating Assessment?

An Overheating Assessment evaluates whether a proposed dwelling is likely to overheat during warmer months, using dynamic thermal modelling software. It analyses internal temperatures against comfort thresholds, typically based on CIBSE TM59 (the Chartered Institution of Building Services Engineers methodology for assessing overheating in homes).

Key inputs include:

Room orientation
Window size and glazing type
Shading (e.g. overhangs, trees, blinds)
Ventilation strategy (natural or mechanical)
Thermal mass and internal heat gains

The goal is to identify overheating risks before construction, so designs can be adjusted accordingly.

🏗️ Why Are Overheating Assessments Required for Planning?

Overheating is more than just a comfort issue—it’s now considered a health and safety risk, particularly in vulnerable groups like the older people or young children. Because of this, many local authorities include overheating checks as part of planning policies, particularly for:

✅ Flats and apartments
✅ Dwellings in high-density or urban areas
✅ Homes with large glazing-to-floor area ratios
✅ Properties with restricted ventilation due to noise or pollution

In cities like London, overheating risk assessments are mandatory for major applications under the London Plan. This requirement is now spreading to boroughs and councils across the UK.

🏢 What Planning Documents Might Require One?

Your local planning authority may request an overheating assessment as part of:

🔎 Sustainability Statements
🏘️ Energy or Environmental Statements
🗂️ Design & Access Statements
🧾 Supporting documentation for detailed planning applications

Even where it’s not explicitly listed, many case officers will flag overheating as a concern—especially in schemes with limited shading or poor airflow.

🔎 What Is TM59 and Why Is It Important?

CIBSE TM59 is the current industry standard for assessing overheating in new homes. It sets two key criteria for compliance:

Living rooms and bedrooms must not exceed 26°C for more than a specific number of hours per year.
Bedrooms must remain cool at night—temperatures must not exceed 26°C for more than 1% of annual night-time hours.

If either condition is breached during simulation, the property is considered at risk of overheating.

🛠️ What Happens If Your Design Fails?

If your overheating assessment identifies a compliance failure, your assessor will provide design recommendations, which might include:

Reducing glazing on south/west-facing elevations
Adding external shading (e.g. louvres, balconies, vegetation)
Improving natural cross-ventilation
Upgrading to solar control glass
Introducing night purging via windows or ventilation systems

The earlier these risks are identified, the easier and more affordable they are to resolve.

📋 What’s Included in an Overheating Assessment?

A professional overheating assessment typically includes:

✅ A dynamic thermal model of the dwelling(s)
✅ Simulation of internal temperatures using current and future climate data
✅ Evaluation of ventilation performance (natural or mechanical)
✅ Comparison against TM59 or equivalent benchmarks
✅ Clear pass/fail outcomes for each habitable room
✅ Detailed design feedback to address risks
✅ A formal report suitable for submission with your planning application

🔗 Get a quote for an Overheating Assessment

🧮 How It Works with SAP Calculations & EPCs

Overheating risk is not fully covered by SAP Calculations or EPCs. While SAP touches on ventilation and glazing, it doesn’t simulate internal temperatures using real weather data.

That’s why most planning departments now require separate TM59 thermal modelling, especially for:

Multi-unit residential schemes
Passive or low-carbon homes
Developments in noise-sensitive areas (e.g. near train tracks, airports, main roads)

💡 Bundle your assessments to save time:

🕒 When Should You Do the Assessment?

You should carry out your overheating assessment:

✅ Before planning submission
✅ At RIBA Stage 2 or 3 (concept to developed design)
✅ Before finalising glazing, layout, or ventilation decisions

Waiting until later in the project can result in:

Redesigns and delays
Failed planning applications
Uncomfortable homes for future occupants

🧠 Why Choose Assessment Hive?

At Assessment Hive, we provide fast, accurate overheating assessments that meet all local authority planning requirements. Our team of experienced assessors uses CIBSE TM59-compliant software to ensure your design is fit for purpose.

We help: