You're about to break ground on a new residential project. Designs are nearly final, planning permission is looming and someone says, “Have you sorted the TM59 yet?”

Cue the panic.

It’s a common moment for developers and architects across the UK. And the next question is almost always, “What’s it going to cost me?”

The TM59 Overheating Assessment has fast become a key requirement in modern housing design. But the price tag varies and so does what you get for your money. This guide breaks it down in plain English, no waffle, no AI buzzwords. Just what you need to know, before you spend a penny.

What is a TM59 Overheating Assessment?

Before we talk numbers, let’s get clear on what TM59 actually does.

TM59 is a standard developed by CIBSE (Chartered Institution of Building Services Engineers). It tests whether your residential design will overheat in summer using detailed computer simulations. Think of it as a digital stress-test for your floorplans.

You’ll need it if you’re:

• Building flats or apartments
  
  
• Designing in dense urban areas
  
  
• Working on student housing, care homes or social housing
  
  
• Submitting for planning where overheating is flagged as a risk
  
  

What’s the TM59 Assessment Cost?

TM59 Overheating Assessments from just £225 + VAT a smart, affordable way to ensure your residential design meets compliance and keeps future occupants comfortable, all year round.

These are typical figures from leading assessors, including our team at Gradwell Group. But don’t be fooled by price alone what's included matters more.

What Affects the Price?

No two projects are the same, but here’s what usually pushes costs up or down:

Factors That Influence Cost

• Number of dwelling types (i.e., how many different layouts need testing)
  
  
• Window configurations and shading complexity
  
  
• Ventilation strategy (natural, mechanical, or hybrid)
  
  
• Speed of delivery – express turnaround adds premium
  
  
• Number of revisions or redesigns
  
  

What’s Included in a Proper TM59 Assessment?

A good assessment doesn’t just hand you a number. It gives you clarity. You should expect:

• Dynamic simulation modelling (DSM) using approved software
  
  
• Use of CIBSE-recommended weather files (like DSY1 2020)
  
  
• Simulated indoor temperatures per room, per hour
  
  
• Review of solar gains, shading, thermal mass
  
  
• Window and ventilation strategy testing
  
  
• Full PDF report, suitable for submission
  
  
• Design advice if overheating risks are flagged
  
  

At Gradwell Group, we also work closely with your design team, offering mitigation strategies that won’t break the budget or ruin your aesthetic.

Explore TM59 Services at Gradwell Group

Why It’s Worth Every Penny?

Here’s the thing: a failed TM59 can cost far more than the assessment itself.

Risk of skipping or doing it late:

• Planning refusal or delays
  
  
• Expensive redesigns at RIBA Stage 4+
  
  
• Unhappy residents dealing with heat
  
  
• Non-compliance with Building Regulations Part O
  
  

 The UK Climate Risk Independent Assessment warns that overheating is now one of the biggest health risks in new homes especially for the elderly and children.

What You Get With Gradwell Group?

A client came to us with a 12-unit project in London. They were behind schedule, under pressure, and worried their high glazing ratio would cause issues.

We:

• Delivered a full TM59 in 5 working days
  
  
• Identified 3 key risks
  
  
• Recommended subtle design tweaks (brise soleil + night purge windows)
  
  
• Passed the simulation without needing air con
  
  
• Got them over the line before planning deadline
  
  

FAQs

1: Can I do a TM59 assessment myself?

A: No. It requires dynamic thermal modelling software and an experienced assessor.

2: Is it mandatory for planning approval?

A: Not always legally, but increasingly required by local authorities to meet Part O or BREEAM.

3: What happens if my design fails?

A: We’ll suggest passive fixes like louvres, thermal mass changes, or better airflow to help you pass.

4: Can I use mechanical cooling to meet TM59?

A: Only as a last resort. Passive design strategies come first.

Final Thoughts

TM59 isn’t a box to tick it’s a design tool, a safety net, and often the final word in planning compliance. When done right, it pays for itself in peace of mind, project progress, and homes that work all year round.

At Gradwell Group, we don’t do off-the-shelf assessments. We do tailored, detailed, and deadline-friendly TM59 reports that keep your project moving.