Part O vs TM59: What's the Difference?

Introduction

If you're involved in residential development in England, you've probably heard the terms "Part O" and "TM59" used interchangeably. This confusion is incredibly common - even among experienced developers and architects. But here's the thing: they're not the same, and understanding the difference is crucial for your project's success.

Quick answer: Part O is the building regulation you must comply with. TM59 is the standard method we use to prove that compliance.

Let me explain in detail.

What is Part O?

Part O is a UK Building Regulation introduced in June 2022 to address overheating risk in new residential buildings. It's part of the Building Regulations 2010 (as amended) and applies to all new residential buildings in England.

Why Part O Exists

The regulation was introduced because:

• Climate change is making UK summers hotter
• Modern buildings with high insulation can trap heat
• Overheating poses serious health risks, especially for vulnerable people
• Reducing overheating reduces the need for energy-intensive air conditioning

Who Part O Applies To

Part O is mandatory for:

• New residential buildings (houses and apartments)
• Extensions that create new dwellings
• Material changes of use that create new dwellings

It applies throughout England. Scotland, Wales, and Northern Ireland have their own regulations.

What Part O Requires

The regulation requires you to demonstrate that your building will not overheat. Specifically, you must show:

1. Limiting solar gains - Through glazing orientation, area, and solar shading 2. Providing adequate ventilation - Natural or mechanical ventilation strategies 3. Assessment against overheating criteria - Using approved methodologies

This is where TM59 comes in.

What is TM59?

TM59 stands for CIBSE Technical Memorandum 59: "Design methodology for the assessment of overheating risk in homes." It's a technical guidance document published by the Chartered Institution of Building Services Engineers (CIBSE).

The Purpose of TM59

TM59 provides:

• A standardized methodology for assessing overheating
• Clear criteria for what counts as "acceptable" thermal comfort
• Dynamic thermal simulation approach using weather data
• Room-by-room assessment requirements

How TM59 Works

The TM59 assessment involves:

1. Dynamic thermal modeling - Using software like IESVE to simulate your building's thermal performance hour-by-hour throughout the year 2. Weather data - Using Design Summer Year (DSY) weather data representing hot conditions 3. Assessment criteria - Checking bedrooms and living spaces against specific temperature thresholds 4. Pass/fail criteria - Clear benchmarks your building must meet

TM59 Criteria

For a home to pass TM59:

Bedrooms must: - Not exceed 26°C for more than 1% of annual hours (approximately 32 hours per year)

Living rooms/kitchens/other spaces must: - Not exceed 26°C for more than 3% of annual hours during occupied hours

These criteria ensure occupants won't experience excessive overheating that could affect health and wellbeing.

Key Differences: Part O vs TM59

Here's the crucial distinction:

| Aspect | Part O | TM59 | |--------|--------|------| | What it is | Building Regulation (law) | Assessment methodology (guidance) | | Created by | UK Government | CIBSE (industry body) | | Purpose | Sets the requirement | Provides the method to prove compliance | | Status | Mandatory | Recommended approach | | Analogy | The speed limit | The speed camera |

A Helpful Analogy

Think of it this way:

• **Part O** is like a speed limit on a road - it's the rule you must follow
• **TM59** is like a speed camera - it's the proven way to measure whether you're following that rule

You must comply with Part O (the regulation). Using TM59 (the methodology) is the standard way we prove that compliance.

Alternative Methods

While TM59 is the standard and most widely accepted method for demonstrating Part O compliance, it's not the only option. Alternative approaches include:

• **Simplified Method** - Using tables in Approved Document O for very simple buildings
• **CIBSE TM52** - Sometimes used for residential, though primarily for commercial buildings
• **Other dynamic simulation methods** - As long as they meet the Part O criteria

However, TM59 is the industry standard for residential buildings and the method most Building Control officers expect to see.

What This Means for Your Project

When You Need Part O Compliance

If you're developing: - New residential buildings - Converting non-residential buildings to residential use - Adding residential extensions

You need Part O compliance. It's not optional.

The Assessment Process

Here's what typically happens:

1. Design stage - We receive your architectural drawings and specifications 2. Modeling - We create a dynamic thermal model using IESVE software 3. TM59 simulation - We run the model against TM59 criteria 4. Results analysis - We identify any rooms that fail 5. Recommendations - If needed, we suggest design modifications (more shading, better ventilation, etc.) 6. Final report - We provide a Part O compliance report using TM59 methodology 7. Building Control submission - Your report goes to Building Control for approval

Timeline and Cost

Most TM59 assessments for Part O compliance take 1-2 weeks from receiving complete information.

Pricing typically ranges from £350-£800+ depending on: - Number of units - Building complexity - How quickly you need results

Common Questions

"Do I need TM59 if I have Part O compliance?"

You don't "have" Part O compliance yet - you need to demonstrate it. TM59 is how we demonstrate it. So yes, you typically need a TM59 assessment to prove Part O compliance.

"Can I just use the Simplified Method in Approved Document O?"

The Simplified Method only works for very straightforward buildings with limited glazing. Most modern developments need the full TM59 assessment because they have: - Large windows - Multiple orientations - Open-plan layouts - Mixed-use spaces

"What if my building fails TM59?"

Don't panic. Most buildings can be made compliant with relatively simple modifications: - Adding external blinds or brise soleil - Optimizing window sizes and positions - Improving ventilation strategy - Using solar control glazing on certain orientations

We provide detailed recommendations and can reassess once changes are made.

"When should I get the assessment done?"

As early as possible in the design process. Getting TM59 assessment done early means: - Any required changes are easier and cheaper to implement - No delays waiting for Building Control approval - Greater certainty about project costs

Conclusion

The difference between Part O and TM59 is simple once you understand it:

• **Part O = The regulation** (what you must comply with)
• **TM59 = The methodology** (how you prove compliance)

You need Part O compliance for your residential development. We use TM59 dynamic thermal simulation to demonstrate that compliance. The result is a report that Building Control will accept, allowing your project to proceed.

Need Part O Compliance for Your Project?

We specialize in TM59 assessments for Part O compliance. Our process is:

• **Fast** - 1-2 week typical turnaround
• **Thorough** - Using industry-leading IESVE software
• **Clear** - Jargon-free reports and recommendations
• **Accepted** - By Building Control across the UK

Get started today: Contact us for a free quote. We'll review your project and provide a fixed price and timeline within 24 hours.

Whether you're a developer, architect, or builder, we make Part O compliance straightforward and stress-free.