Building Regulations Approved Document O
Part O Simplified Method & TM59 Assessments
The Simplified Method is the default route for most homes, but for some homes, it's a design dead-end. We provide the technical evidence to pass Part O. Whether that's a basic prescriptive check or a full TM59 model to save your glazing and bi-folds.
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Part O Overheating Requirements
Quick Reference TL;DR
Do I need Part O?
Yes. All new dwellings need a Part O overheating assessment alongside SAP.
Simplified or TM59?
Most houses pass Simplified. London projects need TM59 (London Plan). Also for high glazing, flats, noisy sites.
What are the glazing limits?
Two checks: total dwelling AND most glazed room. Varies by orientation and ventilation type.
Can I keep my bi-folds?
Often yes, but you may need TM59 modelling to prove compliance.
How long does it take?
Simplified: 2-4 days. TM59: 5-10 days depending on complexity.
What if my design fails?
We'll recommend fixes or switch to TM59. Revisions included until you pass.
Ready to get started?
Upload your plans and we'll confirm which route you need and provide a fixed quote.
Understanding Approved Document O
Introduced to prevent new homes from overheating during summer months, Part O limits solar gain and mandates minimum removal of excess heat. Every new dwelling must follow one of two paths: The Simplified Method or The TM59 Dynamic Route.
Part O Simplified Method
The majority of new-build houses pass using this prescriptive approach. It avoids software simulation by adhering to strict, pre-defined limits on glazing ratios and ventilation free-areas based on your project's risk location.
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1
Glazing Limits & Orientation
Compliance is calculated by comparing total glazing area against the total floor area. Limits are significantly tighter for units without cross-ventilation (openings on opposite facades).
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2
The Free Area Requirement
This is the most common point of failure. You must provide a minimum Equivalent Area of window openings, often equivalent to 12% of the room's floor area.
Glazing Limits - Two Checks
The simplified method has two compliance checks: total dwelling glazing AND the most glazed room. Limits also differ based on whether your dwelling has cross-ventilation (openings on opposite facades).
Cross-Ventilated (Moderate Risk)
Total dwelling glazing by orientation:
- North: 18% (room: 37%)
- East: 18% (room: 37%)
- South: 15% (room: 30%)
- West: 11% (room: 22%)
No Cross-Ventilation (Moderate Risk)
Stricter limits for single-aspect:
- North: 18% (room: 26%)
- East: 18% (room: 26%)
- South: 15% (room: 15%)
- West: 11% (room: 11%)
For complete tables including High Risk (London) limits and free area requirements, see our Part O Simplified Method Cheat Sheet. Note that London projects typically require TM59 regardless of these limits due to the London Plan.
CIBSE TM59 Overheating Assessment
When a design falls outside the prescriptive limits of the Simplified Method, TM59 uses physics based simulation to demonstrate compliance. By building a digital twin of your project to the CIBSE TM59 methodology, we model actual performance rather than relying on rigid glazing percentages.
Design Flexibility
TM59 factors in shading credits for balconies, overhangs, and external shutters. This often allows for significantly more glass than the 11-15% cap mandated by the Simplified Method.
Acoustic & Security Fixes
For noisy or urban sites, we can model mechanical ventilation (MVHR) or external shading to ensure a pass even when windows must remain closed at night.
Communal Infrastructure
As per Section 1.3, this is the only valid route for buildings with communal heating or hot water pipes, as it accounts for constant internal heat gains.
Building Physics
We use local CIBSE weather files and account for thermal mass (brick vs timber) to show how the building actually sheds heat over a 24-hour cycle.
Choosing Your Compliance Route
Approved Document O offers two distinct methods for demonstrating compliance. Selecting the wrong route early in the design phase can lead to late-stage architectural changes.
The Simplified Method
A prescriptive, calculation-based approach that sets absolute limits on glazing area and mandates specific window opening sizes.
- Glazing Limits: 11-18% of floor area depending on orientation. West-facing is strictest
- Cross Ventilation: Tighter limits for non cross ventilated dwellings in the highest glazed room
- Acoustics & Security: Noise or security considerations could reduce the dwelling's free area, pushing towards a TM59
CIBSE TM59 (Dynamic Route)
Advanced 3D thermal simulation that models hourly temperature fluctuations based on local weather data and internal heat gains.
- London Projects: Required by the London Plan regardless of simplified method limits
- High-Glazing Designs: Proves compliance for floor-to-ceiling glass and large bi-folds
- Apartments: Required for communal heating with pipe work passing through the dwellings
- Noisy Sites: Models alternative ventilation strategies for closed window compliance
Part O & TM59 - Frequently Asked Questions
What is Part O of the Building Regulations?
Part O (Approved Document O) is the Building Regulation introduced in June 2022 to prevent new homes from overheating. It sets limits on solar gain through glazing and requires minimum ventilation openings to remove excess heat during summer.
Do all new builds need a Part O assessment?
Yes. Every new dwelling in England requires Part O compliance alongside SAP. This is for both new houses and flats.
What is the difference between Simplified Method and TM59?
The Simplified Method is a prescriptive check against glazing limits and ventilation openings, no simulation is required. TM59 is dynamic thermal modelling that simulates hourly temperatures using weather data. TM59 (DSM) offers more design flexibility but costs more.
When do I need TM59 instead of Simplified Method?
You need TM59 if: your project is in London (London Plan requirement), glazing exceeds the simplified limits, the dwelling is single-aspect, has communal heating, is on a noisy site requiring closed windows, or the ventilation free area requirements cannot be met.
How long does a Part O assessment take?
Simplified Method assessments are typically completed within 2-4 working days. TM59 dynamic modelling takes longer depending on complexity - usually 5-10 working days for a single dwelling.
What happens if my design fails Part O?
We provide clear recommendations to achieve compliance - typically reducing glazing, adding external shading, increasing ventilation openings, or switching to TM59 to prove the design works through simulation. Revisions are included until you pass.
Can I keep my large windows and bi-fold doors?
Often yes, but you may need TM59 modelling to prove it. The Simplified Method has strict glazing caps, but TM59 can account for shading from overhangs, balconies, or external blinds that reduce solar gain and allow larger glazing areas.
Do you provide Part O and SAP together?
Yes. Most new builds need both SAP (Part L) and Part O. We offer combined packages at a reduced rate - one submission, one point of contact.
Need a Part O expert?
Whether it's a single house needing a simplified check or a large development requiring full TM59 modelling, we provide clear, Building Control ready reports.