Part O
Overheating
Simplified Method & TM59 Dynamic Modelling.
Part O overheating assessments for new-build dwellings and residential schemes. Advice on which method is best, multiple recommendations to pass, and one free re-run of TM59.
- Advice on which method is best
- Multiple recommendations to pass (TM59)
- One free re-run of TM59
2 Methods
Simplified & TM59
Part O
2022 Regulations
SI 4
London Plan Ready
1 Re-Run
Of TM59, Free
What is a Part O assessment?
Part O of the 2022 Building Regulations tackles overheating in new residential buildings. Its purpose is to mitigate overheating caused by unwanted solar gains and to provide adequate means of removing excess heat, protecting occupant health and welfare rather than guaranteeing thermal comfort.
Compliance is demonstrated via one of two methods. The Simplified Method sets defined limits on window size and openable area by room type and location — fast and inexpensive. CIBSE TM59 Dynamic Thermal Modelling simulates indoor temperatures hour by hour across a future summer and compares against pass criteria for living rooms and bedrooms — flexible, and often the only route for high-glazing or single-aspect dwellings.
Who needs Part O?
Every new residential building in England and Wales — dwellings, institutional residential buildings, student accommodation, care homes, and the shared communal spaces of multi-unit buildings. London Plan Policy SI 4 additionally requires most London schemes to submit a TM59 assessment regardless of whether the Simplified Method would pass.
When should I book?
At design stage. Overheating is one of the hardest regulations to pass late, because the cheapest fixes (orientation, glazing ratio, external shading, openable area) all sit at the early design stage. Engage us early and we recommend the cheapest compliant combination; leave it late and active cooling may be the only remaining option.
Simplified or TM59.
Part O gives you two routes. We advise which one suits your scheme before you pay for anything.
Simplified Method
Defined limits on maximum window size and minimum openable area per room, by location. Faster and cheaper — suitable for most standard dwellings that sit within the parameters.
- Faster turnaround
- Lower cost than TM59
- Fallback to TM59 if it fails
CIBSE TM59
Full Dynamic Thermal Modelling — hourly simulation across a design summer, comparing modelled room temperatures against Part O pass criteria. Much more flexible; required by London Plan and for complex dwellings.
- Hourly dynamic simulation
- Credit for shading, glazing g-value & ventilation
- One free re-run if first iteration fails
London Plan TM59
Most London boroughs require a TM59 regardless of whether the Simplified Method would pass, under London Plan Policy SI 4 and its cooling hierarchy. We include this as standard in our London Plan packages.
- Policy SI 4 compliance
- Included in London Plan packages
Institutional Residential
Care homes, student accommodation, HMOs and schools with boarding — all fall under Part O. Occupancy and use patterns often push these towards TM59.
- Care homes, student accommodation, HMOs
- Occupancy-aware modelling
Full support if a TM59 fails.
Advice on which method is best, multiple recommendations to pass as outlined in TM59, and one free re-run if the first TM59 iteration does not pass.
If TM59 does not pass, we help
Multiple recommendations to pass as outlined in TM59 — plus one free re-run once the design has been updated.
Elmhurst-Registered SAP Basis
Part O assessments produced by Elmhurst-registered SAP assessors. Methodology consistent with your Part L calculations so numbers line up across documents.
Advice on Which Method Is Best
We advise whether the Simplified Method or TM59 is the right route before you pay for anything — so you only pay for the method that actually fits your scheme.
Multiple Recommendations to Pass
Recommendations to pass as outlined in TM59 — shading, glazing g-value, openable area, ventilation strategy — so you can choose the route that suits your design and budget.
One Free Re-Run of TM59
If the first TM59 iteration does not pass, we give you one free re-run with your updated design choices — so you can test a revised shading or glazing strategy without a second fee.
Design-Stage Advice Included
We engage at design stage (not just verification), flagging high-risk orientations, glazing ratios and ventilation routes before specification is locked in.
London Plan & TM59 Ready
Full London Plan Policy SI 4 compliance, Part O Simplified Method, and full TM59 Dynamic Thermal Modelling — one provider for every scenario.
Where overheating commonly trips up.
Six design patterns that regularly cause Part O failures. Flagging these at design stage gives you the widest and cheapest set of options.
Simplified Method
The faster, cheaper route. Defined limits on window size and ventilation area by room type, orientation and location. Suitable for most standard dwellings.
CIBSE TM59 Dynamic Modelling
The fall-back when the Simplified Method fails. Hourly dynamic simulation across a design summer, with much more flexibility on how compliance is achieved.
High-glazing apartments
Glass-fronted apartments (especially south and west) are common Part O failures. External shading, lower g-value glazing and purge ventilation all help.
Single-aspect dwellings
Flats with only one openable facade struggle for cross-ventilation. TM59 lets us model the specific purge ventilation strategy proposed.
Urban sites
London and other urban locations have higher assumed external temperatures. The Simplified Method applies different window limits for high-risk locations.
Institutional & student housing
Care homes, student accommodation and shared residential buildings all fall under Part O — and often need TM59 given occupancy patterns.
Three steps to Part O compliance.
Advice on which method fits your scheme, then the modelling itself, then design recommendations if a re-run is needed.
Method Advice
Send us plans, elevations and a sketch of the glazing strategy. We advise whether the Simplified Method or TM59 is right for your scheme, and quote the correct method.
We Model
We run the Simplified Method or full TM59 dynamic simulation. If TM59 does not pass, we produce multiple recommendations to pass — shading, glazing, ventilation combinations.
Report & Free Re-Run
You get a full Part O assessment report for Building Control and planning. If your TM59 does not pass, you get one free re-run after the design is updated.
Method-specific pricing.
Simplified Method and TM59 Dynamic Thermal Modelling are priced differently — the Simplified Method is significantly cheaper. Tell us the scheme and we will confirm which applies.
Pricing depends on method (Simplified or TM59), unit count and complexity. Send us plans for a fixed-price quote — with method advice included before you commit.
- Advice on which method is best
- Multiple recommendations to pass (TM59)
- One free re-run of TM59
- London Plan SI 4 compliance
*Priced per scheme. Simplified Method is materially cheaper than TM59. Contact us for an accurate quote.
Get your Part O quote
Takes 60 seconds. No obligation. Working to a planning deadline? Tell us — we'll prioritise your project.
Part O questions.
Everything you need to know about Part O overheating assessments — the two methods, London Plan SI 4, and what happens if a TM59 does not pass.
Still Got a Question? Ask Us What is Part O?
Part O of the Building Regulations (introduced June 2022) tackles overheating in new residential buildings. It aims to mitigate excess internal temperatures caused by unwanted solar gains, and to provide adequate means of removing heat from the indoor environment. It applies to new dwellings, institutional residential buildings, and multi-use residential buildings in England and Wales.
Which method do I need — Simplified or Dynamic Thermal Modelling (TM59)?
The Simplified Method is the usual first route — it's faster, cheaper, and suitable for most dwellings. CIBSE TM59 Dynamic Thermal Modelling is used when the Simplified Method fails, or where the scheme has large glazing, unusual orientations or has to hit tighter London Plan targets. We advise on which method is best for your specific scheme before we start.
How much does a Part O assessment cost?
Part O assessments are quoted individually — the Simplified Method is significantly cheaper than TM59 Dynamic Thermal Modelling. Send us your plans and we will confirm which method applies and a fixed price. TM59 includes one free re-run if the first iteration does not pass.
When should I book my overheating assessment?
As early as possible at design stage. Overheating is often the hardest Part Regulation to pass late in design — the design-level fixes (orientation, glazing ratio, external shading, openable areas) are the cheapest and most effective. Leave it too late and the only remaining fix may be mechanical cooling.
What buildings does Part O apply to?
New-build dwellings, institutional residential buildings where people sleep (care homes, student accommodation, schools with boarding), multi-use residential buildings, live/work units, and the shared communal rooms and common spaces of buildings containing more than one residential unit. If in doubt send us the brief.
What happens if my building fails the Simplified Method?
The fallback is a full CIBSE TM59 Dynamic Thermal Modelling assessment, which offers more flexibility via factors like solar shading, glazing g-value and natural ventilation strategy. TM59 often passes buildings that the Simplified Method flags as non-compliant, particularly in dwellings with high glazing or constrained ventilation.
What is TM59?
CIBSE TM59 is the industry-standard methodology for Dynamic Thermal Modelling of overheating risk in homes. It simulates indoor temperatures across a future summer using detailed hourly weather data, occupancy patterns and the building design, and compares the results against Part O pass criteria in living rooms and bedrooms.
Can I improve the design to reduce overheating risk?
Yes — and it's much cheaper to do it at design stage than late. Key levers: external shading (brise-soleil, overhangs, balconies), lower solar-gain glazing (lower g-value), reducing glazing on south and west elevations, increasing openable window area, and specifying purge ventilation. We recommend the cheapest combination to pass.
Does Part O interact with The London Plan?
Yes. London Plan Policy SI 4 requires overheating risk to be reduced through the cooling hierarchy, and most London boroughs request a TM59 assessment regardless of whether the Simplified Method would pass. Our London Plan compliance packages include TM59 as standard for this reason.
Do you cover all UK regions?
Part O applies in England and Wales. Scotland and Northern Ireland have their own overheating guidance (typically using CIBSE TM52 or TM59 methodology). We deliver overheating assessments across all UK regions.