Name: Part O Calculator
Price: 35.00 GBP
Author: Part O Calculator

Question 1

What is Part O of the Building Regulations?

Accepted Answer

Part O (Approved Document O: Overheating) is a requirement under the Building Regulations 2010 for England, in force since 15 June 2022. It requires all new residential buildings to be designed to limit unwanted solar gains in summer and provide adequate means to remove excess heat. Compliance is demonstrated through the Simplified Method (glazing-to-floor-area ratios and ventilation checks) or Dynamic Thermal Modelling using CIBSE TM59.

Question 2

What are the Part O glazing limits?

Accepted Answer

Glazing limits depend on location risk zone and cross-ventilation. High Risk zones (London, central Manchester): 20% with cross-ventilation, 15% without. Moderate Risk zones (rest of England): 25% with cross-ventilation, 20% without. These are the maximum total glazing area as a percentage of total floor area.

Question 3

Does Part O apply to extensions and loft conversions?

Accepted Answer

Currently, Part O only applies to new residential buildings. It does not apply to extensions, loft conversions, or changes of use added after the building is constructed. However, the Future Homes Standard consultation (published March 2026) is actively considering widening Part O's scope to include extensions and conversions. Industry bodies recommend conducting overheating assessments on extensions and conversions as best practice.

Question 4

What is the Future Homes Standard and how does it affect Part O?

Accepted Answer

The Future Homes Standard (FHS) comes into force on 24 March 2027, requiring new homes to produce 75-80% less carbon with low-carbon heating and enhanced insulation. More airtight, highly insulated homes increase overheating risk, making Part O compliance even more critical. The government's FHS consultation response (March 2026) is considering widening Part O to cover extensions and conversions, and may introduce additional requirements for noise-affected areas.

Question 5

What is the Part O Simplified Method?

Accepted Answer

The Simplified Method (Section 2 of Approved Document O) is a prescriptive compliance route that checks four criteria: overall glazing-to-floor-area ratio, most-glazed room limit, total free area (openable ventilation), and bedroom free area (4% minimum). Limits vary by overheating risk zone and cross-ventilation availability. If a building fails the Simplified Method, Dynamic Thermal Modelling using CIBSE TM59 is required.

Question 6

What is the free area requirement in Part O?

Accepted Answer

Free area is the effective openable ventilation area. Part O requires a minimum free area for the whole dwelling (the greater of a percentage of floor area or 55% of glazing area) and 4% of floor area for each bedroom. The percentage of floor area varies: High Risk with cross-vent 8%, without 6%; Moderate Risk with cross-vent 9%, without 7%. Different window types have different effectiveness factors — rooflights typically provide only 8-15% of their area as free area.

Question 7

What happens if my building fails the Part O Simplified Method?

Accepted Answer

If the Simplified Method criteria are not met, you must either redesign to reduce glazing and increase ventilation, or commission Dynamic Thermal Modelling (DTM) using CIBSE TM59 with approved software such as IES-VE, TAS, or DesignBuilder. DTM simulates hourly temperatures to prove acceptable thermal comfort. Professional Part O tools like partouk.com include AI-powered design optimisers that show you the minimum changes needed to pass before committing to expensive DTM.

Question 8

How do I calculate Part O compliance for a loft conversion with rooflights?

Accepted Answer

Rooflights have significantly lower ventilation effectiveness than standard windows. Centre-pivot rooflights provide approximately 10% free area, top-hung rooflights 15%, and electric rooflights 8%. A professional Part O calculator with rooflight support will apply the correct free area factors automatically. Loft bedrooms with rooflights are particularly challenging for the 4% bedroom free area requirement.

Help & Guidance

Key Definitions

Frequently Asked Questions

Official Resources

UKUK Government Publications

CICIBSE Technical Guidance

+Related Standards & Guidance

Help & Guidance

Key Definitions

What is Part O?

What is the Simplified Method?

When can't I use the Simplified Method?

What is cross-ventilation?

What are g-values?

What is CIBSE TM59?

What determines a High Risk location?

Frequently Asked Questions

Does this tool guarantee Building Control approval?

My building failed - what now?

What if my postcode isn't recognised?

Does Part O apply to extensions and conservatories?

Can I use this for commercial buildings?

What about internal blinds or curtains?

How accurate is the postcode-based risk assessment?

What glazing should I include in my calculation?

Why do high risk glazing limits sometimes allow more glazing than moderate risk?

Which standard should I use when two façades have equal glazing?

Which standard for a façade midway between two compass points?

Do buildings in central Manchester follow high or moderate risk values?

What does '50 degrees altitude cut-off' mean for overhangs?

How does the 650mm window handle standard work?

Do juliet balconies affect equivalent area calculations?

If mechanical cooling is used to pass Part O, does it affect Part L?

Can windows be modelled as open at night if they're made secure?

Is extra guarding needed if windows have restrictors?

Official Resources

UKUK Government Publications

CICIBSE Technical Guidance

+Related Standards & Guidance