Part O cheat sheet for the simplified method

Cheat Sheet – Part O Simplified Method

December 31st, 2025

Ian Kay

3 min read

Table of Contents

Below you’ll find a quick reference guide for the glazing limits for the simplified method of Approved Document O (Part O). Use the table of contents above to quickly navigate to the relevant risk and ventilation type for your project. Part O is to reduce and manage the risk of overheating within homes.

Dwelling With Cross Ventilation in a Moderate Risk Location

This type of dwelling has the least restrictions in the simplified method of Part O. The glazing limits for each facade are in Table 1.1 below.

Maximum Area Of Glazing (% Floor Area)Maximum area of
glazing in the most
glazed room
(% floor area of room)
North1837
East1837
South1530
West1122
Table 1.1- Cross ventilated dwelling in a moderate risk location – maximum glazing area

Minimum free area for cross ventilated dwellings in a moderate risk location are in Table 1.2 below

Total Minimum Free AreaThe greater of the following:
a. 9% of the total dwelling floor area or,
b. 55% of the glazing area
Bedroom Minimum Free Area4% of the floor area of the room
Table 1.2- Cross ventilated dwelling in a moderate risk location – Minimum free area

Dwelling Without Cross Ventilation in a Moderate Risk Location

Non cross ventilated dwellings is when cross ventilation is not possible, e.g. a single aspect or corner flat. These glazing limits are in table two below.

Maximum Area Of Glazing (% Floor Area)Maximum area of
glazing in the most
glazed room
(% floor area of room)
North1826
East1826
South1515
West1111
Table 2.1- Dwelling without cross ventilation in a moderate risk location – maximum glazing area

Minimum free area for non cross ventilated dwellings in a moderate risk location are in Table 2.2 below

Total Minimum Free AreaThe greater of the following:
a. 12% of the total dwelling floor area or,
b. 80% of the glazing area
Bedroom Minimum Free Area4% of the floor area of the room
Table 2.2- Dwelling without cross ventilation in a moderate risk location – Minimum free area

Dwelling With Cross Ventilation in a High Risk Location

Due to local planning guidance in the London Plan, passing overheating compliance on Part O rarely follows the simplified route. The targets, however, are in Table 3.1 below.

Maximum Area Of Glazing (% Floor Area)Maximum area of
glazing in the most
glazed room
(% floor area of room)
North1537
East1837
South1522
West1837
Table 3.1- Cross ventilated dwelling in a high risk location – maximum glazing area

Minimum free area for cross ventilated dwellings in a high risk location are in Table 3.2 below

Total Minimum Free AreaThe greater of the following:
a. 6% of the total dwelling floor area or,
b. 70% of the glazing area
Bedroom Minimum Free Area13% of the floor area of the room
Table 3.2- Cross ventilated dwelling in a high risk location – Minimum free area

Dwelling Without Cross Ventilation in a High Risk Location

Again, it is unlikely that the simplified method would be relevant for a property in a high risk location (London) and a mechanically cooled dwelling would have the highest levels of restriction on glazing volumes and opening sizes. These targets, however, are in Table 4.1 below.

Maximum Area Of Glazing (% Floor Area)Maximum area of
glazing in the most
glazed room
(% floor area of room)
North1526
East1118
South1111
West1118
Table 4.1- Dwelling without cross ventilation in a high risk location – maximum glazing area

Minimum free area for dwellings without cross ventilation in a high risk location are in Table 4.2 below

Total Minimum Free AreaThe greater of the following:
a. 10% of the total dwelling floor area or,
b. 95% of the glazing area
Bedroom Minimum Free Area13% of the floor area of the room
Table 4.2- Dwelling without cross ventilation in a high risk location – Minimum free area

Ian Kay — SAP Assessor

About the Author

Ian Kay is an Elmhurst accredited On Construction Domestic Energy Assessor (OCDEA), Overheating Consultant and the founder of SAPgen. He specialises in Overheating Assessments and SAP Calculations for new builds, conversions and extensions across the UK, helping architects, builders and homeowners achieve building regulations compliance quickly and with zero stress.

When he’s not helping clients, Ian writes clear, practical guides to help the construction industry understand energy compliance, Part L / O rules and best practice build specifications.

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This article offers general guidance based on current SAP 10 and Part O practice. Every project is different, so always check the exact specification and requirements with your assessor before making design or build decisions.