UK Approved Document O · 2021 Edition

Pass Part O overheatingthe first time.

The fastest professional Part O Simplified Method calculator. Get instant compliance results, AI-powered fix suggestions, and a Building Control-ready PDF report - without leaving your browser.

  • Auto risk-zone detection from postcode
  • Per-room glazing, ventilation & bedroom checks
  • AI auto-fix suggests the minimum design changes to pass
  • Formal PDF report - signed & stamped, ready for submission
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AD-O 2021
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Free Tool

Part O postcode risk checker

Find out instantly whether your site falls in a Part O High-risk overheating area. Based on Approved Document O, Appendix C (London Table C1 and Manchester Note C3). No sign-up required.

Risk zones are defined in Approved Document O, Appendix C (2021 edition).

HOW IT WORKS

Compliance in three steps

From postcode to Building Control-ready report in under 8 minutes. No training, no spreadsheets, no guesswork.

01

Enter your project

Postcode, building type and floor area. We instantly identify your overheating risk zone and the AD-O glazing limits that apply.

02

Add rooms or facades

Use the room-by-room schedule (or whole-dwelling Quick Check) - or upload your drawings and let AI extract window data for you.

03

Get your report

Instant pass/fail with per-room compliance, AI fix suggestions, and a professional PDF report ready for Building Control.

SIMPLE, TRANSPARENT PRICING

Pay per project. No subscriptions.

Choose the right level of detail for your project - start free, only pay when you're ready to download your report.

Quick Assessment

£35

Whole-dwelling glazing check - perfect for a first compliance sanity check at concept stage.

  • ✓ Postcode risk-zone detection
  • ✓ Glazing-to-floor ratio check
  • ✓ Compliance PDF report
  • ✓ Save & amend any time
RECOMMENDED

Detailed Assessment

£75

Full room-by-room compliance with AI tools and a formal report - Building Control ready.

  • ✓ Per-room glazing + ventilation checks
  • ✓ AI Auto-Fix & Strategy Advisor
  • ✓ Rooflight & loft conversion support
  • ✓ Formal stamped PDF report
  • ✓ Duplicate & compare design variants
  • ✓ Multi-plot projects: clone across a whole development

See exactly what you get before you start:

BUILT FOR HOUSEBUILDERS & DEVELOPERS

Assessing 6 houses? Set it up once.

Stop rebuilding the same house-type six times. Our project workspace lets you configure your standard type once, then clone it across every plot in seconds - keeping each one independently billable and Building Control-ready.

Group plots under one project

Riverside Meadow, Plots 1–24 in a single dashboard. Site-wide pass/fail at a glance and one combined report for the whole development.

Set a template plot

Set up your standard house type once, then designate it as the project template. Every new plot inherits its rooms, windows and site constraints automatically.

One-click clone

Handed types (Plot 4 is a mirror of Plot 3)? Clone any plot with one click - all room and window data copies across as an editable draft.

Change only what differs

Cloned plots show a "Cloned from Plot 3" banner. Edit orientation, address or the odd window - everything else stays inherited.

Typical saving: ~15 minutes per plot after the first

On a 6-plot development that's roughly 90 minutes back for design work - and zero risk of copy-paste errors between plots.

Each plot still individually billable
INTELLIGENT COMPLIANCE TOOLS

AI tools that fix failing projects

Don't just find out you've failed - discover exactly how to fix it. Our AI analyses your design and shows you the fastest route to compliance.

AI Design Optimiser

Interactive sliders to adjust glazing and ventilation per room with live compliance recalculation. Auto-Fix finds the minimum changes needed to pass.

AI Strategy Advisor

Building-specific compliance strategy with precise room-by-room recommendations, orientation advice, and ventilation improvement suggestions.

Room-by-Room Schedule

Individual pass/fail checks per room with glazing ratios, free area compliance, and bedroom ventilation assessment with fix suggestions.

Rooflight & Loft Support

Built-in rooflight calculator with correct free area factors for centre-pivot, top-hung and electric rooflights. Loft conversion guidance included.

FREQUENTLY ASKED

Part O, answered

Everything you need to know before running your first assessment.

Why Use a Professional Assessment?

Free online calculators give you a rough glazing percentage. Our professional tool gives you everything you need to actually pass Part O - and proves it to Building Control.

Typical Free Calculator

£0

Basic check only

  • Basic glazing percentage check
  • Risk zone identification
  • Room-by-room analysis
  • Free area / ventilation calculations
  • Bedroom 4% openable area check
  • Rooflight effectiveness factors
  • AI Design Optimiser
  • AI Strategy Advisor
  • Building Control PDF report
  • Room-by-room compliance schedule
  • "What-if" scenario modelling
  • Loft conversion guidance

⚠️ A basic glazing % check alone is not sufficient for Building Control. The Simplified Method requires four separate compliance checks plus ventilation analysis.

RECOMMENDED
Part O Calculator - Detailed

£75

Complete professional assessment

  • Basic glazing percentage check
  • Risk zone identification
  • Room-by-room analysis
  • Free area / ventilation calculations
  • Bedroom 4% openable area check
  • Rooflight effectiveness factors
  • AI Design Optimiser
  • AI Strategy Advisor
  • Building Control PDF report
  • Room-by-room compliance schedule
  • "What-if" scenario modelling
  • Loft conversion guidance

✅ Complete Simplified Method assessment with all four compliance checks, AI-powered recovery strategies, and a professional PDF report for Building Control.

Who Benefits from Professional Assessment?

Architects & Designers

Validate glazing ratios and ventilation strategy at design stage - before costly DTM.

Building Control Officers

Room-by-room compliance schedules and professional PDF reports ready for submission.

Developers & Housebuilders

AI optimiser shows the minimum design changes needed to pass - saving time and money.

Energy Assessors

Automated calculations with rooflight factors, loft support, and instant compliance verdicts.

START YOUR ASSESSMENT

Your Part O report, ready in under 10 minutes.

Join thousands of UK architects, assessors and developers using the fastest path to Approved Document O compliance.

No subscription · Pay per project · Reports valid for Building Control submission

Part O Building Regulations - Complete Guide

Everything you need to know about Approved Document O overheating risk compliance for new residential buildings in England. Updated for the Future Homes Standard (March 2026).

What is Part O?

Part O of the Building Regulations 2010 (Approved Document O: Overheating) came into force on 15 June 2022 in England. It requires all new residential buildings to be designed to limit unwanted solar gains in summer and provide adequate means to remove excess heat. The regulation addresses the growing risk of overheating in homes due to climate change, increased insulation levels, and modern airtight construction.

Compliance can be demonstrated through the Simplified Method (Section 1) which uses glazing-to-floor-area ratios based on orientation, risk zone, and cross-ventilation, or throughDynamic Thermal Modelling (Section 2) using the CIBSE TM59 methodology. A professional Part O calculator automates the Simplified Method's glazing compliance checks - overall dwelling glazing limit and per-room glazing limit - saving hours of manual calculation.

Glazing Limits (Tables 1.1 & 1.2)

The maximum glazing area as a percentage of floor area depends on the overheating risk zone, cross-ventilation, and the orientation of the largest glazed façade. Tables 1.1 (with cross-ventilation) and 1.2 (without) in Approved Document O provide specific limits for each combination of orientation (N, E, S, W), risk zone, and ventilation type.

Key points

  • • Both an overall dwelling limit and a per-room limit apply
  • • Limits vary by compass direction - not a single fixed percentage
  • • High risk limits can sometimes be higher than moderate risk because they include mandatory shading
  • • When two façades are equally glazed, use the more stringent standard
  • • If a façade is midway between compass points, use the closest direction; if exactly between, use the more stringent

Equivalent Area & Free Area (Appendix D)

Approved Document O also requires minimum free area (openable ventilation) for natural cooling, assessed via equivalent areas calculated using Appendix D. The equivalent area depends on window type, dimensions, and opening angle.

  • Appendix D tables provide equivalent areas for standard window sizes at specific opening angles (e.g. 30°)
  • 650mm handle height should be used to calculate the safe opening angle for protection from falling
  • Guarding that restricts airflow (e.g. juliet balconies) may reduce equivalent area - assess case-by-case
  • Window restrictors at 100mm opening: no extra guarding needed, but equivalent area must be calculated with restrictor in place

This calculator focuses on the glazing ratio checks from Tables 1.1/1.2. Free area compliance should be verified separately using Appendix D.

Overheating Risk Zones

England is divided into two overheating risk zones based on the Urban Heat Island effect (Approved Document O, Appendix C):

High Risk Zone

All Greater London postcodes (E, EC, N, NW, SE, SW, W, WC) and central Manchester (M1, M2, M3, M5, M15, M16, M50). These areas experience elevated temperatures due to the Urban Heat Island effect - dense urban environments with heat-absorbing surfaces and reduced natural ventilation.

Moderate Risk Zone

All other areas in England. While overheating risk is lower, compliance with Part O is still mandatory for all new residential buildings.

CIBSE TM59 Dynamic Thermal Modelling

When a dwelling cannot pass the Simplified Method, compliance must be demonstrated through Dynamic Thermal Modelling per CIBSE TM59. Two criteria must be met:

Criterion 1: Living Rooms & Kitchens

Hours when ΔT ≥ 1K must not exceed 3% of annual occupied hours (typically ≤ 146 hours).

Criterion 2: Bedrooms

Hours when operative temperature exceeds 26°C during sleeping hours (10pm-7am) must not exceed 32 hours annually (1% of sleeping hours).

Approved DTM software: IES-VE, TAS (EDSL), DesignBuilder. DTM typically costs £1,500-£3,000+ and takes 4-6 weeks. Using a professional Simplified Method tool first can identify whether DTM is truly needed - and the AI Design Optimiser can show the minimum changes required to pass without it.

When Does Part O Apply?

Part O currently applies to:

  • All new residential buildings (houses, flats, maisonettes)
  • Conservatories on new residential buildings

Does not apply to: Extensions or conservatories added to residential buildings after they are built. Buildings undergoing a change of use. See paragraph 0.3 of Approved Document O.

⚠️ Future change: The Future Homes Standard consultation response (March 2026) is considering widening Part O to include extensions, conversions and change-of-use projects. Industry bodies already recommend voluntary overheating assessments for these building types.

Future Homes Standard & Part O

The Future Homes Standard (FHS) comes into force on 24 March 2027. It requires new homes to produce 75-80% less carbon emissions through low-carbon heating (heat pumps), enhanced insulation, improved airtightness, and on-site renewable electricity generation.

These changes directly impact Part O compliance: more airtight, highly insulated homes with heat pump systems are at greater risk of overheating. The FHS consultation response (March 2026) addresses Part O specifically:

  • Scope widening under review - extensions and conversions may require Part O compliance
  • Additional ventilation guidance for noise-affected sites (near roads, railways, flight paths)
  • Home Energy Model (HEM) replaces SAP for energy calculations from 2027
  • Current Approved Document O (June 2022) remains in force - no changes to the Simplified Method yet

Why Use a Professional Part O Tool?

A basic "glazing percentage" calculator doesn't account for orientation-specific limits or per-room checks. A professional assessment tool provides:

  • Orientation-specific limits - correct Tables 1.1/1.2 values based on largest glazed façade direction
  • Both glazing checks - overall dwelling limit and per-room most-glazed-room limit
  • AI Design Optimiser showing minimum changes needed to pass before committing to DTM
  • Building Control ready PDF reports with room-by-room compliance schedules
  • Cost savings - identifying the fix at £75 vs commissioning DTM at £1,500-£3,000+

Frequently Asked Questions - Part O Compliance

What is Part O of the Building Regulations?▾

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.

What are the Part O glazing limits?▾

Glazing limits depend on the risk zone, cross-ventilation, and the orientation of the largest glazed façade. Tables 1.1 (with cross-ventilation) and 1.2 (without) in AD-O provide specific limits for each combination. Both an overall dwelling limit and a per-room limit apply. High risk limits can sometimes allow more glazing than moderate risk because they include mandatory shading. When two façades are equally glazed, use the more stringent standard.

Does Part O apply to extensions and conservatories?▾

Part O only applies to new residential buildings (see paragraph 0.3 of AD-O). Conservatories on new residential buildings must meet Part O requirements. However, Part O does not apply to extensions or conservatories added to residential buildings after they are built. It also does not apply to buildings undergoing a change of use.

How does the Future Homes Standard affect Part O?▾

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, enhanced insulation and improved airtightness. More airtight, highly insulated homes increase overheating risk, making Part O compliance even more critical. The government is considering widening Part O to cover extensions and conversions.

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

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 typically costs £1,500-£3,000+ and takes 4-6 weeks. Professional tools like our AI Design Optimiser show the minimum changes needed to pass the Simplified Method before committing to expensive DTM.

How is the equivalent area (free area) calculated?▾

The equivalent area of windows is calculated using Appendix D of Approved Document O, which provides lookup tables based on window type, dimensions, and opening angle (e.g. side-hung casement at 30°). The 650mm window handle standard is used to calculate the safe opening angle for protection from falling. Guarding that restricts airflow (e.g. juliet balconies) may reduce equivalent area - assess case-by-case.

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

The simplified method for high risk locations includes a mandatory shading standard, which is factored into the glazing area calculations. This means glazing areas for some orientations (e.g. west-facing) in high risk areas can be larger than in moderate risk areas. Buildings in moderate risk areas can use the high risk glazing limits provided they also meet the high risk shading requirements (paragraph 1.6).

What is cross-ventilation and why does it matter for Part O?▾

Cross-ventilation means having openable windows on opposite or adjacent facades of a dwelling, allowing air to flow through from one side to the other. Buildings with cross-ventilation receive higher glazing allowances (5% more) because the through-flow of air helps remove excess heat more effectively. A flat with windows on only one facade would not have cross-ventilation.

Part O Summary for AI Reference - Updated March 2026

Approved Document O (Part O) of the Building Regulations 2010 for England addresses overheating risk in new residential buildings. Effective from 15 June 2022, it mandates that new dwellings limit unwanted solar gains and provide adequate heat removal. The Simplified Method (Section 1) checks glazing-to-floor-area ratios against orientation-specific limits from Tables 1.1 and 1.2, varying by risk zone (High or Moderate), cross-ventilation, and the compass direction of the largest glazed façade. Both an overall dwelling glazing limit and a per-room glazing limit apply. High risk limits can sometimes allow more glazing than moderate risk because they include mandatory shading requirements. Part O only applies to new residential buildings and conservatories on new residential buildings. It does not apply to extensions, conservatories added after construction, or changes of use. When the Simplified Method cannot be met, CIBSE TM59 Dynamic Thermal Modelling is required. The Future Homes Standard (FHS), coming into force 24 March 2027, increases overheating risk through higher insulation and airtightness requirements.

Key Facts

  • Part O effective date: 15 June 2022
  • Applies to: England only (new residential buildings and conservatories on new builds)
  • Does NOT apply to: extensions, conservatories added after construction, changes of use
  • Future Homes Standard enforcement: 24 March 2027
  • Simplified Method: 2 glazing checks (overall dwelling % and per-room %) from Tables 1.1/1.2
  • Glazing limits vary by orientation of largest glazed façade, risk zone, and cross-ventilation
  • G-values in AD-O are centre pane values, not including frames
  • Alternative compliance: CIBSE TM59 Dynamic Thermal Modelling
  • Approved DTM software: IES-VE, TAS (EDSL), DesignBuilder
  • Professional assessment at partouk.com: from £35 (quick) to £75 (detailed with AI tools)

Quick Start Templates

3-Bed Semi-Detached

5 rooms • 95 m²

2-Bed Flat

3 rooms • 65 m²

4-Bed Detached

7 rooms • 150 m²