Postcode checker
Approved Document O (2021), para 1.3 and Appendix C, Table C1, with the central Manchester note and MHCLG FAQ 6. The check happens on this page; the postcode is not sent anywhere.
What the risk band changes
The glazing limits, from Tables 1.1 (cross-ventilated) and 1.2 (not cross-ventilated), by the direction of the most glazed side — % of the floor area, then the cap on the most glazed room:
| Most glazed side faces | Cross-ventilated | Not cross-ventilated | ||
|---|---|---|---|---|
| High risk | Moderate | High risk | Moderate | |
| North | 15% · room 37% | 18% · room 37% | 15% · room 26% | 18% · room 26% |
| East | 18% · room 37% | 18% · room 37% | 11% · room 18% | 18% · room 26% |
| South | 15% · room 22% | 15% · room 30% | 11% · room 11% | 15% · room 15% |
| West | 18% · room 37% | 11% · room 22% | 11% · room 18% | 11% · room 11% |
The minimum free area, from Tables 1.3 and 1.4:
| High risk | Moderate | |
|---|---|---|
| Cross-ventilated, whole home | the greater of 6% of floor area or 70% of glazing | the greater of 9% of floor area or 55% of glazing |
| Not cross-ventilated, whole home | the greater of 10% of floor area or 95% of glazing | the greater of 12% of floor area or 80% of glazing |
| Each bedroom | 13% of the room's floor area | 4% of the room's floor area |
And in a high-risk location, glazing facing anywhere from north-east round to north-west via south must be shaded (para 1.9): external shutters with a means of ventilation, glazing with a g-value of 0.4 or less and a light transmittance of at least 0.7, or — on a due-south wall only — an overhang that cuts the sun off above 50°. That is why some high-risk glazing limits are more generous than the moderate ones: the tables assume the shading is there. A moderate-risk home whose most glazed side faces west may use the high-risk limits if it carries the same shading (MHCLG FAQ 2).
The full list — Appendix C, Table C1
All 207 high-risk postcode districts, grouped by postcode area:
CR Croydon · 2
CR4CR7
E East London · 20
E1E1WE2E3E4E5E6E7E8E9E10E11E12E13E14E15E16E17E18E20
EC City of London · 24
EC1AEC1MEC1NEC1REC1VEC1YEC2AEC2MEC2NEC2PEC2REC2VEC2YEC3AEC3MEC3NEC3REC3VEC4AEC4MEC4NEC4REC4VEC4Y
HA Harrow and Wembley · 2
HA0HA9
IG Ilford · 5
IG1IG2IG3IG4IG11
KT Kingston · 8
KT1KT2KT3KT4KT5KT6KT7KT8
N North London · 16
N1N1CN2N4N5N6N7N8N9N13N15N16N17N18N19N22
NW North-west London · 8
NW1NW2NW3NW5NW6NW8NW10NW11
RM Romford and Dagenham · 3
RM8RM9RM10
SE South-east London · 28
SE1SE2SE3SE4SE5SE6SE7SE8SE9SE10SE11SE12SE13SE14SE15SE16SE17SE18SE19SE20SE21SE22SE23SE24SE25SE26SE27SE28
SW South-west London · 27
SW1ASW1ESW1HSW1PSW1VSW1WSW1XSW1YSW2SW3SW4SW5SW6SW7SW8SW9SW10SW11SW12SW13SW14SW15SW16SW17SW18SW19SW20
TW Twickenham and Hounslow · 16
TW1TW2TW3TW4TW5TW6TW7TW8TW9TW10TW11TW12TW13TW14TW15TW19
UB Southall and Uxbridge · 9
UB1UB2UB3UB4UB5UB6UB7UB8UB11
W West London · 25
W1BW1CW1DW1FW1GW1HW1JW1KW1SW1TW1UW1WW2W3W4W5W6W7W8W9W10W11W12W13W14
WC West Central London · 14
WC1AWC1BWC1EWC1HWC1NWC1RWC1VWC1XWC2AWC2BWC2EWC2HWC2NWC2R
Reproduced from Approved Document O (2021), Appendix C, Table C1. Contains public sector information licensed under the Open Government Licence v3.0. The area names are ours, for finding your way around.
Not all of London
Table C1 is a list of postcode districts, not a boundary. Large parts of outer London are not on it: no district in Bromley (BR), Dartford and Bexley (DA), Enfield (EN) or Sutton (SM) is listed; in Croydon only CR4 and CR7 are; and in north London N3, N10, N11, N12, N14, N20, N21 and NW4, NW7, NW9 are moderate risk. Check the district, not the borough. A few central districts are listed only by sub-district (EC1, EC2, EC3, EC4, SW1, W1, WC1, WC2) — W1, for instance, appears as W1B to W1W — so a site given only as W1 is treated as high risk.
Central Manchester
Appendix C adds a note: the centre of Manchester — M1, M2, M3, M5, M15, M16, M50 — can have elevated night-time temperatures, and the guidance suggests following the high-risk approach there. MHCLG's FAQ is clear that the minimum is still the moderate-risk standard, so either is compliant; the high-risk values are the cautious choice. O-Monkey asks which you want when the postcode is in one of these districts.
Wales, Scotland and Northern Ireland
Approved Document O, and the list above, are for England. Wales has its own Part O and Approved Document O, in force since 23 November 2022, with a simplified method organised by single- or dual-aspect and no London-style list. In Scotland overheating is Standard 3.28 of the Domestic Technical Handbook, and Northern Ireland has its own building regulations. The checker flags postcodes outside England, and gives a gentler note for postcode areas that cross a border, such as Chester, Shrewsbury, Hereford and Berwick.
O-Monkey — free in your browser
The risk band is step one — check the whole home
O-Monkey takes the postcode and runs the rest of the simplified method: cross-ventilation, the glazing limits for the home and the most glazed room, shading, and the free area of every window with the 650 mm rule — then fills in the Appendix B checklist.
Free to check. Building Control-ready PDF with the Appendix B checklist £25 +VAT (£30 inc VAT) per house type, every plot included.
Frequently asked questions
What is a high-risk location under Part O?
One of the 207 London postcode districts listed in Appendix C, Table C1 of Approved Document O (para 1.3b). Night-time temperatures stay high there, so the simplified method asks for more free area to purge heat, a bigger opening in every bedroom, and shading on glazing facing north-east round to north-west via south. Everywhere else in England is a moderate-risk location (para 1.3a).
Is all of London high risk?
No. Table C1 is a list of postcode districts, not a boundary, and much of outer London is not on it: no district in Bromley (BR), Dartford and Bexley (DA), Enfield (EN) or Sutton (SM), only CR4 and CR7 in Croydon, and in north London N3, N10, N11, N12, N14, N20, N21 and NW4, NW7, NW9 are moderate. Check the district, not the borough.
Is central Manchester high risk?
Not as a minimum. Appendix C notes that central Manchester (M1, M2, M3, M5, M15, M16, M50) can have elevated night-time temperatures and suggests following the high-risk guidance there, but MHCLG's FAQ confirms the minimum standard is the moderate-risk one. O-Monkey lets you choose the high-risk values for those districts.
My site has no postcode yet. What do I use?
The district of the street it is on, such as SE16, is enough: the risk band only depends on the district. Table C1 lists a few central districts only by sub-district — W1 as W1B to W1W, EC1 to EC4, WC1, WC2 and SW1 the same way — and every one of those is high risk, so a site given as just W1 is high risk too.
What changes if my site is high risk?
Three things. The glazing limits come from the high-risk columns of Tables 1.1 and 1.2 — tighter facing north or south, and more generous facing west because shading is compulsory. Glazing facing north-east round to north-west via south must be shaded (para 1.9). And the free area changes: for a cross-ventilated home, the greater of 6% of the floor area or 70% of the glazing, with 13% of each bedroom's floor area in that bedroom, against 9%, 55% and 4% at moderate risk.
Does the high-risk list apply in Wales or Scotland?
No. Approved Document O and its Appendix C are for England. Wales has its own Approved Document O, in force since 23 November 2022, with no London-style high-risk list; in Scotland overheating is Standard 3.28 of the Domestic Technical Handbook. The checker on this page flags postcodes outside England.
Next: the equivalent area of a window · the 650 mm handle rule · Part O explained.
Sources: Approved Document O (2021 edition), para 1.3 and Appendix C · Approved Document O frequently asked questions (MHCLG), FAQ 2 and 6.