Glazing limit calculator
Approved Document O (2021), Tables 1.1 to 1.4, with the risk band from Appendix C and MHCLG FAQ 2. It works on this page; nothing is sent anywhere.
The limits, by the way the most glazed side faces
From Tables 1.1 (cross-ventilated) and 1.2 (not cross-ventilated): the most glass for the whole home, as a percentage of its floor area, then the cap on the most glazed room as a percentage of that room’s floor area.
| Most glazed side faces | Cross-ventilated | Not cross-ventilated | ||
|---|---|---|---|---|
| Moderate risk | High risk (London) | Moderate risk | High risk (London) | |
| North | 18% · room 37% | 15% · room 37% | 18% · room 26% | 15% · room 26% |
| East | 18% · room 37% | 18% · room 37% | 18% · room 26% | 11% · room 18% |
| South | 15% · room 30% | 15% · room 22% | 15% · room 15% | 11% · room 11% |
| West | 11% · room 22% | 18% · room 37% | 11% · room 11% | 11% · room 18% |
Between two of the compass points above — a home whose most glazed side faces north-east, say — the stricter of the two rows applies (MHCLG FAQ 4 and 5). In a high-risk location, glass facing north-east round to north-west via south must also be shaded (para 1.9), which is why some high-risk limits look more generous. Is the site high risk?
The opening area it will need
The glass sets a second requirement. The windows and doors must open enough to purge heat: the greater of a share of the floor area and a share of the glass (Tables 1.3 and 1.4), plus a minimum in every bedroom.
| Moderate risk | High risk (London) | |
|---|---|---|
| Cross-ventilated, whole home | the greater of 9% of the floor area or 55% of the glass | the greater of 6% of the floor area or 70% of the glass |
| Not cross-ventilated, whole home | the greater of 12% of the floor area or 80% of the glass | the greater of 10% of the floor area or 95% of the glass |
| Each bedroom | 4% of the bedroom’s floor area | 13% of the bedroom’s floor area |
An opening counts by its equivalent area from Appendix D, not its size: a casement held to 56° by the 650 mm handle rule lets through less air than one open at 90°, and an upper-floor window without guarding counts as a 100 mm opening. Work out any window’s equivalent area.
Using the budget at concept stage
The limit depends on which way the most glazed side faces, so the useful question early on is where the glass goes. A house with most of its glass on a west-facing garden side has the tightest budget outside London; turning the same glass to face north or east raises the limit by more than half. If a plot faces west and the design needs the glass, FAQ 2 allows the high-risk limits with shading. And because more glass raises the opening area the home needs, every extra square metre of glass needs more opening window to match. The common mistakes are worth a look before the window schedule is fixed.
O-Monkey — free in your browser
The budget is the start — check the windows
O-Monkey shows this budget as soon as you enter the postcode and floor area, then checks every window as you add it: glass against the limit, the opening area with the 650 mm rule, every bedroom, every direction a house type could face.
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
How much glazing is allowed under Part O?
It depends on three things: whether the site is in one of the 207 high-risk London postcode districts, whether the home is cross-ventilated, and which way its most glazed side faces. For a cross-ventilated home outside London the limit runs from 11% of the floor area (most glass facing west) to 18% (facing north or east): for an 88 m² house, 9.68 to 15.84 m² of glass. The most glazed room has its own, separate cap.
Is it the glass or the window that counts?
The glass. Approved Document O, Appendix A, defines the glazing area as the area of the transparent material only, so the frames, the bars between panes and any solid panels are left out: the glass is always smaller than the frame, and by more on small windows and windows with several panes. Every pane counts, fixed or opening, and so does the glass in doors, the front door included.
Which side is the most glazed side?
The side of the home with the most glass, in square metres. Its direction picks the row of Table 1.1 or 1.2: the nearest of the compass points, and where it sits exactly between two, the stricter row (MHCLG FAQ 4 and 5). Rooflight glass counts towards the total but does not decide which side is the most glazed.
What does cross-ventilated mean here?
Openable windows on opposite sides of the home, such as front and back, so air can pass straight through (para 1.4). Two sides that meet at a corner do not count, which is why most corner flats use the stricter not-cross-ventilated tables. Most houses are cross-ventilated; many flats are not.
Can a west-facing home have more glass?
In a moderate-risk location, yes, if it carries the high-risk shading. MHCLG FAQ 2 lets such a home use the high-risk glazing limits, which allow more glass facing west (18% rather than 11% when cross-ventilated), provided all the glass facing north-east round to north-west, via south, is shaded: external shutters with ventilation, solar-control glass with a g-value of 0.4 or less and light transmittance of at least 0.7, or a 50° overhang on a due-south wall. The opening areas stay at the moderate-risk values.
Is the glazing limit the only check?
No. The windows also have to open enough: the simplified method sets a minimum opening area for the whole home (a share of the floor area, or of the glass if that is more) and for every bedroom, with the 650 mm handle rule, guarding on upper floors and security at night cutting what each window counts for. More glass raises the opening area the home needs. O-Monkey runs all of it, window by window.
Next: is the site high risk? · the equivalent area of a window · common mistakes · Part O explained.
Sources: Approved Document O (2021 edition), paras 1.3–1.12, Tables 1.1–1.4, Appendices A and C · Approved Document O frequently asked questions (MHCLG), FAQ 2, 4 and 5.