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Using the frame size as the glass
The mistake Glazing area taken from the window schedule’s frame sizes, or the structural opening.
What Approved Document O says. Appendix A defines the glazing area as the transparent material only: frames, the bars between panes and solid panels are left out.
What it changes. Overstating the glass makes the home look further over the glazing limit than it is, and raises the opening area it needs. Understating it, by guessing, is the more dangerous slip.
In O-Monkey. Asks for the glass. If you only have the frame, “Estimate it” takes the frame size and the number of panes and allows for slim frames, so the estimate errs on the side of more glass, and the report says it was estimated.
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Counting outward-opening casements at 90°
The mistake Every casement entered as fully open.
What Approved Document O says. Para 3.9a: the handle of a window that opens outwards should be no more than 650 mm from the inside face of the wall, and MHCLG FAQ 15 says to count the window at the angle where that happens. Nothing exempts the ground floor.
What it changes. A 600 mm wide, 1.2 m high casement with its frame 150 mm back from the inside face counts at 56.4°, not 90°: 9% less opening area.
In O-Monkey. Takes the reveal depth once and applies the rule to every outward-opening window, drawing the counted angle and the 650 mm line on each. Tilt-and-turn windows that open inward, doors and roof windows are not affected.
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Counting upper-floor windows fully open without guarding
The mistake First-floor bedroom windows counted at their full opening.
What Approved Document O says. Paras 3.8–3.9 and Table 3.1: where the floor is more than 600 mm above the ground outside, an opening only counts as more than 100 mm if nobody can fall out: the sill at 1,100 mm or more, or guarding to 1.1 m.
What it changes. The same 600 × 1200 mm casement counts as a 100 mm opening: 9.6°, 68% less than fully open. It often decides whether a bedroom passes.
In O-Monkey. Asks the falling question for every upper-floor window, and until you answer it counts the window as 100 mm open and lists it to check. The one-tap fix shows what guarding would add.
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Using the whole floor area of an open-plan room or kitchen/diner
The mistake A 30 m² kitchen/living/dining room entered at 30 m².
What Approved Document O says. Appendix A: where a room serves more than one activity, such as an open-plan kitchen and living room, the room area is taken to a depth of no more than 4.5 m from the glazed façade.
What it changes. A smaller room area raises the room’s share of glass, which is what the most glazed room cap is checked against. A deep open-plan room can pass on its full area and fail on the 4.5 m area.
In O-Monkey. For an open-plan room, and for a kitchen/diner or living/dining room that is the most glazed room, “Work out the area within 4.5 m” takes the width along the glazed wall and the depth, and counts no more than 4.5 m of it.
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Treating windows on two adjacent sides as cross-ventilation
The mistake A corner flat entered as cross-ventilated.
What Approved Document O says. Para 1.4: cross-ventilation means openings on opposite façades. Two façades that meet at a corner are not opposite.
What it changes. The not-cross-ventilated tables (1.2 and 1.4) are stricter on both glazing and opening area, so a corner flat entered as cross-ventilated can pass when it should fail.
In O-Monkey. Works cross-ventilation out from the openable windows themselves, and says why in the working.
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Counting ground-floor bedroom windows at night
The mistake A ground-floor or easily accessible bedroom given credit for its full opening.
What Approved Document O says. Paras 3.6–3.7: in a bedroom on the ground floor, or one that is easily accessible from outside, only openings that can be left open securely count at night: lockable louvred shutters, or fixed or lockable grilles or railings.
What it changes. Without one of those, the bedroom’s own minimum has to be met by secure openings alone, and often isn’t.
In O-Monkey. Marks every ground-floor bedroom, asks about easily accessible upstairs bedrooms once, and counts an opening there only if you tick that it is secure when open.
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Counting the front door as an opening, or leaving out its glass
The mistake The front door left off the schedule, or counted as a way of purging heat.
What Approved Document O says. Approved Document O counts all glazing, the glass in doors included. The Future Homes Hub Part O guidance treats the entrance door as shut, because it can’t be left open securely.
What it changes. Leaving the glass out understates the glazing; counting the door open overstates the opening area.
In O-Monkey. Has a front door type: its glass counts towards the limits and its opening does not.
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Counting every leaf of a bifold or every pane of a slider
The mistake A three-leaf bifold entered as three doors; a two-pane slider counted on its full width.
What Approved Document O says. Approved Document O assesses sliding sashes on their clear opening (Appendix D) and does not mention sliding or bifold doors. The Future Homes Hub guidance counts one pane of a sash or slider, because only one can be fully open at a time.
What it changes. Entering the leaves as separate doors triples a bifold’s opening area and can turn a fail into a false pass.
In O-Monkey. Takes the clear opening when it is fully open, and checks with you when a bifold or slider is entered more than once.
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Counting restricted windows fully open
The mistake Windows on 100 mm restrictors entered at their full opening.
What Approved Document O says. MHCLG FAQ 8: a window with a restrictor is assessed in its restricted position.
What it changes. On a 100 mm restrictor, a 600 × 1200 mm casement counts for 68% less than fully open.
In O-Monkey. Takes the restrictor gap or the opening angle, and flags anything under 10°, where Appendix D gives no figures and the result is an extrapolation.
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Adding up the rooms instead of the whole floor area
The mistake The floor area taken as the sum of the rooms that have windows.
What Approved Document O says. Appendix A: the floor area is the gross internal area of the whole home, measured to the inside face of the outside walls, halls, stairs, bathrooms and cupboards included.
What it changes. A smaller floor area lowers the glazing limit and the opening area the home needs, so it can push a home over the glazing limit, or under-ask for opening area.
In O-Monkey. Asks for the gross internal area, and checks your entries for slips such as rooms adding up to more than the whole home.
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Choosing the most glazed room by its percentage
The mistake The room with the highest glass-to-floor ratio checked against the room cap.
What Approved Document O says. Approved Document O doesn’t define the most glazed room. The Future Homes Hub Part O guidance takes it as the room with the most glass in square metres, and where rooms tie, the one with the higher ratio.
What it changes. A small room with one window can have the highest ratio without being the most glazed room, and the other way round.
In O-Monkey. Follows the Future Homes Hub definition, shows which room it picked and why, and says so in the report.
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Rounding the orientation the generous way
The mistake A façade facing south-west read from the more generous of the south and west rows.
What Approved Document O says. MHCLG FAQ 4 and 5: use the nearest compass point; exactly between two, use the stricter row.
What it changes. Between south and west the limits differ by several percent of the floor area.
In O-Monkey. Uses 16 compass points and the stricter row between two, and checks a house type in all 16 directions so every plot is covered.
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Do your own Part O assessment
Start from a typical house or flat, turn the picture to face the way it does on the site plan, and add the windows. The answer updates as you type, says what to change if it fails, and 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.
Related: the 650 mm handle rule · equivalent area for any window · how much glass a home can have · high-risk postcodes · how to use O-Monkey · how O-Monkey is tested.
Sources: Approved Document O (2021 edition), Section 1, Section 3 and Appendices A and D · Approved Document O frequently asked questions (MHCLG) · the Future Homes Hub Part O technical guidance (2022), where Approved Document O is silent. The figures on this page are worked out by O-Monkey’s own calculation.