How to Use O-Monkey

The short version. You need five things from the drawings: the postcode, the floor area (GIA), which way the front faces, how far the window frames sit back from the inside face of the wall, and for each window its glass area, how it opens and the size of the part that opens. O-Monkey does the rest: the risk band, cross-ventilation, the most glazed room, the equivalent area of every window and the Appendix B checklist.

Open O-Monkey — free, no sign-up

Every box has an i beside it with the same guidance as this page. The full guide is in the tool under ? Help.

Before you start

The postcode. It sets the risk band: 207 London postcode districts are high risk, everywhere else in England moderate. Check yours.
The floor plans, with the room sizes and the total floor area (GIA).
The site plan, to see which way the front faces.
The window schedule or the elevations: for each window, its glass area, how it opens and the size of the part that opens.
A wall section, to read how far the window frames sit back from the inside face of the wall.

The project name or address, client and reference only print on the report; nothing in the checks uses them.

Step 1: the home

O-Monkey opens by asking whether it is a house or a flat, then for the postcode, the floor area, which way the front faces and the reveal depth. Turn the arrow on the picture to match the north arrow on the site plan, or tap a direction; as the windows go in, the same picture shows the glass on every side of the home. Then pick a start point — a typical semi, terrace, detached house, bungalow or flat — or start from scratch, or import a schedule.

Postcode: Sets the risk band. 207 postcode districts in London are high risk (ADO Appendix C, Table C1); everywhere else in England is moderate risk. High risk changes the glazing limits and the opening areas, and the glass needs shading.
Where to find it: The site address. No postcode for a new plot yet? The district of the street is enough, for example SE16.
House or flat: A flat is assessed on its own, not the whole block. Flats get one extra question, about communal heating pipework (para 1.13).
Floor area (GIA): The total floor area of the home, every floor, measured to the inside face of the outside walls: the gross internal area (ADO Appendix A). Include halls, landings, stairs, bathrooms and cupboards. Rooms without windows count too.
Where to find it: The area schedule on the drawings, or the SAP report. Or add up each floor’s internal length × width.
Which way the front faces: The compass direction the front of the home looks towards, usually the side with the front door. O-Monkey uses it to work out which way every window faces, so you only say whether a window is on the front, back or a side. For a flat, use the side with the main living-room windows.
Where to find it: The north arrow on the site plan. Imagine standing at the front door looking out, and pick the nearest of the 16 points.
Reveal depth: How far the window frames sit back from the inside face of the wall, in millimetres. For windows that open outwards, ADO para 3.9a says the handle must stay within 650 mm of the inside face of the wall, and MHCLG FAQ 15 says to count the window at the angle where that happens. It applies on every floor, the ground floor included. A deeper reveal means less opening counts.
Where to find it: The wall section, or a plan detail through a window. On site, measure across the window board from the wall face to the frame. It is usually the same for every window; you can set a different one for any window.
Cross-ventilation: A home is cross-ventilated if it has opening windows on opposite sides, such as front and back. Two sides that meet at a corner don’t count (ADO para 1.4). It changes every limit, so O-Monkey works it out from your windows.
Where to find it: Leave it on “Work it out” unless you have a reason to set it.
Night-time noise: The method assumes bedroom windows are left open at night. If, with them open, bedroom noise would be over 40 dB L_Aeq averaged over 11pm to 7am, or over 55 dB L_AFmax more than ten times a night, people will shut them (paras 3.2–3.3). Then this method can’t be used and you need dynamic thermal modelling (CIBSE TM59).
Where to find it: The planning permission and any noise report. If the planners raised road, rail, aircraft or venue noise, tick this.
Pollution source: Next to a significant source of air pollution, the design should keep polluted air out (para 3.5), which can stop some windows counting.
Where to find it: The planning air-quality assessment, if there was one.
Communal heating pipework (flats): If the block has communal heating or hot water with significant horizontal pipework, the pipes add heat and ADO para 1.13 says this method isn’t suitable. Use TM59 modelling.
Where to find it: The heating design or M&E drawings.
Central Manchester: ADO Appendix C notes that central Manchester can be hotter at night. The minimum is the moderate-risk standard (MHCLG FAQ 6), but ADO suggests following the high-risk guidance.
FAQ 2: homes facing west: In a moderate-risk area, a home may use the high-risk glazing limits if it also has the high-risk shading on every window facing north-east round to north-west through south (MHCLG Part O FAQ 2). It only helps when the most glazed side faces west: the limit for a cross-ventilated home rises from 11% to 18% of the floor area. The opening areas don’t change.
What counts as shading: Shading means external shutters with ventilation; solar-control glass with a g-value of 0.4 or less and light transmittance of 0.7 or more; or an overhang with a 50° cut-off, which only counts on a due-south wall.

The reveal depth and the 650 mm rule

For a window that opens outwards, paragraph 3.9a of Approved Document O says its handle must 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. So the deeper the frame sits, the less of the window counts: a 600 mm casement with its frame 150 mm back counts at 56°, not 90°. Inward-opening windows, doors and roof windows are not affected. The rule applies on every floor, the ground floor included — paragraph 3.9 sits under protection from falling but gives no exemption for ground-floor windows. Work out the angle for any window.

Cross-ventilation

Openable windows on opposite sides of the home — front and back, or left and right — make it cross-ventilated, and earn the more generous limits of Tables 1.1 and 1.3. Two sides that meet at a corner do not count (para 1.4), which is why most corner flats are not cross-ventilated. Leave the box on “Work it out” and O-Monkey works it out.

Your glazing budget

As soon as the postcode and the floor area are in, Your glazing budget shows the most glass the home can have for each way its most glazed side could face, and the least opening area it will need, overall and for each bedroom — the brief for the windows before they are drawn. It folds away once a window has glass. The glazing limit calculator works out the same figures without opening the app.

Step 2: rooms

Add every room that has a window or glazed door. Rooms without windows still count in the floor area from step 1. A start point puts in the rooms of a typical home, named for their type, with a window for each; type over the names and areas; a room’s type follows its name until you pick one. For an open-plan room, or a kitchen/diner that is the most glazed room, Work out the area within 4.5 m takes the whole room’s area and its depth from the most glazed wall, and counts only the first 4.5 m (ADO Appendix A). Until you have, or have confirmed the room is no deeper, an open-plan most glazed room is flagged.

Room type: Bedrooms get their own check: enough opening to cool them at night. The room with the most glass is checked against the most-glazed-room limit. Choose “Open-plan room” for a kitchen, living and dining space.
Room floor area: Measured to the inside face of the room’s walls, in m². For an open-plan room, enter only the area within 4.5 m of its most glazed wall (ADO Appendix A).
Where to find it: The room sizes on the floor plans.
Upper floor: Tick if the floor is more than 600 mm above the ground outside. A window there only counts as opening more than 100 mm if it is guarded to 1.1 m, so nobody can fall out (para 3.9, Table 3.1).
Easily accessible: ADO Appendix A: any part of the window is within 2 m (vertically) of the ground, a basement level or an access balcony, or within 2 m of a flat or low roof (pitch under 30°) that is itself within 3.5 m of the ground. In such a bedroom, only windows that are secure when open count at night (para 3.6).

Step 3: windows and doors

Add each window and glazed door — fixed panes and the front door included, because all glass counts towards the limits. The home from above, at the top of the step, shows the glass on every side and which way each side faces; tap a side to add a window there. Pick how it opens from the pictures; the drawing beside each window shows how far it swings and where the 650 mm line falls. Copy repeats a window, which saves time on a row of identical ones, and Edit them all in a table shows every window at once, like a schedule.

Which type is my window?

Casement, side-hung (opens out) — Hinged at one side, opens outwards. The usual UK casement. The 650 mm handle rule applies.
Tilt-and-turn or inward-opening casement — Opens into the room, like a tilt-and-turn in its “turn” position. The 650 mm rule doesn’t apply.
Top-hung (opens out) — Hinged at the top, opens outwards: small bathroom and fanlight windows. The 650 mm rule applies. As a rooflight it’s counted in its centre-pivot position at 90°, which most top-hung roof windows also have.
Bottom-hung (opens in) — Hinged at the bottom, tips into the room (a hopper).
Centre-pivot, horizontal axis — Turns about a horizontal bar across the middle, like many roof windows. Counted as two halves.
Centre-pivot, vertical axis — Turns about a vertical bar down the middle. Counted as two halves.
Vertical sliding sash — Slides up and down. Enter the gap when it is open; only one sash counts.
Sliding window or sliding door — Slides sideways: a sliding window or patio door. Enter the gap when it is open; only one pane counts.
Bifold door — Panels that fold back. Enter the clear gap when they are folded open.
French or patio door (hinged) — French or patio doors on hinges. Counted to 90°; the 650 mm rule is for windows, not doors.
Front or entrance door (glass only) — The front door or another entrance door. Its glass counts; the opening doesn’t, because it can’t be left open securely.
Fixed glazing (does not open) — Doesn’t open. Its glass still counts towards the glazing limits.
Tested equivalent area (BS EN 13141-1) — The maker has tested it to BS EN 13141-1 and gives an equivalent area. Keep the certificate.

Glass, and the part that opens

Side of the home: Front, back, or the left or right side as you look at the front from outside. Rooflights are separate. For a wall at an angle, such as a bay, pick its compass direction.
Where to find it: Elevation drawings are usually labelled front, rear and sides.
Window or door type: How it opens. That decides how much air it lets through (ADO Appendix D) and whether the 650 mm handle rule applies. Not sure? See “Which type is my window?” above.
Where to find it: The window schedule, or the elevations: the lines drawn on a window point to its hinge side.
Glass area: Only the see-through glass, in m²: not the frame, the bars between panes or any solid panel (ADO Appendix A). Include every pane, opening or fixed, and the glass in doors.
Where to find it: The window maker’s drawing or quote, or measure the glass on the elevations. “Work it out” adds up panes of different sizes; “Estimate it” works from the frame size when that is all you have.
Size of the opening part: The part that opens: the sash or door leaf, not the whole frame, in millimetres. Width is from the hinge to the handle side; height is the other way.
Where to find it: The window schedule, or measure the sash on the elevation.
Clear opening: For a sash, sliding or bifold opening: the size of the gap when it is fully open, in millimetres. Only one pane of a sash or slider counts, because only one can be fully open at a time (Future Homes Hub guidance).
Where to find it: On a two-pane slider the gap is a little under half the frame width; the maker’s drawing gives the real figure.
How many: How many opening parts of this size: 2 for a pair of casements or French doors. Fixed panes aren’t counted here; their glass is already in the glass area.
How far it opens: “Fully” means as far as the hinges allow, up to 90°. Choose an angle if the stays stop it sooner, or a restrictor if one is fitted. Always use the restricted position: that’s what counts (MHCLG FAQ 8).
Protection from falling: On an upper floor, a window only counts as opening more than 100 mm if nobody can fall out: the sill is at least 1,100 mm above the floor, or there is guarding to 1.1 m that a child can’t climb (para 3.9, Table 3.1). Otherwise it counts as a 100 mm opening.
Secure when open: In a ground-floor or easily accessible bedroom, a window only counts at night if it can be left open securely: fixed or lockable louvred shutters, or fixed or lockable grilles or railings (paras 3.6–3.7). It still counts towards the whole home.
Shading: Needed in high-risk London, or when you use FAQ 2, on glass facing between north-east and north-west through south (para 1.9). It means external shutters with ventilation; solar-control glass with a g-value of 0.4 or less and light transmittance of 0.7 or more; or an overhang with a 50° cut-off, which only counts on a due-south wall.
Tested equivalent area: If the maker has tested the window to BS EN 13141-1 and gives its equivalent area, you can use that instead of Appendix D. Keep the test certificate for Building Control.
Ref (W01, W02…): The window’s mark on the drawings, such as W01. Optional: it appears in the results, the checklist and the report so each window can be matched to the schedule.
Where to find it: The window schedule or the elevations.

Only the frame size on the schedule? Only the frame size? Estimate it works the glass out from the frame, the number of panes and the frame material, on the generous side; Panes of different sizes? Work it out adds up the panes one size at a time. A roof window goes in with Add a roof window: pick its VELUX size code and the glass and the sash that opens fill in.

Want the equivalent area of one window on its own, with the working? The equivalent area calculator uses the same Appendix D tables and calculation as O-Monkey.

Importing a window schedule

Import a window schedule in step 3 reads the schedule straight from Excel, Google Sheets, a .csv file or a PDF: copy the rows with their headings and paste them in, or open an .xlsx, .csv or .pdf file. A PDF has to be one saved from the drawing or spreadsheet program, with real text in it; a scan or a photo of a schedule cannot be read, so save that one as .xlsx or .csv instead. O-Monkey guesses what each column holds — room, type, side or direction, sizes, glass — and shows a preview; correct any column it got wrong, choose millimetres or metres, and import, adding to the windows already there or replacing them.

Schedules usually give the frame size, not the size of the part that opens, and often no glass area. Where O-Monkey had to fill a gap it marks the window for you to confirm. An opening part taken from the frame size, or a type, side or floor the schedule didn't give, could make the result too generous, so it cannot read as a clean pass until you have checked them. Glass estimated from the frame, and a frame wide enough for more than one opening part when the schedule doesn't say how many open (O-Monkey counts one), are on the safe side, so they are listed for you to check without holding back a pass.

Notes in the schedule are read too. A restrictor is counted in its restricted position, and on an upper floor a sill at 1,100 mm or more, a Juliet balcony or guarding to 1.1 m answers the protection-from-falling question. It all happens in your browser; nothing is uploaded. A template with the columns it reads best, including how many parts open, is in the import dialog.

Reading the answer

The answer sits beside the form on a computer, and on its own tab on a phone. It says one of five things:

Passes — every check of the simplified method is met.
Passes — see notes — it passes, but something needs a look before it goes to Building Control: a value carried in from a schedule still to check, or a restrictor below 10°.
1 check fails (or more) — each failing check says what to change and by how much.
Method not suitable — night-time noise, or communal heating pipework in a block of flats, rules the simplified method out. The route is dynamic modelling to CIBSE TM59.
Not finished yet — something essential is still blank, and the list says what.

Every direction shows whether the same design passes built facing each of the 16 compass points — the check for a house type that goes on plots facing different ways. List the actual plots under it, each with the way its front faces (tick handed for a mirror-image plot), and each is checked in its own direction and printed in the report.

Two gauges show the glass against its limit and the opening area against what is needed, and Show the working opens every check with its numbers and the paragraph it comes from.

If it fails, the answer says what to change and by how much, and offers a one-tap fix where one change will do it. Where it takes more, it lists the fewest changes that pass together, and Make it pass makes them. Either way the changes stay listed in the answer, with Undo (or Ctrl+Z), until you change something else. In the working, below the checks, Window swaps that add opening area lists every swap ranked by what it adds — a tilt-and-turn window, guarding, no restrictor, a narrower casement, an openable pane, French doors for a slider — each with Try it. Glass over the limit has to come out on the drawings, so O-Monkey says how much. Entries that look like slips are listed under the answer, with a button to go to each; they never change a figure.

Every plot has to pass in its own direction: if one fails, so does the answer, and it says which plot. Several house types on one site? Add another house type, under the plots, keeps them in one project, and the site summary lists every type and plot with its result. Each house type is its own report.

The Building Control report

The export unlocks once the design is complete and any sizes, types or sides an import had to guess have been checked — it will not sell you a report on half a design. Add the designer's details first (they go into Appendix B Part 2a). The report carries:

Appendix B Part 1 (the building and site, the designer) and Part 2a (the simplified method's figures) filled in, and Part 3 left blank for the builder and the inspector at completion;
the checks with their numbers and paragraph references, and the room and window schedules with the Appendix D method named on every window;
the every-direction table and your plots;
a Report ID and a QR code that Building Control can check at energycount.co.uk/verify.

Part 2a is signed by whoever is competent to design the home (ADO B2) — the report is your evidence, prepared with the tool, not a certificate from us. £25 +VAT (£30 inc VAT) per house type, every plot included.

Change the design after a report and export again: the new report carries the next revision letter and lists what changed since the last one — rooms, windows, sizes, plots and the result — and the export panel shows the list before you pay.

What this tool is, and is not

It implements the simplified method of Approved Document O, 2021 edition, for new homes in England — Section 1, the parts of Section 3 that decide what a window counts for, and Appendices B, C and D — with MHCLG's Part O FAQ and the Future Homes Hub's guidance where the document is silent. It is not a substitute for a building control body's judgement, and it is not the dynamic modelling route. Wales has its own Approved Document O and Scotland its Standard 3.28. Need TM59, or the whole thing done for you? We do both.

More: the common Part O mistakes · the glazing limit · how O-Monkey is tested.

Frequently asked questions

Which floor area does Part O use?

The gross internal area of the whole home: every floor, measured to the inside face of the outside walls (Approved Document O, Appendix A). Halls, landings, stairs, bathrooms and cupboards count, with or without windows. Adding up only the rooms that have windows understates the floor area, and with it the free area the home needs.

Which way does my house face?

The way the front looks out, usually the side with the front door. Find the north arrow on the site plan, imagine standing at the front door looking out, and pick the nearest of the 16 compass points. O-Monkey turns that into a direction for every window. When the most glazed side sits exactly between two compass points, the stricter row of the table applies (MHCLG FAQ 5).

How is the most glazed room chosen?

It is the room with the most glass in square metres, following the Future Homes Hub guidance; if two rooms tie, the one with more glass for its floor area. That room is checked against the room limit in Table 1.1 or 1.2. For an open-plan room, only the floor within 4.5 m of its most glazed wall counts as its floor area (Appendix A), which raises its percentage.

Do rooflights count?

Yes. Their glass counts towards the whole home and the room it lights, and their opening counts towards the free area. They do not decide which side of the home is the most glazed, and in a high-risk location they need shading like other glazing facing between north-east and north-west through south. Top-hung roof windows are counted in their centre-pivot position, fully open at 90°, which most also have and which is the one used for ventilation. Add one with Add a roof window: pick its VELUX size code, such as MK04, and the glass and the sash fill in from the maker’s data; for other makers, type the sizes in.

What if the schedule only gives the frame size?

Use Only the frame size? Estimate it, beside the glass area: enter the frame width and height, the number of panes and the frame material, and O-Monkey estimates the glass from slim frame and mullion widths for that material, so the estimate errs on the side of more glass and never flatters the glazing checks. The window is marked as an estimate until you keep it, and the maker’s figure is better when you have it.

How do I enter a sash window or a sliding door?

By its clear opening: the size of the gap when it is fully open, which is a little under half the frame on a two-pane slider. Only one pane of a sash or slider counts, because only one can be fully open at a time. O-Monkey assesses it on Table D9 at 90°, the way Appendix D treats sliding sashes.

What does “easily accessible” mean?

Approved Document O, Appendix A: any part of the window is within 2 m (vertically) of the ground, a basement level or an access balcony, or within 2 m of a flat or low roof (pitch under 30°) that is itself within 3.5 m of the ground. In an easily accessible bedroom, only openings that can be left open securely count towards that bedroom at night.

What if a window has a restrictor?

Count it in the restricted position (MHCLG FAQ 8): enter the gap at the opening edge, typically 100 mm, and O-Monkey works out the angle it allows and the equivalent area at that angle. Below 10° Appendix D gives no figures, so the result is an extrapolation and the tool says so.

Can I use the window maker’s tested figure?

Yes. If the window has been tested to BS EN 13141-1 and the maker gives its equivalent area, choose “Tested equivalent area” and enter it instead of letting Appendix D work it out. Keep the test certificate for Building Control.

Where the method comes from

The documents behind every box in the tool.

Approved Document O: Overheating (2021 edition) Section 1 and Tables 1.1–1.4 for the limits, Section 3 for noise, security and protection from falling, Appendix A for the definitions (floor area, glazing area, easily accessible), Appendix B for the checklist, Appendix C for the high-risk postcodes and Appendix D for equivalent areas.
Approved Document O: frequently asked questions (MHCLG) Orientation and ties (4–5), central Manchester (6), restrictors (8), shading on west-facing homes (2) and the 650 mm handle (15).