Obscure glazing conditions and how they are enforced
Side windows built under permitted development must be obscure glazed and non-opening below 1.7 metres above the internal floor. Breaching that takes the whole extension outside the right, not just the window.
An obscure glazing planning condition on a side window is doing two jobs at once, and most householders only notice one of them. The glass has to be obscure, and the window has to be non-opening below 1.7 metres above the floor of the room it serves. Fitting obscure glass in a window that opens at waist height satisfies half the requirement and breaches the other half.
Where the extension was built under permitted development rather than a planning permission, that breach is more serious than it looks. It does not merely break a condition. It takes the whole extension outside the permitted development right.
Where the requirement comes from
Class A of Schedule 2, Part 1 to the General Permitted Development Order 2015 permits the enlargement of a dwellinghouse, subject to conditions. One of those conditions applies to any window on a wall or roof slope forming a side elevation of the house: it must be obscure-glazed, and it must be non-opening unless the parts that open are more than 1.7 metres above the floor of the room in which the window is installed.
Class B, which covers roof additions such as dormers, carries the same condition in the same terms. So a side-facing dormer window gets treated exactly like a side window in a ground floor extension.
Note what the condition does not say. It does not specify a level of obscurity, it does not mention film or applied coatings, and it says nothing about rooflights on a rear or front slope. Those gaps are where the arguments happen.
The 1.7 metre rule and how it is measured
The measurement is from the floor of the room, upward, on the inside. Not from external ground level, not from the window cill, and not from the ceiling down.
That matters on a split-level extension or a room with a raised floor build-up. A window set at a comfortable height above external ground can sit well below 1.7 metres above an internal floor that has been raised for insulation and underfloor heating. Check the figure against the section drawing rather than the elevation.
The rule allows a window to open above 1.7 metres. A tall window with a fixed lower pane and a top-hung opening light whose openable part sits entirely above that height complies, which is the usual way to get ventilation into a bathroom or landing on a side elevation.
Trickle vents and non-glazed ventilation are unaffected. The condition is about openable parts of the window.
Obscurity levels and specifying one
Because the Order sets no level, councils fill the gap through the conditions they attach to planning permissions. The industry obscurity scale runs from 1 to 5, with 1 barely diffusing and 5 the most obscure. Most authorities ask for level 3 as a minimum, and a good number ask for level 4 or 5 where the window faces a neighbouring garden or a habitable room window at close range.
Write the level into the application yourself. A drawing annotated "obscure glazed to a minimum of level 3, fixed shut below 1.7 metres above internal floor level" usually results in a condition in those terms, which is far easier to discharge than a condition requiring details to be submitted later.
Applied film does not satisfy an obscure glazing requirement in practice. It is removable, it degrades, and no officer treats it as permanent. Specify obscure glass in the unit.
For a proposal where the privacy argument is doing real work, set it out in the Design and Access Statement with a plan showing the sight lines rather than leaving the officer to infer them.
Rooflights and side-facing dormers
A side-facing dormer window inserted under Class B carries the obscure glazing and 1.7 metre condition in full.
A rooflight is treated differently. Alterations to a roof under Class C, including inserting a rooflight, do not attract the obscure glazing condition, although Class C has its own limits on projection above the roof plane and on windows in a side-facing roof slope. Where a loft conversion needs light on the flank elevation, a conservation-style rooflight set into the slope is frequently the simplest answer, because it avoids the condition and tends to look better than a side dormer.
Read the classes separately. People assume an extension and a loft conversion sit under one set of rules, and they do not.
Conditions in the Order against conditions on a permission
This is the part worth understanding properly, because the consequences differ.
| Route | What the obligation is | What breach means |
|---|---|---|
| Permitted development under Class A or B | A condition built into the Order itself | The development falls outside the right and is unauthorised in its entirety |
| Planning permission with a condition | A condition on the grant | Breach of condition, enforceable by breach of condition notice or enforcement notice |
Under the first route, a clear-glazed opening side window can mean the extension was never permitted development. The council can then take enforcement action against the extension, not just the glazing. That is a much bigger problem than a window, and it surfaces most often when a buyer's solicitor asks for a lawful development certificate.
Under the second route, the council can serve a breach of condition notice under section 187A of the Town and Country Planning Act 1990. There is no right of appeal against a breach of condition notice, and failing to comply with one is a criminal offence.
Swapping the glass later
Replacing obscure glass with clear glass after the works are finished is the commonest way people fall into this. It usually happens years later, when a unit fails and a glazier replaces it like for like without knowing the history, or when a new owner decides the bathroom is gloomy.
If the window was installed under permitted development, the swap takes the extension outside the right from that point. If it was installed under a planning permission with a condition requiring the glazing to be retained, the swap is a breach of condition.
Keep the paperwork. The decision notice, the approved drawings and the glazing specification belong with the deeds, and handing them to a purchaser prevents the problem repeating. The same discipline applies to any window work in a sensitive area, as set out in the note on replacement windows in an Article 4 area.
Alternatives that avoid the condition
Four designs sidestep the issue rather than managing it.
A high-level window with the whole opening above 1.7 metres, which complies without obscure glass being a practical inconvenience because nobody looks out of it anyway. A rooflight instead of a side window. An oriel or angled window facing away from the neighbouring boundary, which is often accepted as clear glazed because the sight line does not reach the neighbour. Or simply reorienting the room so the window it needs faces the rear.
Screening works less well than people hope. A fence or hedge relied on for privacy can be removed, and councils know it, so a proposal that depends on planting to justify clear glazing usually attracts the condition anyway.
Where a garage or outbuilding is being converted and a new side window is proposed, the same thinking applies from the start, and the wider set of traps in that situation is covered in the note on converting a garage to a habitable room.
Only a Lawful Development Certificate confirms that what you have built is lawful, and it is worth applying for one before a sale rather than during it.
To see which documents a householder project is likely to need, enter your postcode on PlanScope and answer the guided questions.
This is general guidance, not legal advice, so check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Why does my side window have to be obscure glazed?
Class A and Class B of Schedule 2, Part 1 to the General Permitted Development Order 2015 both impose the same condition on any window in a wall or roof slope forming a side elevation. The window must be obscure glazed and non-opening unless the openable parts are more than 1.7 metres above the floor of the room it serves.
How is the 1.7 metre height measured?
From the floor of the room the window serves, upward, measured inside. Not from external ground level and not from the cill. On split-level extensions or where the floor build-up has been raised for insulation and underfloor heating, a window that looks high enough on the elevation can fall short in section.
What level of obscure glazing do councils require?
The Order does not specify a level, so councils set one by condition. The industry obscurity scale runs from 1 to 5, with 5 the most obscure. Level 3 is the usual minimum, with level 4 or 5 asked for where the window faces a neighbouring garden or a habitable room window at close range.
Can I use obscure film instead of obscure glass?
In practice no. Film is removable and degrades, so officers do not treat it as permanent compliance with an obscure glazing requirement. Specify obscure glass within the sealed unit. The same applies to blinds, screens and planting, none of which satisfy a condition that requires the glazing itself to be obscure.
What happens if I replace obscure glass with clear glass?
If the window was installed under permitted development, the change takes the extension outside the right and the whole extension becomes unauthorised. If it was installed under a planning permission with a retention condition, it is a breach of condition, enforceable by a breach of condition notice with no right of appeal.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.