Obscure glazing and the 1.7 metre opening restriction
The 1.7 metre line is measured from the floor inside, not the ground outside. Get it wrong and the whole extension drops out of permitted development.
If you build a two-storey extension under permitted development and it has a window in the side wall, that window has to be obscure glazed, and any part of it that opens has to sit at least 1.7 metres above the floor of the room it serves. Not 1.7 metres above the ground outside. Above the floor inside. That single measurement catches more householder schemes than any other condition in Part 1.
The condition comes from Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015. It is a condition, not a limitation, which means a side window that breaches it does not simply need its own permission: the whole extension falls outside permitted development and becomes unauthorised.
Which windows the condition applies to
It bites on windows in a wall or roof slope forming a side elevation, and only on the parts of the house covered by the relevant class.
Class A covers extensions. Where the extension has more than one storey, or forms an upper storey, a window in a side elevation must be obscure glazed and non-opening below the 1.7 metre line. Class AA, which allows upward extension of certain houses, carries its own version. Class B, which covers roof enlargements including dormers, applies it to any window inserted in a side elevation wall or roof slope. Class C, covering other roof alterations such as rooflights, carries the same requirement for a window in a roof slope forming a side elevation.
The Order has been amended repeatedly, so check the current consolidated text rather than an old copy of the technical guidance.
What the condition does not catch:
- Ground floor side windows under Class A, where the extension is single storey
- Windows in the front or rear elevation, at any level
- Windows in the original house that you are not altering
- Windows forming part of a scheme with full planning permission, unless a condition on that permission says otherwise
That last point trips people up in the opposite direction. If you have planning permission for the extension, the GPDO condition is irrelevant; what binds you is the approved drawings and any condition the council imposed.
How the 1.7 metre height is measured
From the finished floor level of the room in which the window is installed, up to the lowest part of the window that can be opened.
So a first-floor side window with a fixed lower pane and a top-hung opening light is compliant if the bottom of that opening light sits at 1.7 metres or more above the first-floor finished level. A full-height side window with a side-hung casement is not, because the openable part starts at cill level.
Two practical consequences. In a room with a raised floor, perhaps over a step up into the extension, the datum moves up with the floor. And on a stairwell or a double-height space, the floor in question is the floor of the room the window serves, which can be ambiguous on a split-level layout. Where it is ambiguous, that is a reason to apply for a certificate rather than to assume.
For the related question of how heights are measured from the ground outside, see the note on measuring height under Part 1.
What obscure glazing means in practice
The Order does not define it. It says obscure glazed, and stops.
The convention that has grown up, and the one most councils work to, is the obscuration scale used by glass manufacturers, running from level 1 through to level 5. Levels 4 and 5 give the most privacy. Level 3 is widely treated as the minimum acceptable, and many authorities will say so if asked, though that figure is convention rather than law.
What is clear is what obscure glazing is not. It is not clear glass with a film applied, because a film can be peeled off. It is not a frosted lower pane with a clear upper pane, because the condition applies to the window, not to part of it. And it is not net curtains.
Where the authority has issued its own householder design guidance, follow the figure in that document. Where it has not, specify a level and record it on the drawing.
Opening lights above and below the line
The condition allows a window to open, provided the openable parts are above 1.7 metres. That gives you three workable arrangements.
- A fixed obscure glazed window with no opening parts at all, with ventilation provided elsewhere in the room
- A tall obscure glazed window with a top-hung or top-pivot opening light whose lowest openable edge sits above 1.7 metres
- A high-level obscure glazed window positioned entirely above 1.7 metres, which can then open however you like
Check Part F of the Building Regulations at the same time. A bedroom or bathroom still needs its purge ventilation and, where the room relies on that window for it, the opening area above the line has to be enough. Designing a compliant planning solution that fails Building Regulations is a familiar sequence.
Rooflights in a side roof slope
A rooflight in a side-facing slope is caught, which surprises people because a rooflight at normal roof pitch rarely overlooks anything. The condition applies regardless.
Position is the usual answer. A rooflight set high in the slope, with its lowest openable part above 1.7 metres from the floor beneath, satisfies the condition without obscure glass being a problem for daylight, because most of the glazing is above head height anyway. On a loft conversion with a low collar, that calculation needs doing before the rooflight is specified.
Removing or altering the glazing later
The condition runs with the development permanently. Replacing the obscure glass with clear glass five years later is a breach, and the breach is of the condition under which the extension was built rather than of a planning permission.
Development carried out in breach of a condition imposed on permitted development is not itself permitted, which is the point covered in the note on development contrary to a condition. An authority can serve a breach of condition notice or an enforcement notice, and the time limits for doing so are set out in the piece on enforcement time limits for permitted development breaches.
The practical risk arrives at sale. A buyer's solicitor asks for evidence that the extension was lawful, the window is clear glass, and the answer is a retrospective application or an indemnity policy.
An illustrative case
A rear two-storey extension on a 1930s semi, with a first-floor bathroom window in the flank wall facing the neighbour. The builder fits a standard side-hung casement in clear glass because the client wanted light on the landing. The extension is otherwise within every Class A limit.
The whole extension is now unauthorised, because a condition has been breached rather than a limitation exceeded. The fix is either to replace the window with an obscure glazed unit whose opening light sits above 1.7 metres from the bathroom floor, restoring compliance, or to apply for planning permission for the extension as built. Replacing the window is faster and costs less. The example is illustrative.
Annotating the condition on the elevations
Make it unambiguous on the drawing, because that is the document anyone will look at in ten years.
On the elevation, label each side window "obscure glazed, obscuration level 3 minimum". On the section, show the finished floor level of the room and dimension from it to the lowest openable part of the window. Add a note to the drawing citing the condition in Part 1 of Schedule 2 to the GPDO 2015. If a window is entirely fixed, say so.
Where any part of the analysis is finely balanced, a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. A short Planning Statement setting out the measurements against each limitation and condition makes that application much easier to determine.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Is the 1.7 metres measured from the ground or the floor?
From the finished floor level of the room in which the window is installed, up to the lowest part of the window that can be opened. Ground level outside is irrelevant. On a room with a raised floor the datum moves up with the floor, and on a split-level layout the correct floor can be open to argument, which is worth resolving early.
What counts as obscure glazing?
The Order does not define it. In practice councils work to the glass industry obscuration scale, and level 3 is widely treated as the minimum, with levels 4 and 5 giving more privacy. Applied film is not accepted because it can be removed, and a part-frosted window does not satisfy a condition that applies to the whole window.
Does the rule apply to a single storey side extension?
Under Class A the obscure glazing condition applies where the extension has more than one storey or forms an upper storey. A ground floor side window in a single storey extension is not caught by it. Other classes have their own versions, and Class B catches any window inserted in a side elevation during a roof enlargement.
Are rooflights in a side roof slope caught?
Yes. A window in a roof slope forming a side elevation is caught even at normal roof pitch, where it may overlook nothing. Positioning the rooflight high in the slope so its lowest openable part sits above 1.7 metres from the floor below usually solves it without losing useful daylight.
What happens if clear glass is fitted by mistake?
The condition is breached, which means the development was not permitted development at all, so the whole extension is unauthorised rather than just the window. Replacing the glazing with a compliant unit restores compliance and is normally quicker and less costly than applying for permission for the extension as built.
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.