Upper floor side windows and the obscure glazing condition

If you add a second storey to the side of your house, or put a dormer in a side-facing roof slope, the window in that side elevation has to be obscure-glazed, and any part of it that opens has to sit more than 1.7 metres above the floor of the room. That pair of requirements is the obscure glazing side window condition, and it is written into the permitted development right itself rather than added by a planning officer. Break it and the whole extension stops being permitted development.

The wording sits in Schedule 2 Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015. It appears in three places, with small differences that decide whether your window is caught at all.

Which windows the condition catches

Class A covers enlargement of a dwellinghouse. Its condition applies only where the enlarged part has more than a single storey. So a two-storey side extension with a landing window facing the neighbour is caught, while a single-storey rear extension with a small side window is not.

Class B covers additions to the roof, which is where most dormers live. Its condition applies to any window inserted in a wall or roof slope forming a side elevation of the dwellinghouse. There is no single-storey let-off, because a Class B dormer is in the roof by definition.

Class C covers other alterations to the roof, including rooflights. Its condition applies to any window located on a roof slope forming a side elevation.

The phrase that does the work is "side elevation". It is about orientation, not about which builder put the wall up. A flank wall of a two-storey side extension that faces the boundary forms a side elevation even though it did not exist last year.

The obscure glazing requirement and glazing grades

The Order says obscure-glazed and stops. No British Standard, no grade, no measurement. The government's technical guidance for householders fills the gap by explaining that the glass should be such that it is not possible to see through it.

In practice councils and glazing suppliers work to the industry obscuration scale, which runs from 1 to 5, and officers generally accept level 3 or above. Level 1 patterned glass still gives a workable view of next door's patio, and enforcement officers treat it as non-compliant.

Two things people try that do not satisfy the condition. A privacy film stuck to clear glass is not obscure glazing, because the glazing itself is clear and the film comes off with a scraper. A blind or a shutter is not glazing at all.

Obscure glazing is not the same as dark glazing. Good level 4 glass transmits plenty of daylight. That matters if the side window is lighting a stairwell or a bathroom with no other opening.

The restriction on opening parts below 1.7 metres

The second half of the condition is about what opens. The window must be non-opening unless the parts that can be opened are more than 1.7 metres above the floor of the room in which the window is installed.

Fixed obscure glazing at any height is compliant. A top-hung vent whose opening section begins at 1.8 metres is compliant. A side-hung casement that opens from 900 millimetres is not, and the fact that the glass is obscure makes no difference to that limb of the condition.

The measurement applies to the opening part, not to the frame or the head of the window. Which is why the standard answer is a tall fixed pane with a narrow opening light across the top.

Then check it against Building Regulations before you order anything. A loft room often relies on a window for escape, and a window that cannot open below 1.7 metres cannot be your escape window. Purge ventilation under Part F also needs a reachable opening. Those two documents have beaten more side windows than planning ever has.

Measuring the height from internal floor level

The datum is the floor of the room in which the window is installed. Internal finished floor level, not external ground level, not the top of the joists before the screed and the boards go down.

This catches people on sloping sites, where the ground outside is a metre lower than the floor inside, and in loft conversions where a new deck is laid over the old ceiling joists and the floor finishes 60 millimetres higher than the drawing assumed.

Dimension it on the elevation and again on a section through the window, with the finished floor level labelled. If an officer ever asks whether the right was properly used, that drawing is your evidence.

As an illustrative example, take a 1930s semi with a two-storey side extension. The bathroom window in the flank wall is 1,200 millimetres tall. Sitting the cill at 1,100 millimetres above finished floor level puts the whole opening section above 1.7 metres only if the bottom 600 millimetres is fixed. Raise the cill to 1,750 millimetres instead and the entire window can open, but you lose the view of the garden and the room feels like a cupboard. Most people take the fixed pane.

Rooflights and whether they are included

Yes, when the roof slope faces the side. A conservation-style rooflight over a side-facing bathroom slope is a window on a roof slope forming a side elevation, so the Class C condition applies in full: obscure glazing, and nothing opening below 1.7 metres.

A rooflight in a rear slope is outside the condition, which is one reason a rear-facing rooflight plus a fixed side light costs less to detail than two side rooflights.

Hipped roofs make the judgement harder. A slope that reads as part of the front or rear plane is not obviously a side elevation, and councils differ on where they draw the line. If the answer changes your design, ask the duty planner in writing before you build, in the same way you would for the drainage condition that attaches to hard surfaces under Class F.

Replacing obscure glazing later and the consequences

The condition runs with the development for as long as the development exists. Swapping the obscure pane for clear glass five years later is a breach of planning control, and the council can require the obscure glazing to be reinstated.

That is a genuine risk, not a theoretical one, because the breach is visible from the street and from the neighbour's bedroom. Enforcement time limits for breaches of condition in England were extended under the Levelling-up and Regeneration Act 2023, so do not assume a short immunity period, and check the current position with your local planning authority.

A buyer's solicitor may ask for evidence that the right was used properly. The formal answer is a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 for proposed works, or section 191 for works already done. Nothing else, including a builder's assurance, has the same standing.

Designing a side light that still works

The usable options are narrower than most suppliers suggest, and they are all about putting the glass where the overlooking is not.

1. A fixed obscure pane with a top-hung opening light whose lowest opening edge is above 1.7 metres.
2. A high-level clerestory strip sitting entirely above 1.7 metres, clear glazed only if it is not in a side elevation.
3. A rooflight in the rear slope instead of the side, which escapes the condition altogether.
4. A sun tunnel, which is not a window in a side elevation and brings daylight into an internal bathroom.
5. A full planning application for a clear side window, arguing separation distance and oblique angles, which is the honest route when the side window is the only daylight the room will get.

Option five is a real choice rather than a fallback. Where the flank wall faces a blank gable 14 metres away, a clear window is often acceptable, and the same logic that governs what counts as incidental to the enjoyment of a dwellinghouse applies here: the test is the actual relationship on the ground. A short Planning Statement setting out the separation distance, the angle of view and the neighbouring window positions does most of the work.

If you want to see which documents a householder application for a two-storey side extension is likely to need, enter your postcode on PlanScope and work through the guided questions. Keep the glazing specification and the floor level dimensions on the drawings either way, because once the scaffold comes down they are hard to prove. And check the design against the conditions attached to domestic solar equipment if panels are going on the same roof.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate is the only formal way to confirm that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.