A rooflight against a roof lantern on a flat roof

On a single storey flat roof extension, the choice between a flush rooflight and a raised roof lantern is a planning question as much as a design one, because the lantern adds height and the height limits in permitted development are measured to the highest part of the extension. A lantern on an existing flat roof is a separate case again: alterations to the roof of a house are permitted under Class C of Part 1 only where the alteration does not protrude more than 150mm beyond the plane of the existing roof, which a lantern will exceed.

So on a new extension the lantern eats into the 4m height allowance. On an existing roof it usually needs an application. The flush rooflight rarely raises either issue.

Daylight delivered by each option

Horizontal glazing is the most efficient way of getting daylight into a deep plan room, which is why both options work better than another window in the rear wall. The difference between them is the quality of the light rather than the quantity.

A flush rooflight gives a view of the sky directly above and a bright pool of light beneath it. A lantern, because its glazing is pitched on four sides, admits light from lower angles too, so the light reaches further back into the room and changes through the day. In a room with a glazed rear wall, that difference matters less than it does in a room lit only from above.

Glazing area is the bigger lever. Two rooflights give more even light than one of the same total area, and splitting the area also reduces the structural span.

Upstand heights and how they read from above

Every rooflight on a flat roof sits on an upstand, because the glazing has to be above the water line and above any ponding. A kerb of around 150mm is the usual minimum for a flush unit, and some systems require more.

A lantern adds its own frame and pitch on top of that kerb. The combined height is commonly in the region of 400mm to 700mm above the roof surface depending on the plan size and pitch, and that is what someone looking down from the first floor window above actually sees.

Those ranges reflect common products rather than any standard, so take the dimensions from the system you are specifying and draw them.

Overlooking from the window above

This is the detail that surprises people. A lantern does not create overlooking in the usual sense, because nobody stands on the roof. What it does is make the roof visually busier when seen from the bedroom window above, and it introduces light spill upwards at night.

Neither point is normally a reason for refusal on its own. Both become relevant where the extension is close to a boundary and the neighbour's first floor window looks across the roof, because an officer assessing the effect on amenity will consider what the neighbour sees and whether the glazing lights their room at night.

Where the extension itself sits near a boundary, the eaves height limits bite first, and our note on eaves height limits when an extension sits near a boundary sets those out.

Solar gain and the overheating consequence

Horizontal and near horizontal glazing collects more solar radiation per square metre than a vertical window, particularly in midsummer when the sun is high. A large lantern over a south facing kitchen is the most reliable way to make a new extension uncomfortable in July.

The mitigations are worth deciding at design stage rather than buying later:

• Solar control glazing with a lower solar factor in the horizontal panes, accepting a small loss of daylight transmission.
• An openable vent in the lantern or in one rooflight, which gives a stack effect and clears warm air at the highest point.
• External or integral blinds, which work better than internal ones.
• Splitting the glazed area and keeping some of it away from the most exposed orientation.

Approved Document O deals with overheating in new dwellings rather than extensions to existing houses, but the physics is identical and the approach in an Overheating Risk Assessment is a sensible model even where no assessment is required.

Structural openings and deflection

The structural difference is in how the load gets to the perimeter. A flush rooflight sits on a kerb formed within the roof structure, with trimmers either side, and the load path is simple.

A lantern of any size concentrates load on its kerb, which has to be supported on the structure below, and the glass is unforgiving about movement. Deflection limits for glazed assemblies are tighter than the limits that keep plaster from cracking, so the beams supporting a large lantern are usually deeper than they would otherwise be. On an extension with a glazed rear wall and a large lantern, the roof is doing very little structural work and the perimeter beams are doing all of it.

Two practical points. Agree the deflection criteria with the glazing supplier before the structural design is fixed. And check the head height consequence, because a deeper beam at the junction with the existing house either lowers the ceiling or raises the roof, and raising the roof takes you back to the 4m height limit.

Walk on and flush glazed options

Where there is accommodation above and the flat roof could become a terrace, walk on glazing is worth considering, though a roof terrace is a planning matter in its own right. Forming a balcony or terrace is excluded from permitted development for extensions, so a usable terrace needs permission and will be assessed on overlooking in both directions.

Flush glazed systems with minimal framing suit contemporary extensions and read well in conservation areas precisely because they disappear. Where the extension is visible from the street or from a neighbouring upper floor, that invisibility is the argument to make in the application.

Showing the glazing on plan and section

Councils refuse householder applications on height far more often than on glazing type, so the drawings should make the height position unarguable.

1. A roof plan showing the position and size of each rooflight or lantern, with the roof falls and the outlet.
2. A section through the extension and the glazing, with the height above existing ground level dimensioned to the highest point of the glazing, not to the roof surface.
3. The 4m overall height and, where the extension is within 2m of a boundary, the 3m eaves height marked as dimensions on the section.
4. The upstand height dimensioned, with the system named.
5. A rear elevation and, where a neighbour's upper floor window faces the roof, a section showing the sight line from that window.

Where permission is needed, a short Planning Statement explaining the daylight rationale and the amenity position does more work than extra drawings. The parallel rules for other roof mounted items are covered in our notes on solar panels and the projection limit and on flues and soil vent pipes on a front roof slope.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works are lawful without permission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.