Roof lanterns: planning and Building Regulations

Class C of the permitted development rules covers rooflights and roof windows on a house in England. They are usually allowed if they stick out no more than 150 mm, frame included, beyond the plane of the original roof slope, and sit no higher than the highest part of the original roof. On a pitched roof the plane is the line of the tiles or slates. On a flat roof it is the surface of the covering as first built.

A lantern starts its climb from that surface. First comes the kerb, the insulated upstand that lifts the frame clear of standing water and gives the roof covering something to turn up against. The frame sits on the kerb, and above the frame the glazed sides run up to the ridge. The sum of those three is what gets set against 150 mm. Because the kerb depends on how the roof is built up, with insulation above the deck needing a taller upstand than insulation between the joists, one lantern can stand noticeably higher on one roof than on its neighbour's.

Why a flat rooflight can pass when a lantern cannot

A low fixed rooflight on a shallow kerb can come in under the limit. A lantern, with sloping glass on every side, usually cannot, which puts it outside Class C. It then normally needs planning permission unless the council confirms otherwise. Lowering a lantern to fit tends to lose what it was chosen for, since the height is the reason people choose one over a flat pane. The sensible route is to ask the council, and we help you do that.

If the position is borderline, for instance a slim lantern on a very low kerb, you can ask the council for a certificate of lawfulness, which is its written confirmation that a proposed job needs no application. We measure the build-up on your roof first, so the drawing sent to the council shows the real height.

A flat or maisonette carries no householder permitted development rights, so a lantern over the kitchen of a top-floor flat goes to the council whatever its height. The same goes for a house created by converting an office, a shop, a barn or a similar building under the change-of-use permitted development routes.

An extension that has its own planning permission is a separate case. If the approved drawings show a lantern, building it as drawn forms part of the permission. If they show a plain flat roof, a lantern added afterwards needs its own answer. Either way, the decision notice is worth reading, because permitted development rights may have been removed from the house by a condition on the original permission. We ask to see the notice and the approved drawings at the survey, and we never assume what they say.

Conservation areas and listed houses

Standing in a conservation area does not by itself remove Class C; only an Article 4 direction does, and your council's website shows whether one covers your road. A lantern is usually past the height limit already, so in a conservation area the practical question is how it looks from public places. Our guide to Article 4 directions and rooflights explains what a direction changes.

A listed house still has Class C rights on paper. In practice a new or altered roof window or lantern will almost always need listed building consent, and that application is the one that matters, with its own drawings and its own officer. Our page on rooflights on listed buildings goes into it.

Pitched side slopes

Lanterns belong on flat roofs, but a pitched roof has a rule of its own that people often assume applies to everything. A window on a roof slope forming a side elevation of the house must be obscure-glazed and non-opening, unless the parts that open are more than 1.7 m above the floor of the room it is in. That condition concerns roof windows. Our skylight planning guide covers it alongside the rest of the Class C wording.

A lantern goes to the local planning authority as a householder application, made through the Planning Portal or directly to the council. The council charges a fee, publicises the proposal to neighbours, and decides. The officer looks at how the lantern sits against the house and what it does for the people next door, so a lantern at the back of a single-storey extension, set away from the boundary, is a different proposition from one close to a neighbour's bedroom window.

The drawings carry most of the weight. The council needs to see exactly how tall the thing is, so a lantern application is more about sections than a window application would be. We prepare the drawings from our survey measurements, or work alongside your architect if you have one, and the application goes in under your name.

Light at night

Glass that glows is something to think about at the design stage. A lantern over a kitchen that is lit in the evening can be seen from upstairs windows next door, which an officer may raise. Frame colour, glass type and the position on the roof can all be adjusted before the application is made, and blinds inside are an option for the owner afterwards.

Timing the order

A lantern is made to the size of your kerb, so a change to the drawing becomes a change to the order. For that reason we place the order after the council has decided. That way the glass made in the factory matches the consent.

Planning and Building Control run separately, and a planning consent does not stand in for a Building Control notification. Cutting a lantern opening through the roof is itself building work. The structure must take the weight of the glazed unit, the roof has to stay weathertight around it, the energy performance of the glazing has to meet the standard, and fire safety and escape come in where the room calls for them. We handle the Building Control notification where the work is notifiable. The council's building control team, or a registered building control approver (the name private inspectors have carried since April 2024), inspects the job and signs it off.

The energy limit for a lantern

Under Part L of the Building Regulations, a new or replacement rooflight or roof lantern in an existing home takes a maximum U-value of 2.2 W/m²K, assessed for the whole unit with its frame. A roof window in a pitched roof takes the tighter window figure of 1.4 W/m²K, or a Window Energy Rating of band B at minimum. In the Approved Document L glossary a rooflight is a glazed unit installed out of plane with the roof surface, on a kerb or upstand, and a roof lantern is one. We put the whole-unit figure on the quote so that the glass specification and the regulation can be read side by side.

Light is a design choice here

The Building Regulations set no daylight requirement for houses in England, so nothing in them says how large a lantern has to be or whether a room needs one. Size is limited by the structure beneath, by the planning height and by what you want the ceiling to look like. The inspector looks at how the job is built.

Replacing the polycarbonate or glass roof of a conservatory with a solid, insulated roof and a lantern changes what the structure is. That is building work, so the frames and base are checked for the new load and the job goes through Building Control, besides the planning question. An orangery built on that pattern is judged on the same two counts.

An old lantern swapped for a new one on the same kerb is a different matter, but it is not automatically free of questions. Planning rules look at what stands on the roof after the work, so a replacement that rises higher than the old lantern, perhaps because the new frame is deeper or the kerb has to be raised to take current insulation, raises the height question again. We compare the old and new sections before ordering and look at the planning history of the extension for any approved drawings that show the original lantern. The new unit has to meet today's energy limit, whatever the old one achieved.

If your extension has no lantern yet and the roof is already finished, adding a lantern to an existing roof covers what the survey checks on the structure. Where the choice is between a lantern and a pitched-roof window, our short answer on a roof lantern or a VELUX window sets out how the two are treated differently by both sets of rules.

The survey comes first, and it is free. We stand on the roof with a tape, read the build-up, measure the opening and look at the room underneath, then check the planning history that applies to your address. The fixed written quote follows, with the Building Control notification and any planning fee shown as lines of their own.

Where an application is needed it goes in before anything is ordered. Once the council has decided and the order is placed, the lantern is made while the Building Control notification is submitted and any structural design is settled. Fitting day takes the roof covering back around the opening, builds the kerb, fits and glazes the frame and finishes the reveals inside. The inspection follows, and our 10-year workmanship guarantee is put in writing once the job is complete.

To start, send a WhatsApp message to 07880 284133 with a photo of the roof and of the room underneath, and add a photo of the planning decision if you have it. You can also request a survey slot and we will bring the tape.