Can a roof lantern be fitted to an existing extension?

A roof lantern needs three things from the extension underneath it: a flat or very shallow-pitched roof, a structural opening it can span, and enough depth in the roof build-up to form a kerb. Most single-storey rear extensions in south Essex already tick the first box, because the standard warm-deck flat roof was built to take foot traffic and a decent snow load, and a lantern is not a heavy addition once you spread it across a timber kerb.

The structural question is about the opening, not the glass. Cutting a large hole in a roof deck removes some of what was tying the joists together, so the timbers around the opening are trimmed and doubled up, and where the span is wide the edges are picked up on a beam. On a deck built with the lantern already in mind this is done from the start. On an existing roof it means opening up a section, seeing what is actually there, and sizing the trimming to suit. A roof built in the last couple of decades to current standards rarely gives any trouble. An older, thinner deck sometimes needs a bit more timber, which is normal and not a reason to walk away.

Height matters too. A lantern reads best when it stands proud of the roof on a kerb of 150mm or so, which throws rainwater clear of the glass and keeps the frame above any standing water. If the extension sits tight under a first-floor window or a boundary, that height has to be checked against what is above it before anything is ordered.

The kerb, sometimes called the upstand, is the raised timber box the lantern bolts down onto. On a new extension it is built as the roof goes on. On an existing one it is built after the fact, and this is the part of a retrofit that has to be done carefully, because the join between the new kerb and the old waterproofing is where a badly done job lets water in.

The sequence is: mark and cut the opening, trim the structure, build the timber kerb up off the deck, then dress the roof covering up and over the kerb so the waterproof layer runs continuously from the flat roof onto the timber. The lantern sits on top of that, on its own seal, so there are two lines of defence and neither of them relies on a bead of silicone doing structural work. Get the covering dressed properly up the kerb and the lantern is weatherproof for its full life. This is the single detail that separates a lantern that stays dry from one that does not, which is why it is worth reading how the job is done on our roof lantern installation page before you commit.

Most existing extensions have a felt, EPDM rubber or GRP fibreglass roof, and all three take a retrofit kerb well. The covering is cut back, the kerb is built, and the same material is dressed up to meet the new frame so the finish matches the rest of the roof rather than looking bolted on.

Adding a lantern to an existing extension is notifiable under Building Regulations, the same as any new or enlarged rooflight. Part L covers the thermal performance of the glazing, so the unit has to meet the U-value ceiling for a replacement rooflight, and Part K covers safety glazing where the glass is within reach or over a walk-on area. We make the Building Control notification to Southend-on-Sea City Council, or to your local authority if you are outside the city, so the paperwork is handled as part of the work rather than left to you.

Planning permission is usually not needed. A rooflight or lantern generally counts as permitted development on a house as long as it projects no more than 150mm above the existing roof plane and does not sit above the highest part of the roof. There are real exceptions. Flats and maisonettes do not get the same rights, listed buildings always need consent, and conservation areas change the picture. In Leigh, the Leigh Cliff and Leigh conservation areas carry an Article 4 Direction that removes permitted-development rights for roof changes, so a planning application is needed there where it would not be a mile away. Treat all of this as general guidance and check your own address with the council before ordering glass.

Here is the part that gets skipped. A lantern going onto an existing rear extension is almost always going over a kitchen or a kitchen-diner, and across a great deal of the 1930s semi belt in Southend, Westcliff and Thorpe Bay the back of the house faces south towards the estuary. That is a lovely aspect and a real problem at the same time, because horizontal glazing sees the high summer sun almost square on, and a clear-glazed lantern over a room that already makes its own heat from cooking will turn that room into a greenhouse by mid-afternoon.

Retrofitting is actually the good news here, because you are choosing the glass from scratch. A south-facing lantern wants solar-control glazing with a G-value down around 0.3 or below, which lets the daylight through while stopping most of the heat, rather than the clear double glazing at roughly 0.6 that a builder will fit by default. The open southern horizon along the estuary and the light bouncing off the water both add to the load, so this is not a detail to leave to chance. Specifying the glass to the orientation is the whole basis of our energy-efficient skylight installation work, and on a retrofit it costs you nothing to get right because you are ordering the unit anyway.

Start by looking at your roof and answering three things: which way the extension faces, what the roof is covered in, and whether anything above it limits how high a kerb can go. If the extension faces anywhere from east through south to west, plan on solar-control glass rather than clear. If you want a shape that is not a standard rectangle, a custom skylight design can be built to the opening you have.

If your existing flat roof is tired or the covering is near the end of its life, it is worth thinking about the lantern and the roof covering together, since the covering has to be dressed up the new kerb regardless. Our flat roof skylight installation page covers how the two jobs fit together, and the skylight installation costs page sets out where the numbers land.

When you are ready, tell us the extension’s orientation, the rough size of the lantern you want and what the roof is covered in, and we will come back with a glass specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole you want to put in it.