Rooflights for Single-Storey Extensions

People describe an extension by its room name: the kitchen-diner, the snug, the utility. Light does not care about room names. It cares about where somebody will be standing or sitting. So before choosing a unit, mark four or five spots on the floor plan. The worktop where you chop vegetables. The end of the dining table. The reading chair. The spot by the back door where coats and shoes pile up.

Each of those wants a different quality of light. A worktop wants bright, even light without a shadow falling across the board. A dining table is happier with softer light that does not glare on the plates. A reading chair needs steady light that is not so strong it washes out the page. A dark utility corner mostly needs enough light to find things.

This is the idea behind everything we do. The room decides what you ask for, but the spot decides where the rooflight goes. A rooflight placed centrally because it looks tidy on a plan may throw its light onto a patch of floor nobody uses. The same unit shifted a little towards the worktop can transform the way the room works.

Look at the room in its three zones

A deep rear extension tends to split into three zones: the back of the old house, the middle, and the garden end. The garden end is usually bright thanks to glazed doors. The back of the house is often the darkest, because it is furthest from the doors and shadowed by the old walls. The middle is where a rooflight earns its keep. We note how each zone will be used, then decide whether one large unit or two smaller ones serves the room better.

The flat roof of an extension takes several kinds of unit, and the choice changes how the room looks from both sides of the glass. The hub page for flat roof rooflights sets out the whole family. In this guide we focus on how each one behaves in a new room.

Fixed flat glass

A fixed unit, with no opening, is the simplest and the most discreet from the garden. It lets in daylight without any ventilation, so it suits a room that already has windows or doors you can open. It is also the lowest-maintenance choice because there are no moving parts.

Opening flat rooflights

An opening unit helps in a kitchen, where steam and cooking heat rise. It lets that air out at the top of the room, which is where it collects. The motorised versions are covered under electric opening rooflights, which can be run from a wall switch or a remote when the unit is out of reach.

Roof lanterns

A lantern is a raised, glazed structure that stands up from the flat roof. It brings daylight in from several directions and gives a room a ceiling with real height in the middle. It suits a larger extension with a generous footprint, and it is the unit most people picture when they say they want a feature in the ceiling. See roof lanterns for the detail.

Walk-on glass

Where the extension roof doubles as a terrace, or a first-floor window looks out onto it, a walk-on unit takes foot traffic. That is a different build, with thicker glazing and a different frame, covered in walk-on rooflights.

Rooflight size is not simply a matter of the bigger the better. A very large unit over a south-facing kitchen can make the room warm in summer and bright enough to be uncomfortable in the afternoon. A very small one can look lost in a large ceiling and leave the corners dark. The sensible size depends on the floor area, the ceiling height and how much glazing the room already has in the walls.

Position matters as much as size. A unit placed close to a wall lights that wall, which can look good because it washes the surface with light that grazes down it. A unit placed in the centre lights the middle of the floor. A unit placed over the worktop lights the worktop. We talk this through on the survey, standing in the room, because it is easier to agree where the light should fall when you can point at the spot.

Orientation

Which way the roof faces changes the character of the light. A rooflight on a rear extension facing the sun brings in strong light and warmth for much of the day. One on the shaded side brings in a more even, cooler daylight that is kinder to a desk or a reading corner. Neither is wrong, and the glazing specification can balance them, which is where the comparison of glazing types matters. How glass choices change the heat entering and leaving is explained under energy efficient rooflights.

The depth of the light well

On an extension, the ceiling below is often only a short distance from the roof, so the light well is shallow and the daylight spreads widely. If the roof has been built up with extra insulation, the well gets deeper. A deeper well narrows the spread of light, and splaying the sides helps. This is worth knowing at the planning stage, because a deeper roof can change which size of unit gives the result you hoped for.

An extension is often open-plan to the old house, so any heat gain or loss from the roof affects a large space. Rooflights are the weakest thermal part of a roof, so the specification deserves thought.

On the legal minimum, a flat roof unit counts as a rooflight, lanterns included, and for an existing dwelling Approved Document L (2021), Table 4.2 sets 2.2 W/m²K for it. Roof windows are held to the tighter window limit instead. Many glazed units perform better than the minimum, and for a large extension it is worth choosing one that does.

Controlling the sun

The other half of the question is solar gain. Glazing can be specified to let in less of the sun's heat while still passing daylight, which helps a south-facing kitchen. Some owners add blinds on the inside, and some opening units can be vented to release the warm air that collects below the glass. None of these removes the need to think about orientation early. If a large unit is going over a spot that gets direct sun for long hours, we say so before the order is placed, rather than leaving you to find out in July.

Condensation

A new extension is a damp building for its first season, as plaster and screed dry out. Condensation on a cold pane in that period is common and is not a sign the unit has failed. Good ventilation helps, and so does a well-insulated kerb and a sealed light well, which stop warm moist air reaching the cold surfaces. Our guide to kerbs and upstands for flat rooflights explains that part of the build.

If the extension is not yet built, the best time to plan the rooflight is now. The joists can be laid to suit the opening, the kerb can be built as part of the roof, and the covering can be laid around it in one go. That is cheaper in labour and cleaner in finish than cutting into a roof later, and it avoids a second set of junctions.

If the extension is already standing, adding a rooflight is still straightforward, but it is a separate piece of work with its own structural steps. We cover that in a short answer: adding one to a roof already built.

Talking to your builder

If you have a builder running the extension, bring us in early. We can read the plans or visit the site, agree the unit and the opening size, and give your builder the dimensions they need for the structure. The rooflight is then supplied and fitted to the roof as it is built. Getting the opening size wrong at the framing stage is the one error that costs real time, because trimming has to be redone.

Garage conversions and rear bays

The same thinking applies to a garage that has been turned into a living room, or to a rear bay with a flat roof. Garages are usually deep, narrow rooms that lose their original door and gain only a small window, so the middle of the room is dark. A rooflight, or a pair of them, fixes that. The roof is typically flat or low-pitched, and the joists run in a known direction, which makes the survey quick.

There is a lot of fear around paperwork, and most of it is avoidable. Here is the plain version.

Planning. House rooflights normally come under Class C permitted development, and its conditions are laid out in our planning guide. Flats, listed homes and conservation areas are where to ask the council before you order; in Derby that is Derby City Council, at derby.gov.uk.

Building Regulations. A fresh opening in the roof is building work, notified through the council or a registered building control approver. If the extension already has a building control application open, the rooflight can sit inside it. We take care of the notification where one is due.

Daylight. English rules set no daylight target for houses, so a rooflight is a choice about comfort and use, not a box to tick.

Escape. Sleeping space in a roof brings in the escape rules of Approved Document B, where a roof window serves as an escape only on storeys up to 4.5 m above the ground. A flat-roofed single-storey extension seldom raises this. A bedroom added into a bungalow roof does.

The survey is free, the quote is written and fixed, and the 10-year workmanship guarantee is in writing. To start, book your survey and we will look at the roof and the spots you use.