Can you put a roof lantern on a garage?

A roof lantern is heavier than people expect. A double-glazed unit runs to something like 25 to 30 kilograms per square metre of glass, and a four-pane lantern over a two by one and a half metre opening is carrying a serious load before you add the frame and the kerb. Most garages were never built with that in mind. A lot of them have a roof deck sized to hold up felt, a bit of insulation and the odd person standing on it during maintenance, not a permanent point load with people underneath.

So the first job is to look at what the roof is made of. A concrete-decked garage roof is usually the easiest case, because the deck already spans well and spreads load. A timber joisted flat roof is workable, but the joists may need doubling up around the opening or replacing at a deeper section, and a new structural kerb has to be built to frame the hole and carry the lantern. Trimming an opening into any flat roof means the joists you cut are no longer doing their job, and the trimmers around the opening take up that load, so the timber has to be sized for it. A garage with an asbestos cement or thin corrugated roof is a different proposition and generally means a new flat roof structure first, at which point the lantern is designed into the build rather than dropped onto what is there.

None of this is a reason to give up on the idea. It is a reason to have the structure looked at before anyone orders glass. If the span is long or the existing joists are shallow, a structural engineer will specify what the opening needs, and that calculation is cheap next to the cost of getting it wrong.

This is the question that changes the whole job, and it is worth being honest about before you start. If the garage stays an unheated store or a workshop and you simply want more daylight in it, a rooflight is a modest piece of work and the regulations are light. The moment the garage becomes a room you sit in, a home office, a snug, a kitchen extension, a playroom, it is habitable space, and a new or enlarged rooflight over habitable space is notifiable under Building Regulations.

Two parts matter here. Part L covers thermal performance, so the glazing has to meet a ceiling on how fast heat escapes through it, measured as the U-value. Part K covers safety glazing and guarding, which is why overhead glass in a room is toughened or laminated rather than ordinary annealed glass. There is also the wider garage conversion to think about: turning a garage into a living room usually means insulating the walls and floor, dealing with damp and ventilation, and often replacing the up-and-over door with a wall. The lantern is one line on a bigger list. On a new build or a full conversion we make the Building Control notification to Southend-on-Sea City Council or the relevant local authority on your behalf, so that side is handled rather than left for you to chase.

Here is where a garage lantern catches people out, and it is the part we care most about. A great many garages in this area sit at the rear or side of a 1930s semi, single storey, low, with the garden behind. Along the estuary the back of the house very often faces south, which means the garage roof does too. Put a lantern in a flat roof and the glass is nearly horizontal, so it sees the high summer sun almost square on and orientation stops rescuing you. A south-facing garden room under a clear-glazed lantern can become the hottest room on the plot by mid-afternoon, which rather defeats the point of building it.

The fix is in the glass, not the blind. For anything that faces south or sits flat, ask for solar-control glazing with a G-value of around 0.3 or below, so that less of the sun’s energy passes through as heat while the room stays bright. Reflected light off the water adds to the load on a clear day, so seafront and clifftop conversions feel it more than most. Getting the orientation and the glass specification right at the start is the whole basis of our energy-efficient skylight installation work, and a garage conversion is exactly the kind of deep, single-storey room that benefits from it. If the roof faces north, you can relax the figure and enjoy the free winter warmth instead.

For most houses, a rooflight in a flat garage roof is permitted development, provided it projects no more than 150mm above the surrounding roof plane and the garage is not being enlarged in the process. That covers a lot of ordinary cases. The exceptions are the ones to check. A detached garage counts as an outbuilding, which has its own permitted-development rules on height and use. Flats and maisonettes are excluded. Listed buildings need consent. And in a conservation area, or anywhere covered by an Article 4 Direction, the normal permitted-development rights can be removed, so a planning application is needed where it usually would not be.

Locally that bites hardest in the Leigh Cliff and Leigh conservation areas, where an Article 4 Direction removes permitted-development rights for changes to roofing and windows, and it is worth a check in Clifftown and Leigh Old Town too. Treat all of this as general guidance and confirm your own case with the local authority before you commit, because the line between permitted and notifiable turns on details specific to your plot.

Start with the structure and the use, in that order. Work out whether the garage roof is concrete, timber or something thinner, and be clear with yourself about whether this is a store getting more light or a room you intend to heat and sit in, because that decides which regulations apply. Then look at which way the roof faces and be realistic about summer heat before you fall in love with a big lantern over a south-facing conversion.

If you would like us to look at it, we will check whether the roof can take a lantern, tell you what the opening needs, and specify the glass for the orientation rather than selling you a standard unit. Our roof lantern installation page sets out how we approach the job, the skylight installation costs page shows where the money goes, and a custom skylight design makes sense where the garage roof is an awkward size. Ring 01702 898232 or request a quote and we will look at the whole roof, not just the hole you want to put in it.