Most roof lanterns on a rear extension do not need separate planning permission, because they are assessed as part of the extension’s own permission or permitted development. Retrofitting a lantern onto an existing roof is different: permitted development for roof alterations allows a projection of no more than 150mm above the existing roof plane, and a lantern exceeds that. Listed buildings always need listed building consent. Conservation areas and Article 4 areas need checking before you order.
Building Regulations are a separate matter and apply either way. Plenty of lantern installations need no planning permission at all and still need Building Control sign-off.
The 150mm rule and why lanterns sit awkwardly against it
Permitted development for alterations to a roof generally allows an addition projecting no more than 150mm above the existing roof plane, and not standing higher than the highest part of the roof. Flat rooflights fit inside that easily at 100mm to 150mm. A lantern does not: a 150mm upstand plus a 250mm to 450mm pitched glazed section puts you at 400mm or more above the plane.
Where this rarely bites is on a new rear extension, because the extension as a whole is being assessed. The lantern forms part of that build and is dealt with in the same permission or the same permitted development calculation, not as a separate roof alteration afterwards.
Where it does bite is retrofit. Cutting a lantern into a flat roof that has been there for fifteen years is a roof alteration in its own right, and the 150mm allowance becomes directly relevant. In that situation a lawful development certificate from the local authority is worth having, both for peace of mind and for the conveyancing questions that arrive when you sell.
| Scenario | Planning | Building Regulations |
|---|---|---|
| Lantern in a new rear extension with planning permission | Covered by the extension permission | Covered by the extension application |
| Lantern in a new extension under permitted development | Part of the PD calculation | Building notice or full plans |
| Lantern retrofitted into an existing flat roof | Exceeds the 150mm allowance; certificate advisable | Notifiable in its own right |
| Listed building, anywhere on the property | Listed building consent required | Applies as normal |
| Conservation area, lantern visible from the highway | Likely to need permission | Applies as normal |
Conservation areas and listed buildings in Suffolk
Suffolk and north Essex carry an unusually high density of conservation areas and listed buildings, which makes this a live question here far more often than it would be in a modern suburb elsewhere. Lavenham, Aldeburgh, Southwold, Dedham, Coggeshall, Clare and a substantial part of central Ipswich are all constrained, and Article 4 directions in some areas remove permitted development rights that would otherwise apply.
- Listed buildings. Listed building consent is required for a lantern regardless of where it sits, including on a modern rear extension attached to the listed structure. There is no permitted development route and the listing covers the whole building, not just the pretty front of it.
- Conservation areas. A lantern to the rear, not visible from a public highway, is often acceptable. Anything visible from the street usually is not, or will be asked to use a conservation-style unit with more traditional bar proportions and a painted finish.
- Article 4 directions. These vary street by street and are not obvious from looking at a property. Check with the council rather than assuming your neighbour’s extension sets a precedent.
- Material and profile. Timber frames with a painted finish are considerably more likely to be approved in a sensitive setting than slim anthracite aluminium. Our page on timber roof lanterns for period and listed properties covers what officers tend to ask for.
Talk to the conservation officer before you order anything. Ipswich Borough, East Suffolk, Mid Suffolk, Babergh and Colchester all have officers who will discuss a scheme informally, and an early conversation costs nothing. A refused application after a lantern has been manufactured to your surveyed size costs a great deal.
Where Building Regulations apply even when planning does not
This catches people out. A lantern retrofitted into an existing roof is notifiable to Building Control because it involves two regulated changes at once: a structural alteration, since cutting an opening removes joists and transfers load, and a change to a thermal element, since you are replacing insulated roof with glass.
Three parts of the regulations are in play. Part L governs the U-value, with a whole-unit figure of 2.2 W/m²K or better required for replacement rooflights in existing dwellings. Part K governs guarding where glazing sits at low level. Part B matters where the room below is part of a loft conversion escape route. The structural opening is assessed against a structural engineer’s calculations, which you will need on any opening over about 1.5m across.
We handle the Building Control notification for you: retrofit lantern work goes through a building notice or a full plans application to your local authority Building Control. We tell you which route your job needs at survey, and we work to the engineer’s design rather than designing structure ourselves.
Related questions
- Traditional or contemporary roof lantern? covers the profiles that satisfy conservation officers.
- What U-value does a roof lantern need? deals with the Part L side.
- Is an aluminium roof lantern cheaper than timber? matters when consent pushes you towards timber.
See also timber roof lanterns for period properties, under roof lantern installation.
Find out which category your property is in
We check listing status, conservation area boundaries and Article 4 coverage at survey, and tell you which consents your job needs before anything is ordered. Request a fixed quote for your property in Ipswich, Suffolk or north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.