Guide · Skylight & Rooflight Installation & Replacement Chelmsford
Permitted development limits for rooflights
For most houses in England, a rooflight is permitted development and no planning application is needed. The permission is granted by the General Permitted Development Order rather than by the council, and it comes with conditions attached. Two of them do nearly all the work: the unit must not protrude more than 150mm beyond the plane of the original roof slope, and no part of it may sit higher than the highest part of the original roof.
Those two sentences settle the majority of domestic jobs. What is worth understanding is where the measurements are taken from, which classes of the Order apply to which situation, and the several ways the rights can be removed without anyone writing to tell you. This page works through all of it, and it sits under the wider guide to rooflight and skylight installation in Chelmsford.
The two limits that define the whole permission
The projection limit and the height limit. Everything else in the class is either a definition or an exception to those two. A flush glazed unit set into a pitched slope, and a flat rooflight sitting on a modest kerb in a flat roof, both clear them comfortably in ordinary circumstances.
What fails them is a raised structure. A roof lantern with a pitched glazed superstructure standing 400mm above the roof plane exceeds the projection limit on its own, before anyone considers the ridge. That is why lanterns on a house are a different planning conversation from flush rooflights, even though both are glass in a roof.
Measuring the 150mm, and what it is measured from
The measurement is taken perpendicular to the external surface of the original roof, not vertically and not from the tile face at the nearest point. Picture a line at right angles to the slope: the unit may extend 150mm along that line and no further.
On a pitched roof this is generous. A conservation profile unit sits at perhaps 30 to 50mm above the tile line, and a standard roof window with its flashing kit is typically under 100mm. On a flat roof, where the measurement is effectively vertical, the number is consumed by the kerb rather than by the unit. A 150mm upstand with a flush unit on top of it is already at the limit before the glass is counted, which is the single most common way a flat roof job leaves permitted development without anyone noticing.
Why the word “original” changes the answer
The Order refers to the original roof, which means the roof as the house was first built, or as it stood on 1 July 1948 for anything older. It does not mean the roof as it is today.
That matters on a house that has already been extended. A rear addition added in the 1990s is not original, so measuring the projection from its slope is the wrong datum, and the highest part of the original roof is the ridge of the house rather than the ridge of the extension. On the older Chelmsford and Colchester terraces, where rear additions have often been rebuilt more than once, establishing which roof counts as original is a documentary exercise using historic mapping and the planning record.
The ridge rule, and the units that break it
No part of the alteration may be higher than the highest part of the original roof. On a standard slope this is academic, because a rooflight near the eaves is nowhere near the ridge. It becomes live in three situations.
A rooflight positioned very close to the ridge on a shallow pitch, where the frame and flashing can creep above the ridge tile. A lantern or raised unit on a flat roof that stands above the main house ridge, which happens on bungalows across Canvey, Jaywick and the older parts of Clacton where the main roof is low. And a unit in the roof of a two storey rear addition on a house whose original ridge is lower than expected. Each is measurable in advance from a section drawing.
Side elevations, obscure glazing and the 1.7m rule
There is a third condition that people miss entirely. A window installed in a wall or roof slope forming a side elevation of the dwellinghouse must be obscure glazed, and any opening part must be more than 1.7m above the floor of the room it serves.
A roof window in a hipped side slope is caught by this, and hipped roofs are the default on the interwar semis that fill the Essex commuter towns. The practical answer is usually to fit the unit as fixed and obscure glazed, or to move it to the rear plane. A clear glazed opening roof window in a side slope, looking across a neighbour’s garden, is outside the permission and needs an application.
Class C, and the wording that actually governs
Rooflights sit in Schedule 2, Part 1, Class C of the Order, described as any other alteration to the roof of a dwellinghouse. It is the residual class, which is why it covers both a small roof window and a substantial flat rooflight.
Class C also excludes a few specific things that have nothing to do with glass: chimneys, flues, soil vent pipes and certain solar installations are handled elsewhere. And it does not apply where the dwelling itself only exists by virtue of a prior approval conversion, which catches some of the office to residential conversions around Chelmsford and Basildon town centres.
Flat roofs under the same class
Class C makes no distinction between a pitched roof and a flat one. Both are the roof of a dwellinghouse, and both get the same two limits. What changes is how quickly the projection limit is reached.
A warm roof build-up with a 150mm kerb, a flush unit and the covering dressed up the sides puts the top of the glass at or slightly above the limit, measured from the original roof surface. Where the flat roof itself is an addition rather than original, the datum moves again. This is worth resolving at design stage, because reducing the upstand to stay within the limit is a decision with weathering consequences and the two constraints pull against each other. The kerb side of that argument is set out on flat roof rooflight installation.
Rooflights in a new extension roof under Class A
Where the rooflight is going into an extension that is itself being built under permitted development, the extension is Class A and the rooflight forms part of it. Class A carries its own limits: depth from the original rear wall, eaves height, total height, and a maximum eaves height of 3m where the extension sits within 2m of a boundary.
The relevant point for glass is that the extension’s own height limits control the roof, and the rooflight then has to sit within the roof rather than above it. A flush unit does this without difficulty. A lantern on a single storey rear extension close to a boundary can push the overall height past what Class A allows, which converts the whole project into an application rather than just the glass.
Dormers, Class B, and where the rights stop
Enlarging a roof, which is what a dormer does, is Class B rather than Class C, and Class B is more restricted. It carries volume allowances of 40 cubic metres for a terrace and 50 for a detached or semi detached house, it is not permitted on the principal elevation, and it is not permitted at all on designated land, which includes conservation areas.
This is why a loft conversion in a conservation street so often uses roof windows rather than a dormer. Roof windows stay in Class C, which survives designation, while the dormer route requires an application. The daylight strategy for a converted roof space is covered on roof window installation.
Garages, garden rooms and outbuildings under Class E
An outbuilding is Class E, and its constraints are about the building rather than the glass: single storey, maximum eaves height 2.5m, maximum overall height 4m for a dual pitched roof or 3m otherwise, and 2.5m overall where any part sits within 2m of a boundary.
Rooflights in an outbuilding roof are permitted as part of the building, provided the finished structure stays inside those heights. On a detached garage converted to a home office in a Great Notley or Beaulieu Park garden, the height check is usually the binding one, particularly where the building sits tight to a fence line. Building Regulations are a separate question and apply to a habitable conversion regardless.
Conservation areas, and the rights that survive designation
A common assumption is that designation removes rooflight rights entirely. It does not. Class C remains available on designated land, which is why flush conservation units go into rear slopes in Thaxted, Coggeshall and the Chelmsford conservation streets without a planning application.
What designation removes is Class B, so dormers go. It also brings the visual expectations that shape the specification: a slim dark frame sitting low in the roof rather than a white surround standing proud. And on a listed building, permitted development for planning purposes has no bearing on listed building consent, which is required separately. That whole subject is worked through on conservation areas and listed buildings in Essex.
Article 4 directions, and how to check for one
An Article 4 direction is the mechanism by which a council withdraws a permitted development right for a defined area or a defined type of work. Where one is in force, work that would otherwise be permitted needs a planning application, and the application is free of the usual fee.
Directions are common in Essex conservation areas and they are specific in scope. One may remove only front elevation alterations, another may remove roof alterations generally. The council’s planning policy pages list them by area, and the property search on the planning map will normally flag one against the address. The check takes minutes and it is the single most useful thing to do before ordering.
Planning conditions on the newer Essex estates
The other route to lost rights is a condition attached to the original planning consent for the estate. Developers of large schemes frequently accept conditions removing permitted development rights so that the appearance of the estate stays consistent, and those conditions run with the land indefinitely.
This affects the newer housing around Beaulieu Park, Channels, Great Notley and the expansion areas at South Woodham Ferrers. The house is modern, the roof is a standard trussed roof and everything about the job looks routine, and there is a condition in the decision notice from twelve years ago that requires an application for any roof alteration. It shows up in the planning history for the site, filed against the original outline consent rather than the individual plot.
Why Part 1 rights stop at the dwellinghouse
Permitted development under Part 1 is granted to dwellinghouses. A flat is not a dwellinghouse for these purposes, so none of the classes above apply and any rooflight needs a planning application, however small.
That covers purpose built blocks and the converted Victorian houses common around Moulsham Street and New London Road. There is normally a second consent to obtain as well, because the roof is usually retained by the freeholder and the lease governs alterations to it. Both consents are separate from Building Regulations, which apply in every case.
Lawful development certificates, and what one is worth
Where work is permitted development, no permission is needed and therefore none is issued. That leaves nothing in the file to show a future buyer. A lawful development certificate closes the gap: it is an application to the council asking them to confirm formally that the work is lawful, and it produces a document.
It is optional and it is cheap insurance on anything marginal, which in this trade means a unit close to the ridge, a flat roof where the kerb takes the projection near the limit, or a house whose original roof is genuinely ambiguous. Solicitors accept it readily. An argument with a buyer’s conveyancer about where 150mm was measured from is considerably more expensive than the certificate would have been.
Doing the check before anything is ordered
The sequence that works is short. Confirm the property is a house rather than a flat. Establish which roof is the original one. Take the projection from a manufacturer’s section rather than from the brochure image. Check the planning map for a conservation boundary and the planning history for an Article 4 direction or a condition.
All of that happens before glass is specified, because the answer sometimes moves the unit to a different slope and occasionally changes the type entirely. Sunspire runs the check as part of the survey, alongside the structural and access questions set out on how a rooflight survey works, and the Building Regulations side is handled as part of the job.
The 150mm is measured perpendicular to the original roof, and on a flat roof the kerb has usually spent it before the glass arrives.
Keep reading
More on skylight & rooflight installation & replacement chelmsford
Sizing a rooflight for the room
Read the guide →Roof window or rooflight, and the difference
Read the guide →Flat roof against pitched roof rooflights
Read the guide →What happens on installation day
Read the guide →Cleaning and maintaining a rooflight
Read the guide →All about skylight & rooflight installation & replacement chelmsford →
Get a quote
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements