Conservation rooflights and how they are specified

A conservation rooflight specification comes down to four things: a frame that sits no more than 150 millimetres above the roof plane, a flush or recessed fixing into the slope, a single vertical glazing bar down the middle, and a portrait proportion in a dark finish. Get those right on a house in a conservation area and the rooflight usually falls within permitted development under Class C. Get them wrong, or put one on a listed building without consent, and you have an enforcement problem rather than a design discussion.

The 150 millimetre figure is the one people reach for, and it is necessary but not sufficient. Conservation officers are looking at how the window reads from the street as much as how far it projects.

Why rooflights sit in a different class from dormers

Dormers enlarge the roof, so they fall under Class B of Schedule 2 Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015. Class B is withdrawn entirely on article 2(3) land, which covers conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites. A dormer there needs planning permission.

Rooflights are an alteration to the roof rather than an enlargement, so they sit under Class C. And Class C is not withdrawn on designated land. That difference is why a loft conversion in a conservation area is so often drawn with rooflights on the front slope and a dormer only at the back.

Class C also excludes chimneys, flues, soil and vent pipes, and solar equipment, which are dealt with elsewhere in the order. So a scheme combining a rooflight with a new flue is testing two different sets of limits at once.

The protrusion limit and what satisfies it

Class C is unavailable where the alteration would protrude more than 150 millimetres beyond the plane of the roof slope, measured perpendicular to the external surface of the original roof, or where the highest part of the alteration would sit above the highest part of the original roof.

Measure from the surface of the slates or tiles, not from the rafters or the sarking. The dimension that matters is the overall build-up: the upstand of the frame, the depth of the glazed unit and any flashing collar. Several standard domestic rooflights sit at around 90 to 110 millimetres above the tile face when fitted to the manufacturer's detail, so they pass. Several others, particularly units with an insulated kerb, do not.

Two practical traps. Fitting over existing tiles rather than cutting them back adds depth to the whole assembly. And a rooflight installed on a flat roof or a very shallow pitch behind a parapet raises the "highest part" question rather than the protrusion question, so check both limbs.

One condition attaches to any Class C alteration. A window in a roof slope forming a side elevation has to be obscure-glazed and non-opening below 1.7 metres above the floor of the room, which is the same logic described in our note on Juliet balconies and overlooking.

Flush frames and recessed detailing

A conservation rooflight is a product type rather than a planning category, and councils describe it in design guidance rather than in legislation. The features that recur are consistent enough to specify.

• A thin external frame, traditionally cast iron and now usually steel or powder-coated aluminium, in a dark finish that reads as a shadow gap rather than a white line.
• A flush or recessed installation, with the glass set close to the plane of the tiles and the surrounding tiles or slates dressed up to the frame.
• A code 4 or code 5 lead flashing or a proprietary soaker system, not a visible plastic surround.
• No external trickle vents, no white uPVC and no projecting handles on the outer face.

The visual test an officer applies is simple. From the pavement opposite, does the eye read a window or a box sitting on the roof? A unit that is technically within 150 millimetres but finished in white plastic will still draw an objection in a conservation area, and the council can then use its design policies on a subsequent application for anything else on the building.

Central glazing bars and proportions

Historic rooflights were made in standard small sizes because cast iron frames and single glazing could not span far. That is why the central vertical glazing bar matters: it reproduces the proportion of the original pattern, and it keeps the apparent width down on a wide unit.

Portrait orientation is the norm. A rooflight that is wider than it is tall reads as a modern insertion whatever its frame. Where more light is needed, two or three narrow units spaced to line through with the window openings below will nearly always be accepted in preference to one large opening.

Keep the sizes modest and repeat them. On a two-bay terraced slope, two identical units set symmetrically about the party wall line look deliberate. Three different sizes do not.

Position on the slope and visibility

Position is the element most often left to the roofer, and it is the element most often conditioned.

Set rooflights low on the slope where the roof is visible from a public vantage point, so the frame is seen against the tiles rather than against the sky. Align the vertical edges with the heads and jambs of the windows below. Keep a reasonable margin to the ridge, the eaves, the verge and any party wall, both for weathering and because a unit crowding the ridge looks wrong.

Rear slopes are easier, but not automatically. Where a terrace backs onto an open space, a lane or a churchyard, the rear slope is the public elevation and the same discipline applies. Look at the roof from every road, footpath and open space within sight of it before deciding which slope takes the glass.

Listed buildings and the internal evidence

Permitted development rights do not displace the need for listed building consent. Any works to a listed building that affect its character as a building of special architectural or historic interest need consent under the Planning (Listed Buildings and Conservation Areas) Act 1990, and that includes cutting a hole in a historic roof structure.

Expect to evidence three things. What the roof structure is and where the proposed opening falls in relation to rafters, purlins and any historic carpentry. Whether the roof space has ever been used or lit, since an existing historic opening is a far stronger starting point than a new one. And what the proposal does to fabric, including how much lath, plaster and original slate is lost.

A short Heritage Statement covering significance, impact and justification does most of the work here, and it is worth photographing the roof void before anything is disturbed. Material alterations to appearance may also need a parallel planning application, which is where our note on rendering and cladding decisions overlaps.

Specifying so the condition can be discharged

Where permission or consent is granted, the materials condition will ask for the rooflight to be approved in writing. Give the officer enough to approve first time.

1. Name the product range, model and size, and state the external finish and colour reference.
2. Give the protrusion above the tile face as a dimension on a section at 1:5, including the flashing build-up.
3. Show the glazing bar, and confirm the glass specification and whether any vent is internal.
4. Mark each unit on a roof plan with its distance to ridge, eaves and verge.
5. Confirm the flashing material and how the surrounding slates or tiles are dressed.

That drawing takes an hour and saves a fortnight. Enter your postcode on PlanScope to see which documents a loft conversion on your house is likely to need.

This is general guidance, not legal advice: check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.