The 200 mm projection rule explained
Class A permits solar that protrudes no more than 0.2 metres beyond the plane of a wall or pitched roof slope. Standard on-roof mounting is comfortably inside it. The installations that fail are the ones deliberately tilted away from the roof — usually to fix a shallow pitch or a bad orientation.
0.2 metres, measured perpendicular from the surface of the wall or pitched roof slope, to the outermost part of the equipment.
Ordinary on-roof mounting is not close to it — rails and modules together typically stand something like 100 to 160 mm proud of the tiles.
What breaches it is tilting panels off the plane of the roof, which is exactly what gets proposed to rescue a shallow pitch or an east-facing slope. Since December 2023 the limit applies only to pitched roofs; flat roofs have their own 0.6 metre rule.
The provision
Development is not permitted by Class A if the equipment:
would protrude more than 0.2 metres beyond the plane of the wall or [in the case of a pitched roof,] the roof slope when measured from the perpendicular with the external surface of the wall or [pitched] roof slope
The bracketed words were inserted on 21 December 2023 by S.I. 2023/1279. Before that, the limb applied to roofs generally; it now applies to walls and to pitched roofs.
What is measured, and from where
Three things people get wrong.
It is the whole assembly, not the panel. Measure from the roof surface to the outermost face of the module, including the mounting rails, brackets and frame beneath it. A 35 mm module on 100 mm of mounting is a 135 mm protrusion.
It is perpendicular to the surface. Not vertical. On a 35° roof the measurement is taken at right angles to the slope, so a panel lying parallel to the roof has a constant protrusion along its length.
The reference is the external surface. The tile or slate surface, not the rafters and not the battens.
| Component | Typical contribution |
|---|---|
| Roof hooks and brackets standing off the tile | 30–60 mm |
| Mounting rail | 30–50 mm |
| Module thickness | 30–40 mm |
| Total, panel parallel to the roof | roughly 100–160 mm |
| Allowance remaining | roughly 40–100 mm |
That is enough headroom for ordinary variation, and not enough for a tilt frame.
Why tilting fails the test
The interesting case, because tilting is what gets suggested when a roof is poorly angled.
A panel lying flat on the slope protrudes the same amount along its whole length. Tilt it, and the upper edge rises: on a module roughly 1.7 metres up the slope, each degree of additional tilt lifts the top edge by about 30 mm.
So two or three degrees of tilt consumes the remaining allowance. Anything that looks like a tilt frame on a pitched roof is almost certainly outside Class A.
The argument for tilting is usually a shallow pitch. Our modelling says the gain is not worth the planning application: at 20° facing south a roof already achieves 96% of the 35° optimum.
Tilting from 20° to 35° buys about four percentage points, and costs you permitted development. On an east or west-facing roof the case is worse still, because flat beats pitched off-south — steepening a west-facing array makes it worse, not better.
Flat roofs are a different rule
Since December 2023, a flat roof is governed by the separate limit inserted at the same time: the equipment must not result in the highest part being more than 0.6 metres higher than the highest part of the roof, excluding chimneys.
That is the allowance that makes tilted mounting on a flat roof possible — and it is tighter than it sounds. A 1.7 metre module tilted at 20° rises about 0.58 metres before you add the ballast or frame beneath it. So a 20° tilt on a flat roof is close to the limit, and steeper angles will exceed it.
Which matters less than you might think: on a flat roof, a shallow mounting angle often produces more output per square metre of roof anyway, because tilted rows shade each other and need spacing. The 0.6 metre limit and good design point in the same direction.
Note also that on article 2(3) land — conservation areas, National Parks, areas of outstanding natural beauty, the Broads, World Heritage Sites — a flat roof installation additionally requires an application for a determination as to whether prior approval is needed.
Walls
The same 0.2 metre limit applies to wall-mounted equipment, and the measurement is from the external surface of the wall.
Wall mounting is uncommon for rooftop solar but routine for plug-in and balcony systems, where the panel is often fixed to a balustrade or a wall face. Note that in a conservation area or World Heritage Site there is a separate prohibition on installing on a wall which fronts a highway — covered on our conservation areas page.
Practical checks
Ask for the system’s stand-off dimension. Every mounting manufacturer publishes it. Add the module thickness and you have your number.
Be suspicious of any proposal to tilt on a pitched roof. Ask what it gains, in kWh, and compare that against needing planning permission. Our orientation matrix gives the figures.
On a flat roof, ask for the overall height above the roof surface, including ballast, and check it against 0.6 metres.
If it is close, get a lawful development certificate. Protrusion is a measurable fact, which makes it one of the easier things to have determined in advance.
Sources
- The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 14
- The Town and Country Planning (General Permitted Development etc.) (England) (Amendment) (No. 2) Order 2023 (S.I. 2023/1279)
Contains public sector information licensed under the Open Government Licence v3.0.
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