Matching roof pitch and eaves on a rear extension

A rear extension roof pitch has to match the original house in only one narrow legal sense. Where the enlarged part has more than one storey and you are relying on permitted development, paragraph A.3(c) of Class A, Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 requires the roof pitch of the enlarged part to be the same as the roof pitch of the original dwellinghouse, so far as practicable. Everywhere else, pitch and eaves height are design judgements. The judgement a case officer is making is whether the addition reads as subordinate to the house it hangs off.

Eaves are usually the tighter constraint. Two separate limits bite under Class A: the eaves of the enlarged part cannot exceed the eaves height of the existing house, and where any part of the extension sits within 2 metres of the boundary of the curtilage, its eaves cannot exceed 3 metres. Miss either and you are into a householder application.

Why pitch and eaves are assessed at all

Roof geometry is what makes an extension read as part of the house or as a shed bolted onto the back of it. Pitch controls apparent bulk, because a steeper roof on the same footprint throws up a much larger volume. Eaves height controls where that bulk begins, which is what neighbours notice from a garden and what a 45-degree daylight test measures against.

The National Design Guide treats form and scale as one of its ten characteristics of well-designed places, and most local authorities carry a householder design guide that repeats the same instruction in blunter terms: keep the addition lower than the original, set it back where you can, and do not compete with the main roof. Those documents are the ones an officer quotes in a delegated report.

There is a second reason, less often stated. A matched pitch is a low-effort way of showing you looked at the host building at all. An application that arrives with a shallow mono-pitch on a 1930s semi and no explanation invites the officer to assume the roof was chosen by the trusses available rather than by anything on site.

Reading the geometry of the host dwelling

Start from a measured section through the existing house, not from the elevation. You need the pitch in degrees, the eaves height above a fixed datum such as the threshold, the ridge height, and the span between the wall plates. Interwar suburban houses commonly carry a fairly steep main roof and a shallower catslide or outrigger at the back, and those two pitches are different, which matters when you decide which one you are matching.

The arithmetic then constrains you. Ridge height equals eaves height plus half the span multiplied by the tangent of the pitch. Narrow the span and hold the pitch, and the ridge drops. Hold the span and steepen the pitch, and the ridge climbs above the original.

Illustrative example: a rear extension spanning 4 metres, with eaves set at 2.4 metres and a 40-degree pitch, produces a ridge roughly 4.1 metres above the datum. Pull the span in to 3 metres and the same pitch gives a ridge of about 3.65 metres, which is often the difference between clearing the 4-metre single storey height cap and failing it. The figures are illustrative arithmetic, not a substitute for a surveyed section.

That is the calculation to do before anyone draws anything. And it is the calculation most rejected schemes never did.

What Class A actually says

Read the pitch condition carefully. It applies only where the enlarged part has more than one storey, so a single storey rear extension has no pitch requirement under Class A at all, only the 4-metre height cap and the eaves limits. The materials condition in A.3(a) sits alongside it and asks for exterior materials of similar appearance to the existing house, which is where render colour and tile profile are argued.

The larger home extension route in A.4 allows a deeper single storey rear extension, 8 metres for a detached house and 6 metres otherwise, subject to prior approval and the neighbour consultation procedure. It does not relax the height or eaves figures.

An Article 4 direction can remove any of this, and designated land, which includes conservation areas, strips out some Class A rights entirely. A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works are lawful, and it is worth the fee where the eaves figure is close.

Flat against pitched on a two storey extension

A flat roof on a two storey rear extension is not the same pitch as the original by any reading, so the permitted development route closes and you need permission. Officers are rarely hostile to the idea, but they look hard at the parapet line, because a flat roof at first floor level usually brings the top of the wall up close to the main eaves and the addition stops looking subordinate.

Three moves tend to work. Set the extension down from the main eaves by a course or two so the abutment is visible. Hip the ends of a pitched roof rather than gabling them, which pulls the apparent mass away from the boundary. Or drop the ridge of a matched pitch well below the original ridge by narrowing the span, which is the most conventional answer and the easiest to defend in writing.

Parapets, fascias and rainwater goods

Height is measured to the highest part of the structure, so a parapet upstand counts towards the 4-metre and 3-metre figures. People lose permitted development on 100 millimetres of coping.

Below that, the details that make a matched pitch convincing are unglamorous. Fascia depth and soffit projection, gutter profile, whether the verge is mortar bedded or a dry system, the position of downpipes, and the flashing at the abutment with the original wall. Half-round gutter on a cast iron original reads wrong at close range even when the roof plane is perfect. A brief material and rainwater goods schedule on the drawings settles most of it and heads off a condition requiring samples.

Where a contrasting form works better

On a period house with a rear outrigger, a matched pitch can be the wrong answer. Historic England's guidance on extending historic buildings supports new work that is legible as new, provided it is subordinate and reversible in principle, and a low flat or single-slope roof set behind a parapet often harms less than a fake replica pitch. Glazed links and zinc-clad boxes sit in the same category, and a Heritage Statement is the place to argue the case where the building is listed or in a conservation area.

Be honest that this is a judgement where authorities differ. Some design guides insist on matching pitches in suburban streets and accept contemporary forms only in back gardens invisible from the street. Others take the opposite view. Reading the adopted design guide and the recent decisions on similar houses in the same road tells you more than any general rule.

Drawing the comparison for the officer

Put the existing and proposed sections on the same sheet at the same scale, with pitch angles annotated in degrees and eaves and ridge heights dimensioned from one datum. Add a street or garden elevation that shows the original roof and the new one together. A roof plan helps where valleys or catslides are involved.

Then write the reasoning down rather than leaving the officer to infer it. A short Design and Access Statement covering the host geometry, the pitch chosen and why the eaves sit where they do carries more weight than three extra drawings, and for anything contentious the policy case belongs in a Planning Statement. If you are weighing the roof form itself, the trade-offs are set out in more detail in our note on choosing a flat or pitched roof for a single storey extension, and the heritage angle in glazed link extensions to a period house.

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. Enter your postcode to see which reports your extension is likely to need.