Cat slide roofs on a single storey rear extension

A cat slide roof extension design continues the main roof slope down over a single storey rear extension instead of stopping it at the eaves and starting again. Done well it looks like the house always had it. Done without checking two numbers, it either breaks the permitted development eaves limit at the boundary or leaves you with a strip of floor you cannot stand up in.

The two numbers are the 3 metre eaves limit where any part of the extension sits within 2 metres of a boundary, and the 4 metre overall height limit for a single storey rear extension, both in Class A of the General Permitted Development Order 2015. A continuous slope interacts with both at the same time, which is why the section drawing has to come before the plan.

What a cat slide roof does to the elevation

The appeal is unity. A roof that runs from the ridge to a low eaves at the rear reads as one form, which suits houses with a steep original pitch and a simple rear elevation. Cottages, some 1930s semis and anything with an existing catslide over a rear outrigger all take it comfortably.

It reads badly in two situations. On a house with a shallow roof pitch, extending the slope produces either a very low eaves or a very shallow extension, because the geometry is fixed by the pitch. And on a terrace where every other house has a flat roofed or lean-to rear extension, a single catslide stands out from the rear alley in a way that officers in a conservation area will comment on.

So the first question is the existing pitch. At 45 degrees, a 3 metre deep extension drops the eaves 3 metres below the point where the slope starts. At 30 degrees it drops 1.7 metres. That difference decides whether the design works at all.

Eaves height at the boundary and the 3 metre rule

Class A permits a single storey rear extension, but if any part of it comes within 2 metres of a boundary of the curtilage, the eaves height of that part must not exceed 3 metres. On a terrace or a close-set semi, the side wall of a rear extension is almost always within 2 metres of the boundary.

A catslide helps here, because the eaves at the rear and the sides are lower than they would be on a flat roofed extension with a 2.7 metre internal height. But the eaves at the point where the slope meets the existing side wall may be higher than 3 metres, and that is the part people forget to check.

Measure the eaves height at the highest point of the eaves within the 2 metre zone, not at the lowest. On a catslide running down a flank wall, the eaves line is sloping, so the measurement varies along it. Where any part of that sloping eaves exceeds 3 metres within 2 metres of the boundary, Class A does not apply and a planning application is needed.

The 4 metre overall height limit then applies independently. A catslide that starts partway up a steep roof can approach 4 metres at its highest point over the extension.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works would be lawful without permission.

Headroom under a low eaves

This is where catslide extensions disappoint people. A sloping ceiling that meets the external wall at 2.1 metres gives you full headroom only where the slope is above about 2 metres, and the band between the eaves and that line is floor area you cannot use for standing.

Work out the usable depth before signing off the plan. At a 40 degree pitch, the ceiling rises roughly 0.84 metres for every metre you move back from the eaves, so a 2.1 metre eaves height reaches 2.4 metres about 360 millimetres in. At a 25 degree pitch the same rise takes nearly 650 millimetres.

The practical answer is to put the things that do not need headroom at the low edge: a run of base units, a window seat, a bench, built-in storage, or a sofa back. What does not work is a dining table at the eaves, or a door into the garden positioned where the ceiling is lowest, because the door head then sets the eaves height for you.

Note that England's building regulations do not set a minimum ceiling height for habitable rooms in an extension, though Approved Document K requires 2 metres of headroom over stairs. So this is a usability judgement rather than a compliance test, which is exactly why it gets skipped.

Light to the upper floor windows above

The slope has to pass below the first floor window sills, with enough clearance for the roof covering, the abutment flashing and a reasonable upstand. On a house with low first floor sills, which is common on cottages and on some 1930s houses, there may not be room.

Three checks settle it:

1. Measure the existing first floor sill height above the ground floor ceiling level, from the survey rather than from the original drawings.
2. Set the roof slope on the section and check the clearance at the sill, with an allowance for the flashing upstand the covering requires.
3. Confirm the first floor room still has acceptable outlook, because a roof slope rising towards a bedroom window changes the view from it even where the light is adequate.

Where the clearance is tight, the usual resolutions are a shallower pitch over the extension, a reduced extension depth, or an inset where the slope steps around the window.

Where officers prefer a flat roof instead

Officers are not hostile to catslides, but they will ask for an alternative in three situations. Where the slope would obscure or crowd a first floor window. Where the resulting eaves at the boundary would overshadow a neighbour's ground floor window more than a flat roof would. And where the rear elevations of the terrace or the pair have a consistent character that the slope breaks.

A flat or shallow mono-pitch roof has compensating advantages worth putting in the submission: a consistent internal ceiling height, more usable floor area for the same footprint, and the option of a green roof, which brings its own rainwater and biodiversity benefits on a householder extension.

Front extensions face a different set of objections again, and the reasons they are harder than rear extensions rarely apply to a rear catslide.

Rainwater goods and the neighbour's wall

A low eaves at the boundary brings the gutter close to, and sometimes over, the boundary line. A gutter or fascia oversailing a neighbour's land is a trespass regardless of what the planning permission says, and discharging rainwater onto their land is a separate civil matter.

The fixes are straightforward if designed in. Set the wall face back far enough for the gutter to sit within your own land. Use a secret or concealed gutter behind the fascia line where the setback is tight. Or build to the boundary with a parapet or a dry verge detail that throws water back onto your own roof plane.

Discharge the rainwater to a soakaway or to the existing surface water system, following Approved Document H, and keep the soakaway the required distance from foundations. On a small garden with a deep extension, that distance is often the binding constraint.

An illustrative example

A 1930s semi with a 42 degree rear roof pitch proposes a 3 metre deep catslide extension. The slope reaches 2.15 metres at the rear eaves and 3.1 metres at the eaves adjoining the party boundary, which exceeds the 3 metre limit within 2 metres of the boundary, so a householder application is submitted. The kitchen run is placed along the low edge, giving 2.4 metres of headroom across the circulation zone. The rear wall is set back 180 millimetres from the boundary so the gutter sits within the site. This example is illustrative rather than a real approval.

Drawing the section that proves the headroom

One section does most of the work. Draw it through the deepest part of the extension, at a scale where the dimensions are legible, and put the following on it: existing and proposed ground levels, finished floor level, the roof pitch in degrees, the eaves height dimensioned from ground level, the overall height, the headroom at the rear eaves and at 1 metre in from it, the first floor sill with its clearance, and the boundary position with the 2 metre zone marked.

A short Planning Statement is then the place to explain why a continuous slope suits this house rather than a flat roof, which is the question an officer will be asking.

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