What Roof Pitch Does a Single-Storey Extension Need? UK Guide

• Lean-to (mono-pitch) rear extension: usually 15° to 22.5°, because the roof has to meet the house below the upstairs window sills.
• Dual-pitch (gable) extension: usually 25° to 35°, often matched to the main roof.
• Below about 10°: treat it as a flat roof with a membrane, not a tiled roof.

Takeaway: pick the pitch the house allows first, then pick a tile rated for it.

Every tile and slate has a manufacturer's minimum pitch, and BS 5534 (the UK code of practice for slating and tiling) requires the roof to be designed and fixed to suit it. These are Marley's published minimums:

These minimums are rafter pitches, the angle you set out on site. If your existing roof is plain tiles at 40° and the extension can only manage 17.5°, you can't match the tile. You'll need a low-pitch tile in a similar colour.

Takeaway: if matching the house roof matters, check the existing tile's minimum pitch before the architect fixes the extension height.

A lean-to has one slope running from the house wall down to the new outside wall. Its pitch depends on two heights and one distance:

• the eaves height on the outer wall
• the highest point the roof can meet the house wall, usually 150mm or more below the first-floor window sills to leave room for flashing
• the horizontal run between the two walls

A dual-pitch extension has a ridge in the middle and two slopes, like a small house. For the same eaves height it can be much steeper before it reaches the same ridge height, because each slope only covers half the width.

Rise per metre of run for common pitches:

Look at the last column. A 4 m wide gable extension with 2.5 m eaves goes over 4 m at 40°, but stays under at 35°. The lean-to column shows why lean-tos are shallow: a 3.5 m deep extension at 30° needs 2 m of height at the house wall, which most two-storey houses don't have below their bedroom windows.

You can convert any rise and run to an angle with our roof pitch calculator.

Takeaway: for a lean-to, measure the house wall first. That measurement usually decides the pitch.

Under permitted development in England, a single-storey extension must stay within these limits, according to the Planning Portal:

The 4 m limit is measured to the highest point of the roof, so allow for the rafters, battens and ridge tiles on top of your setting-out heights. If you need a steeper or taller roof than this allows, you'll need a full householder planning application. Wales, Scotland and Northern Ireland have their own rules, and designated land (conservation areas, AONBs, National Parks) is stricter.

Takeaway: if your extension is near the boundary, keep the eaves at 3 m or lower and work the pitch back from there.

Tom and Sarah want a 3.5 m deep rear kitchen extension on their semi-detached house, with Marley Modern tiles to match the main roof.

• Their first-floor window sills are 3.60 m above the finished floor. Allowing 150mm for the lead flashing, the roof can meet the house wall at 3.45 m.
• The builder wants 2.50 m eaves on the new outer wall.
• The run from the house wall to the outer wall is 3.50 m.

Step 1: rise. 3.45 − 2.50 = 0.95 m

Step 2: pitch. Pitch = arctan(rise ÷ run) = arctan(0.95 ÷ 3.50) = arctan(0.271) = 15.2°

Step 3: check the tile. Marley Modern needs 17.5°. At 15.2° it fails.

Step 4: options.

1. Lower the eaves. For 17.5° the rise must be 3.50 × tan(17.5°) = 1.10 m, so the eaves drop to 3.45 − 1.10 = 2.35 m. That leaves a low ceiling at the outer wall once you add the roof build-up and insulation.
2. Change the tile. Marley Wessex or Mendip are rated to 15°, so 15.2° works, but with almost no margin. Mendip 12.5 gives a 2.7° margin at the same pitch.
3. Change the roof. A flat roof with roof lights avoids the problem, at the cost of the matching tiles.

They choose option 2 with Marley Mendip 12.5 in a colour close to their existing tiles. The rafter length on the slope is √(3.50² + 0.95²) = 3.63 m before the overhang. Work it out with your own overhang in the rafter length calculator, then count tiles with the roof tile calculator.

Takeaway: run the numbers before you choose the tile. On lean-tos, the window sill height often rules out the tile you wanted.

Most guides tell you the minimum pitch for each tile and stop there. The bigger constraint on a lean-to is the abutment: where the new roof meets the house wall. You need room for a lead or proprietary flashing below the first-floor sills, and on a typical two-storey house that often leaves only 3.4 to 3.6 m. In practice, that's what forces a low-pitch tile on most UK lean-tos, not the budget or the look.

Ventilation changes at low pitches too. Standard contour eaves vents usually only work down to about 20°, so below that you'll need a dry-fix eaves vent system that's rated for the pitch.

Takeaway: measure the window sills and plan the flashing before you buy a single tile.

Even under permitted development, the roof still needs Building Regulations approval. The key parts:

• Approved Document A (Structure): rafter size, spacing and span must suit the pitch and the roof load. Roofs below 30° have to be designed for more snow load than steeper ones.
• Approved Document L (Conservation of fuel and power): the roof must meet the current U-value target, which affects how deep the insulation and rafters need to be.
• BS 5534: tiles, battens, underlay and fixings must suit the pitch and your site's wind exposure. Batten gauge changes with pitch, so check it with our roof batten spacing calculator.

To budget the whole build, use our extension cost calculator.

Takeaway: get the pitch, rafter size and insulation depth checked together. Changing one changes the other two.

It depends on the covering. Low-pitch concrete tiles such as Marley Mendip 12.5 go down to 12.5°, standard interlocking tiles to 15° to 17.5°, natural slate to 20° and plain tiles to 35°. Below about 10°, use a flat roof system instead.

Most tiled lean-tos are 15° to 22.5°. The limit is usually the height where the roof meets the house wall below the first-floor windows, not the tile.

Yes, on a dual-pitch roof. A 4 m wide gable with 2.5 m eaves reaches about 3.66 m at 30°, under the 4 m permitted development limit. A 3.5 m deep lean-to at 30° needs about 2 m of rise at the house wall, which is rarely available.

In England, up to 4 m to the highest point, with eaves no higher than 3 m if you're within 2 m of a boundary. Designated land and the other UK nations have different rules.

Divide the rise by the run and take the arctan. For example, a 0.95 m rise over 3.5 m gives arctan(0.271) = 15.2°. Our roof pitch calculator does it from millimetres.

Only if the extension can reach the tile's minimum pitch. Plain tiles need 35°, so a shallow lean-to usually needs a low-pitch tile in a similar colour instead.

This guide is general information based on manufacturer data, BS 5534 and Planning Portal guidance as of October 2026. It's not a substitute for advice from an architect, structural engineer or your local building control. Always confirm the minimum pitch with the tile manufacturer and check planning rules for your property before you build.