Eaves, ridge heights and the 2 metre boundary rule

Three height limits govern most single storey rear extensions built under permitted development: an overall maximum of 4 metres, eaves no higher than the eaves of the existing house, and eaves no higher than 3 metres where any part of the extension sits within 2 metres of the curtilage boundary. That last one is the extension height boundary rule, and it is the limit that most often turns a permitted development scheme into a planning application.

Height sounds like the easy part of Class A. It is not, because the answer depends entirely on where you measure from, and the ground under most extensions is not level.

Where heights are measured from

Part 1 of Schedule 2 to the general permitted development order defines how height is measured. It is taken from ground level, meaning the level of the surface of the ground immediately next to the building. Where that surface is not uniform, it is measured from the highest part of the ground next to the building.

Read that twice, because it works in the applicant's favour on a sloping site. If the ground falls away from the house across the width of the extension, you measure from the high side, which gives a smaller number than measuring from the low side would.

It also means you cannot improve the position by digging out. Lowering the ground on the high side does not lower the measured height, because the measurement still comes from the highest part of the surface next to the building.

State on your drawings which point you measured from, with a level reference. An officer looking at a section with a single dimension and no datum has to assume the worst case.

The 3 metre eaves cap within 2 metres of a boundary

The rule is specific: where the enlarged part of the house is within 2 metres of the boundary of the curtilage, the height of the eaves of the enlarged part must not exceed 3 metres.

Two points decide whether it bites.

The 2 metres is measured to the boundary of the curtilage, which is the legal edge of your property, not the face of a fence panel that may have been moved. On a terrace or a narrow semi, an extension running the full width will almost always be within 2 metres of at least one side boundary.

And the cap applies to the whole of the enlarged part rather than the bit near the boundary alone. If any part of the extension is within 2 metres of the boundary, the eaves height limit applies to the enlarged part. Designing a stepped eaves line to keep a higher section away from the boundary is possible, and it needs to be drawn precisely rather than described.

Eaves height is measured to the point where the wall meets the roof. On a flat roof that means the top of the external wall, and a parapet above it raises the measurement. Designers reach for a parapet to hide a roof outlet and lose the permitted development position in the process.

Overall height against the existing house

Two separate limits apply to the overall height and they are easy to conflate.

A single storey rear extension must not exceed 4 metres in height. That is an absolute cap regardless of how tall the house is.

Separately, no part of the extension may be higher than the highest part of the roof of the existing house, and the eaves of the extension may not be higher than the eaves of the existing house. On a bungalow with a low eaves line, that second limit is tighter than the 4 metre cap and it is the one that catches people who have designed to the headline figure.

Sloping sites and the highest ground level

On a site that falls from front to back, the useful question is what counts as the ground next to the building.

Take a house where the garden drops 600 millimetres from the rear wall to the end of a proposed 4 metre extension. The ground next to the extension is not at one level, so the measurement is taken from the highest part of that surface, which is at the house. A roof that clears 4 metres from the low end may still be within the limit.

That is a favourable reading, and it is also the reading the legislation supports. What it does not permit is measuring from a point that is not next to the building, or from a level created by the works themselves.

Where the fall is significant, commission a level survey rather than estimating. A spot height taken from a drain cover and a tape measure is not evidence, and if there is any doubt about the position, a lawful development certificate settles it before you build rather than after a complaint.

Flat roofs, parapets and plant

Flat roofed extensions produce most of the disputes, because the roof build-up is where heights get lost.

A firring, an insulation layer, a membrane and a paving system on top add up. A design that sits at 3 metres to the underside of the structure can arrive at 3.2 metres to the finished surface, and the eaves limit is not measured to the structural slab.

Parapets, upstands and guarding all count as part of the enlarged part of the house. So does an access hatch. Roof-mounted equipment is a different question and the definition of what is included in a height measurement is in the interpretation provisions of Part 1, so check it rather than assuming a flue or an outlet is ignored.

Rooflights on a flat roof are usually fine within the overall height. A raised lantern is often what pushes a scheme over 4 metres.

Retaining walls and made-up levels

Raising the ground next to an extension to gain height is a well-worn idea and it does not work.

Building up levels with retained fill is itself an engineering operation, and an engineering operation is development. It may be permitted under other rights, or it may not, but you cannot rely on a level you created to justify a height measurement under Class A. The measurement is of the surface of the ground next to the building, and an officer will ask what it was before.

Retaining walls bring their own issues. A structure over a certain height near a boundary is a separate question from the extension, a wall retaining soil against a neighbour's land may engage the Party Wall etc. Act 1996, and surcharging a neighbour's boundary wall is a civil matter you will hear about quickly.

Where an earlier extension has already used part of the allowance, the position is different again, and that is covered in extending a house that has already been extended.

Drawing the section so the limits are provable

The submission that never gets queried has one thing in common: a section that proves each limit rather than asserting it.

1. A long section through the extension showing existing and proposed ground levels with a datum, and the finished floor level.
2. Dimensions to the eaves and to the highest point of the roof, taken from the stated ground level.
3. A cross section at the point nearest the boundary, showing the distance to the curtilage boundary and the eaves height at that point.
4. A site plan with the curtilage boundary drawn from the title plan, not from the fence position.
5. A roof build-up note giving the finished level rather than the structural level.

Where the appearance of the house is also changing, materials and cladding have their own rules, set out in rendering or cladding a house.

If you want certainty, a lawful development certificate is the only formal way to confirm that works are lawful. A short Planning Statement setting out which class you rely on and how each limit is met turns a set of drawings into an application an officer can approve quickly.

Enter your postcode on PlanScope to see which documents an extension application 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.