How High Can a Single Storey Extension Be?

Four metres. Under Class A of the permitted development order a single storey extension must not exceed four metres in height, and where any part of it comes within two metres of a boundary the eaves must not exceed three metres.

Unlike depth, that figure does not move with the house type. A detached house is allowed a deeper extension than a semi or a terrace, but it is not allowed a taller one. Four metres is four metres on all of them, and on most Colchester plots it is the eaves figure rather than the four that ends up designing your roof.

Where the four metres is measured from

Not from your floor. From the ground outside, and where the ground is not level, from the highest part of it.

The Order measures the height of a building from ground level, meaning the surface immediately next to it, and where that ground is not uniform it takes the highest part adjacent to the building. On a garden that falls away, measuring from the low end flatters your drawing and is not how the council reads it.

The commoner mistake is quieter. Drawings are usually dimensioned from finished floor level, and the finished floor of an extension sits above the ground outside it: the damp proof course wants to be around 150mm clear of the finished ground, and the slab, insulation and screed build up from there. Design a building four metres tall from your new floor and you have built something over four metres tall from the ground.

Two ceilings above the four metres

Class A imposes two further caps that have nothing to do with the number everybody quotes, and on a low house they bite first.

• The enlarged part must not be higher than the highest part of the roof of the existing house.
• Its eaves must not be higher than the eaves of the existing house.

Read the second one again if you own a bungalow. Bungalow eaves are often somewhere around two and a half metres, so a bungalow extension is capped at that, well under the three metre eaves figure and nowhere near four. The headline number is simply not the constraint. There is more on what that does to a single storey house in our guide to extending a bungalow.

Note the word existing rather than original. Depth is measured from the original house, which is what catches people who bought a house somebody else had already extended. Height is measured against the house as it stands today. The two conditions use different words deliberately, and they are not interchangeable.

The eaves rule, and what a boundary does to your ceiling

Three metres to the eaves within two metres of a boundary is the condition that decides more extension roofs around here than any other.

It applies to a boundary, not to the side boundary. A short garden brings the rear boundary into play just as a tight side passage brings the neighbour’s fence into play. And it is written against the enlarged part as a whole, so bringing one corner of the extension within two metres is enough to engage it.

The consequence depends entirely on your roof. Eaves height is taken where the external wall meets the roof, so on a flat roof the eaves and the top of the wall are effectively the same line. A flat roofed extension within two metres of a boundary is therefore capped at roughly three metres overall, not four. The extra metre exists only for a roof with a slope on it, where the eaves can sit at three metres and the ridge climb toward four further from the boundary.

Nothing in the Building Regulations sets a minimum ceiling height for a habitable room in England, so 2.3 metres is lawful. It is also noticeably low across a wide room with a large area of glass at one end, and it is the sort of thing nobody notices on a drawing and everybody notices in the finished room.

The usual way out is a mono pitch falling to the boundary: eaves held at three metres on the neighbour’s side, the roof rising away from them, and the ceiling opened up to follow it. You get the height where you are standing and the neighbour gets the lower edge.

The prior approval route does not buy you height

The Larger Home Extension prior approval scheme relaxes one thing only, which is depth. The four metre cap applies unchanged, and so does the three metre eaves rule near a boundary. A deeper extension under prior approval is a longer building rather than a taller one, which is why a long extension needs its roof working hard to get daylight into the far end.

A single storey side extension is capped at four metres in the same way and carries a width condition of its own, set out on whether a side extension needs planning permission. Anything of more than one storey leaves these conditions entirely and picks up its own, including a minimum distance from the rear boundary.

What height does when you have to apply

If the extension falls outside permitted development and goes in as a householder application, height stops being a pass or fail number and becomes a judgement about your neighbour’s daylight. Officers test it with sectional rules of thumb, covered in our guide to the 45 and 25 degree rules.

The practical point is that height, not depth, is what makes an extension feel overbearing from next door. A three metre eaves line on the boundary reads as a garden wall with something behind it. Four metres of blank flank wall on the same line reads as a building, and that is the one that draws an objection.

The practical answer

Four metres overall, three metres to the eaves within two metres of any boundary, and never higher than the ridge or the eaves of the house you already have. Measured from the ground outside, at its highest point, rather than from your new floor.

Where a project fits, applying for a lawful development certificate afterwards is cheap and gives you the piece of paper a buyer’s solicitor will want. We check the height, the eaves line and the planning history before a single storey rear or side extension gets drawn, because the roof form falls out of that answer rather than the other way round.

These limits change from time to time and how they apply depends on your specific property. Treat this as an explanation of the system rather than confirmation of what you can build.

Last reviewed August 2026.