How Deep Can a Single Storey Extension Be?
The limits, where they are measured from, and the word that catches almost everybody out.
Under permitted development a single storey rear extension can normally project 3m from the original rear wall on a semi or terrace and 4m on a detached house, doubling to 6m and 8m through the Larger Home Extension prior approval route. Height is capped, and depth is measured from the ORIGINAL rear wall, not the wall you have today.
The depth you are allowed depends on three things: whether the house is detached, whether you use the prior approval route, and what the house looked like when it was originally built. The third one is where most people go wrong.
The standard permitted development limits
Under the permitted development rules that govern extensions, a single storey rear extension may normally project from the original rear wall by:
- 4 metres for a detached house
- 3 metres for a semi detached or terraced house
Alongside the depth limit, the overall height is capped at 4 metres. Where any part of the extension comes within 2 metres of a boundary, the eaves height is capped lower, at 3 metres. On a narrow plot that eaves limit often dictates the roof design more than the depth limit does.
Going deeper, through prior approval
The Larger Home Extension scheme roughly doubles those depths:
- 8 metres for a detached house
- 6 metres for a semi detached or terraced house
This is not automatic and it is not a planning application either. You notify the council, the council writes to the adjoining neighbours, and they have a period in which to object. If nobody objects you may proceed. If somebody does, the council decides whether the impact on their amenity is acceptable.
The process runs to 42 days. The council may only consider the effect on neighbouring amenity, not the full range of planning matters, which is why it succeeds more often than a full application would. But a neighbour with a genuine loss of daylight complaint can stop it.
Measured from where, exactly
Depth is measured from the original rear wall: the house as it was built, or as it stood on 1 July 1948 if it is older than that. Not from the wall you have today.
If a previous owner added a 2 metre extension in the 1990s and you own a semi, you have 1 metre of permitted development depth left, not 3. The allowance belongs to the house rather than to the owner, and it does not reset when the property is sold.
This is the single most common reason a project that “definitely does not need permission” turns out to need one. It is checked against the planning history for the address, not by measuring the house as it stands.
The other conditions that apply at the same time
Depth is only one condition. All of the following must also be satisfied, and missing any one of them means the whole extension needs a planning application:
- The extension must not cover more than half the area of land around the original house, counting any previous extensions and outbuildings.
- It must not extend beyond a wall forming the principal elevation of the original house. Separately, it must not extend beyond a wall that fronts a highway and forms a side elevation. Only the second limb mentions a highway, and the two get merged constantly.
- Materials must be similar in appearance to the existing house.
- It must be single storey to qualify for these depths. Two storey extensions have their own, shallower limits and a minimum distance from the rear boundary.
- No verandas, balconies or raised platforms.
When none of this applies to you
Permitted development rights do not exist at all for flats and maisonettes. Listing works differently, and it is widely got wrong: it does not remove permitted development, it adds listed building consent as a separate requirement under separate legislation. Rights are reduced in conservation areas, and an Article 4 direction can remove them for a defined area.
They may also have been removed by a condition on the original planning permission, which is very common on estate housing built from the 1980s onwards. Almost nobody checks this, and it is worth ten minutes with the council’s planning search.
Separately, a restrictive covenant on your title is private law rather than planning. The council will not check it and will grant permission for something your deeds forbid.
What the ground allows is a different question
Planning tells you how deep you are permitted to build. It says nothing about how deep the foundations need to go, which around Colchester is usually the bigger cost question.
Most of the ground here is shrinkable clay, and near a mature tree the foundation may need to go to two metres or more, or move to piles. A 6 metre extension approved under prior approval and founded too shallow will still crack away from the house in its second summer.
Is deeper actually better?
Not always, and this is worth saying because the prior approval route tempts people into building the maximum they are allowed rather than the amount they need.
A very deep single storey extension across the full width of a house produces a room with a large area of floor a long way from any window. Daylight falls off quickly with distance from a glazed opening, so the far end of an 8 metre extension is dim unless the roof is doing serious work with rooflights. It also swallows garden, and garden is the thing people miss afterwards.
The other cost of depth is what it does to the rooms you already have. Extend deeply across the back of a terrace or a narrow semi and the old middle room loses its only source of daylight entirely, becoming a corridor between the front room and the new one. That is the most common regret we see, and it is designed out at drawing stage with a light well, a set back, or rooflights over the line of the old external wall.
A well designed 3 metre extension with light from above frequently produces a better house than a 6 metre one that used every millimetre it was permitted. The question worth asking is not how deep you are allowed to go, but how much room you actually need and where the light is coming from.
The practical answer
On a typical Colchester semi, 3 metres without any notification, or 6 metres through the prior approval route, provided the house has not been extended before and nothing removes your rights. On a detached house, 4 metres or 8 metres on the same basis.
Before designing to those numbers, check the planning history, the designation and the deeds. We do all three as the first step on every job, because the answer decides the design 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.
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