Planning Guide · Single Storey Extension Planning Permission
Single storey extension planning permission in London: which limit applies to yours
Single storey extension planning permission is often not needed, and the schemes that go wrong are usually built a few centimetres over the line. The limits depend on which wall you build from: the rear wall, a side wall, or both. This guide works through each case with worked dimensions. It shows where conservation land and earlier extensions change the answer. It also uses council data to show how often London boroughs refuse the larger schemes.
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Do you need single storey extension planning permission?
Usually it does not, when it is built to the limits in Class A of Part 1 of Schedule 2 to the 2015 General Permitted Development Order. The Planning Portal states that single storey rear extensions cannot exceed four metres in height. They are limited to 4 metres in depth for detached houses and 3 metres for other houses. Prior approval is available to go further. All the rules apply to houses only, not to flats or maisonettes.
The word that causes trouble is original. The Order measures from the house as it stood on 1 July 1948, or as built if later. A previous extension, whether built under permission or under these rights, counts towards the totals when a new one joins it. Paragraph A.1(ja) adds the proposed work to the earlier work and tests the total against the same limits.
Everything below tests your own scheme against those rules. If the answer is no, the fall-back is a householder planning application, which the council decides on design and neighbour amenity instead.
What are the Class A limits for a single storey extension?
The limits on a single storey extension change with the type of house, the wall you build from and the land the house stands on. The table below shows each case in one place.
Part of the house
Normal limit
On article 2(3) land such as a conservation area
Rear extension, detached house
4m beyond the rear wall; 8m with prior approval
4m, and no prior approval route
Rear extension, any other house
3m beyond the rear wall; 6m with prior approval
3m, and no prior approval route
Side extension
One storey, up to 4m high, no wider than half the original house
Not permitted: a side extension needs planning permission
Height of the extension
4m at the highest point; not higher than the existing roof or eaves
Same
Eaves within 2m of a boundary
3m maximum
Same
Garden coverage
Buildings other than the original house on no more than 50% of the curtilage
Same
Materials
Exterior of a similar appearance to the existing house
Also no cladding in stone, render, pebble dash, timber, plastic or tiles
Sources: Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class A, paragraphs A.1, A.2 and A.3 (latest revised version, read 13 October 2026); Planning Portal, Extensions: planning permission. The detached depth limits are 4m and 8m; other houses are 3m and 6m.
How do the limits work on a side or wraparound extension?
A side extension is one that extends beyond a wall forming a side elevation of the original house. It must be a single storey, no higher than 4m, and no wider than half the width of the original house, measured at its widest point. No enlargement may extend beyond the principal elevation of the original house, and on a corner plot none may extend beyond a side wall that fronts a highway.
An extension that fills the gap between a side wall and the rear wall is a combined, or wraparound, extension. The government's technical guidance says both sets of limits then apply: the rear projection limit and the half-width side limit. A scheme can be under half the house width and only 3m deep at one rear wall, and still fail because it runs more than 6m beyond the other rear wall.
Here is a worked example with assumed dimensions. A semi-detached house is 6m wide at its widest point. The side part can be no wider than 3m, and the L-shaped extension can go no deeper than 3m beyond any rear wall it extends beyond without prior approval, or 6m with it. Measure from the base of the original rear wall, not from a later addition.
What does neighbour prior approval add for a deeper single storey extension?
Prior approval lets a single storey rear extension run to 6m, or 8m for a detached house, if the council does not stop it. The owner gives the council a written description, a site plan and the addresses of adjoining premises. The council serves notice on adjoining owners and occupiers, who have at least 21 days to respond. If any of them objects, prior approval is required as to the impact of the extension on the amenity of any adjoining premises, and the council considers all adjoining premises. The scheme may begin if 42 days pass from receipt without a decision. The route does not apply on article 2(3) land or on sites of special scientific interest.
21 days Neighbour notice period
42 days Decision period
8m Detached house depth
How often do London boroughs refuse large single storey extensions?
Among the 24 London boroughs with a published rate, the share of large home extension prior approvals refused in 2025-26 ran from 8.3% to 48.1%, so the same 6m rear extension has very different odds depending on the borough.
Borough
Prior approval decisions
Refused
Share refused
Greenwich
158
76
48.1%
Kingston upon Thames
77
35
45.5%
Lewisham
103
46
44.7%
Barnet
282
103
36.5%
Harrow
438
108
24.7%
Redbridge
521
93
17.9%
Hillingdon
151
13
8.6%
Bexley
96
8
8.3%
Large householder extension prior approval decisions, financial year 2025-26 (2025 Q2 to 2026 Q1). Source: MHCLG district planning application statistics, tables PS1 and PS2, Open Government Licence v3.0, derived in scripts/aggregate/ps1_ps2.py and held in content/data/borough-planning-stats.json. Boroughs with too few decisions for a published rate are omitted. The figures cover all large extension prior approvals, not only single storey ones, and say nothing about why each was refused.
What does that spread mean for your scheme?
The statistics cannot say whether a high refusal share reflects the schemes submitted or how the borough reads amenity, so treat the figures as a warning rather than a verdict. Greenwich refused 76 of 158 decisions, close to half, Lewisham 46 of 103, while Bexley refused 8 of 96. A high share is a warning to take pre-application advice, not proof of how the council reads amenity.
The test is the same everywhere: the impact on the amenity of any adjoining premises, and an objection from one neighbour is enough to bring the council's assessment in. Sections through the boundary, window positions and accurate levels are what answer it. Our guide to whether you need planning permission for a rear extension sets out the three routes in one place.
Can a borough's design guide be stricter than permitted development?
Yes, once you need a planning application. Permitted development sets what you can build without asking. A householder application is judged against the local plan and any design guidance, which is often tighter. Kingston upon Thames' Residential Design SPD, adopted in July 2013, says single storey rear extensions should generally not exceed 3m to 3.5m in depth. At least 50% of the rear garden should remain. A flat roof should not exceed 3m in height, a parapet roof 3.3m and a pitched roof 4m.
The same guidance says side extensions should in general be no more than 50% of the width of the original house, and should normally be set back at least 500mm from the front wall and set in from the boundary. A 6m extension that could proceed through prior approval may therefore draw an objection if the council is asked to approve it in full.
What other rules apply even when planning is not needed?
A permitted extension is still subject to separate regimes, and each has its own approval.
Building regulations
Only a conservatory, porch, covered yard, covered way or open carport at ground level up to 30m² is exempt under Class 7 of Schedule 2 to the Building Regulations 2010. A kitchen extension is not in that list.
Party Wall etc. Act 1996
Planning permission and party wall notices are separate regimes. Section 6 requires notice at least one month before excavating within 3m of a neighbouring building below its foundations, or within 6m on a 45 degree line.
Conditions on earlier permissions
Article 3(4) of the Order means a right cannot be used contrary to a condition on an earlier permission, so read the decision notice for the house.
Previous extensions
Paragraph A.1(ja) adds new work to any earlier enlargement it joins, so an existing deep extension can use up your allowance.
How do you check a single storey extension against the limits?
Work through the checks in this order, because an early failure usually settles the route.
1
Confirm it is a house and find the original
Flats and maisonettes have no rights. For a house, establish what stood on 1 July 1948, or the house as built.
2
Check the land
Conservation areas and other article 2(3) land lose side extensions, cap rear projection at 3m or 4m and ban the listed cladding. An Article 4 direction can remove Class A as well.
3
Measure from each wall you extend beyond
Rear projection runs from the base of the original rear wall, and the half-width limit uses the widest point of the original house.
4
Test height and eaves
No more than 4m at the highest point, eaves no more than 3m within 2m of a boundary, and nothing above the existing roof or eaves.
5
Add earlier extensions
Combine the proposal with any existing enlargement it joins, and retest the totals.
6
Choose the route and the paperwork
Permitted development with a lawful development certificate, prior approval for 6m or 8m, or a householder application. Run building control alongside.
How do you settle single storey extension planning permission quickly?
Measure the original house, find out whether the land is article 2(3) land, and test each wall you extend beyond. Those three steps settle single storey extension planning permission for most houses before any design work starts.
If the numbers sit close to a limit, send us the address and your sketch dimensions. We check the constraints for free, and tell you whether permitted development, prior approval or a full application is the safer route.
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Frequently asked questions
Do I need single storey extension planning permission?
Often not. A single storey extension to a house is permitted development if it meets the limits in Class A of Part 1 of the 2015 Order. It must be no more than 4m high and 3m deep (4m if detached) beyond the rear wall. Eaves must be 3m or less within 2m of a boundary. Conservation areas, flats and houses with removed rights need an application.
How high can a single storey extension be without planning permission?
The Planning Portal gives four metres as the highest a single storey rear extension can be. Where any part is within 2m of the boundary of the curtilage, the eaves can be no higher than 3m. The extension must also not exceed the height of the highest part of the existing roof, or the eaves of the existing house, so a bungalow often has less headroom than the 4m headline.
How wide can a single storey side extension be?
No wider than half the width of the original house, measured at its widest point, and no higher than 4m, in one storey. The width limit counts any earlier side enlargement the new one joins. On article 2(3) land, which includes conservation areas, a side extension is not permitted development at all and needs planning permission whatever its size.
Is a wraparound extension permitted development?
Sometimes. The government's technical guidance says that where an extension fills the area between a side elevation and a rear wall, both the rear and the side restrictions apply. It must be one storey, no higher than 4m, project no further than the rear limit and be no wider than half the original house. Most wraparounds fail one of these limits, so many are built under planning permission.
What is the 6m rule for a single storey rear extension?
It lets a single storey rear extension on a house that is not detached extend up to 6m beyond the rear wall, or 8m for a detached house, if the neighbour consultation scheme in paragraph A.4 is followed. Adjoining owners get at least 21 days to object, and the council has 42 days to respond. It does not apply on conservation land or sites of special scientific interest.
Do I need building regulations approval for a single storey extension?
Yes in almost every case. The exemption in Class 7 of Schedule 2 to the Building Regulations 2010 covers a conservatory, porch, covered yard, covered way or open carport at ground level with a floor area up to 30m². A kitchen or living extension is not in that list. Building regulations approval is separate from planning and covers structure, drainage, ventilation and energy.
Can I extend a house that already has an extension?
Possibly, but the old and new work are added together. Paragraph A.1(ja) of Class A says the total enlargement, being the proposed work and any existing enlargement it joins, must stay within the same limits on height, projection, eaves and side width. A previous deep extension may already use your allowance, so a lawful development certificate is a sensible check.
Does a conservation area stop single storey extensions?
It tightens the rules. On article 2(3) land, which includes conservation areas, a side extension needs planning permission, a rear extension is limited to 3m, or 4m for a detached house, and the prior approval route is not available. Cladding the exterior in stone, render, pebble dash, timber, plastic or tiles is not permitted either. The council can still grant a full application.
Do I need to tell my neighbours about a single storey extension?
For a 6m or 8m prior approval scheme the council notifies adjoining owners and occupiers, and an objection brings in an amenity assessment. For anything else, planning law does not require you to tell them, but the Party Wall etc. Act 1996 can. Building on the boundary or excavating within 3m below a neighbour's foundations, or within 6m on a 45 degree line, needs notice.
What happens if my single storey extension is built over the limit?
It stops being permitted development, so it needs planning permission and may be unauthorised until it gets one. The council can enforce. For operational development substantially completed on or after 25 April 2024 the limit in England is ten years under section 171B of the 1990 Act, and works finished before that date stay on the older four-year limit. Applying for permission retrospectively is possible but risky, so measure against the Class A limits before you pour the foundations.
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