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Side extension rules: width, outriggers and wraparounds
A side extension can be permitted development under Class A, but only if the complete proposal meets the rules. The main side limits are one storey, no more than 4 m in height, and a width no greater than half the width of the original house.
There is a less obvious trap: an extension beside an original rear outrigger can be a side extension even when it stays inside the overall width of the main house. We explain that below.
Need help with permitted development?
Permitted development can avoid a full planning application, but only if the right applies and every relevant limit and condition is met. Planning Geek can check the planning history, Article 4 directions and other restrictions, and help with a lawful development certificate or prior approval where needed.
Which wall is a side elevation?
Start with the original house: as it stood on 1st July 1948, or as first built if later. Government guidance describes a side-elevation wall as one which cannot be identified as a front or rear wall. A house can have more than two such walls.
The long wall along an original outrigger can be a side wall. It does not cease to be a side wall because the main body of the house extends farther across the plot. If the proposal projects beyond that wall, the side-extension rules can apply.
In this example the original house is 8 m wide. The 2 m wide infill is below the 4 m half-width limit, and its 3 m rear projection meets the ordinary rear-depth limit. This illustrates those size tests only. Height, boundaries, protected land and every other relevant requirement still need checking.
How the half-width rule works
Under A.1(j), an enlargement extending beyond an original side-elevation wall must not be wider than half the original house. Government guidance measures the original house at its widest point. Do not use the width of the plot or add a later extension to the original width.
The side enlargement may be exactly half that original width, but it must not exceed it. The same paragraph also limits it to a single storey and a maximum overall height of 4 m.
Why a full-width wraparound often fails
Once a proposal extends beyond an original side wall, the width test applies to the enlargement as a whole. It is not enough to measure a slim strip beside the outrigger and treat a connected rear section as unrelated.
For an 8 m wide original house, a single full-width wraparound enlargement measuring 8 m across exceeds the 4 m allowance. Keeping the work behind the house does not rescue it. Nor does using the larger 6 m or 8 m rear-extension route: that route increases the rear depth allowance, not the side width allowance.
Wraparound is a description of shape, not a separate GPDO class. Some limited side-and-rear combinations can comply; many full-width schemes cannot. Use our rear extension guide to check all relevant original rear walls too.
What if a side extension already exists?
If new work will join an existing enlargement, A.1(ja) applies the relevant limits to the total enlargement: existing and proposed work together. A side extension which is lawful on its own can therefore cause a new joined rear extension to fail.
For example, join a full-width rear extension to a 3 m side extension on an 8 m wide original house and the resulting enlargement can be 11 m across. That is greater than half the original house width. Separate applications or building the work in stages do not avoid the joined-enlargement rule.
Genuinely separate enlargements on opposite sides of a house may each satisfy Class A. But do not assume that leaving a small gap necessarily solves every issue: the actual development and every remaining limitation must be assessed.
Height and boundaries
- A side enlargement must be single-storey and no higher than 4 m overall
- It must not exceed the highest part of the existing house’s roof or the existing house’s eaves height
- Where any part is within 2 m of a curtilage boundary, the enlargement’s eaves must not exceed 3 m
Eaves are measured at the meeting of the external wall and the upper roof surface, not at the gutter. A flat-roof parapet can count in the overall height. The 2 m rule is an eaves restriction, not a universal requirement for a 2 m gap.
Side extensions on protected land or beside a highway
On article 2(3) land, Class A does not permit an enlargement beyond an original side-elevation wall. That includes infilling beside an original outrigger. Conservation areas, National Parks, the Broads, National Landscapes and World Heritage Sites are included in this designation.
Elsewhere, Class A still excludes an enlargement beyond an original side wall that fronts a highway. Corner plots need care. Government guidance considers matters such as angle, distance and intervening land when deciding whether an elevation fronts a highway.
Check the rear depth as well
If the side enlargement also goes beyond an original rear wall, the rear-extension limits apply too. Ordinary single-storey limits are 4 m for a detached house and 3 m for any other house. A qualifying deeper proposal may use the larger home extension procedure, which must be followed before starting.
A side extension can engage the rear-depth limit even if it is not physically attached to the rear wall. Do not assess the wall it touches and ignore the walls it projects beyond.
Before relying on permitted development
Use the main Class A checklist for dwellinghouse eligibility, excluded conversion classes, Part 20 houses, planning conditions, Article 4 directions, the 50% ground-coverage rule, materials and excluded features. Listed building consent, Building Regulations and other property rights are separate matters.
There is no prior approval procedure simply for an ordinary side extension. Where the original walls, combined width or planning history are uncertain, a Lawful Development Certificate can give a formal decision on the proposal before you build.
Sources and Class A legislation
Current GPDO Part 1 and article 2 definitions provide the statutory wording. The government householder technical guidance explains measurements and examples, but must be read alongside later legislative amendments.
Read the Class A legislation
Class A – enlargement, improvement or other alteration of a dwellinghouse
Permitted Development
A. The enlargement, improvement or other alteration of a dwellinghouse.
Development not permitted
A.1 Development is not permitted by Class A if—
(a)permission to use the dwellinghouse as a dwellinghouse has been granted only by virtue of Class G, M, MA, N, P , PA or Q of Part 3 of this Schedule (changes of use);
(b)as a result of the works, the total area of ground covered by buildings within the curtilage of the dwellinghouse (other than the original dwellinghouse) would exceed 50% of the total area of the curtilage (excluding the ground area of the original dwellinghouse);
(c)the height of the part of the dwellinghouse enlarged, improved or altered would exceed the height of the highest part of the roof of the existing dwellinghouse;
(d)the height of the eaves of the part of the dwellinghouse enlarged, improved or altered would exceed the height of the eaves of the existing dwellinghouse;
(e)the enlarged part of the dwellinghouse would extend beyond a wall which—
(i)forms the principal elevation of the original dwellinghouse; or
(ii)fronts a highway and forms a side elevation of the original dwellinghouse;
(f)subject to paragraph (g), the enlarged part of the dwellinghouse would have a single storey and—
(i)extend beyond the rear wall of the original dwellinghouse by more than 4 metres in the case of a detached dwellinghouse, or 3 metres in the case of any other dwellinghouse, or
(ii)exceed 4 metres in height;
(g)for a dwellinghouse not on article 2(3) land nor on a site of special scientific interest, the enlarged part of the dwellinghouse would have a single storey and—
(i)extend beyond the rear wall of the original dwellinghouse by more than 8 metres in the case of a detached dwellinghouse, or 6 metres in the case of any other dwellinghouse, or
(ii)exceed 4 metres in height;
(h)the enlarged part of the dwellinghouse would have more than a single storey and—
(i)extend beyond the rear wall of the original dwellinghouse by more than 3 metres, or
(ii)be within 7 metres of any boundary of the curtilage of the dwellinghouse being enlarged which is opposite the rear wall of that dwellinghouse;
(i)the enlarged part of the dwellinghouse would be within 2 metres of the boundary of the curtilage of the dwellinghouse, and the height of the eaves of the enlarged part would exceed 3 metres;
(j)the enlarged part of the dwellinghouse would extend beyond a wall forming a side elevation of the original dwellinghouse, and would—
(i)exceed 4 metres in height,
(ii)have more than a single storey, or
(iii)have a width greater than half the width of the original dwellinghouse;
(ja)any total enlargement (being the enlarged part together with any existing enlargement of the original dwellinghouse to which it will be joined) exceeds or would exceed the limits set out in sub-paragraphs (e) to (j);
(k)it would consist of or include—
(i)the construction or provision of a verandah, balcony or raised platform,
(ii)the installation, alteration or replacement of a microwave antenna,
(iii)the installation, alteration or replacement of a chimney, flue or soil and vent pipe, or
(iv)an alteration to any part of the roof of the dwellinghouse ; or
(l)the dwellinghouse is built under Part 20 of this Schedule (construction of new dwellinghouses).
A.2 In the case of a dwellinghouse on article 2(3) land, development is not permitted by Class A if—
(a)it would consist of or include the cladding of any part of the exterior of the dwellinghouse with stone, artificial stone, pebble dash, render, timber, plastic or tiles;
(b)the enlarged part of the dwellinghouse would extend beyond a wall forming a side elevation of the original dwellinghouse; or
(c)the enlarged part of the dwellinghouse would have more than a single storey and extend beyond the rear wall of the original dwellinghouse;
(d)any total enlargement (being the enlarged part together with any existing enlargement of the original dwellinghouse to which it will be joined) exceeds or would exceed the limits set out in sub-paragraphs (b) and (c).
Conditions
A.3 Development is permitted by Class A subject to the following conditions—
(a)the materials used in any exterior work (other than materials used in the construction of a conservatory) must be of a similar appearance to those used in the construction of the exterior of the existing dwellinghouse;
(b)any upper-floor window located in a wall or roof slope forming a side elevation of the dwellinghouse must be—
(i)obscure-glazed, and
(ii)non-opening unless the parts of the window which can be opened are more than 1.7 metres above the floor of the room in which the window is installed; and
(c)where the enlarged part of the dwellinghouse has more than a single storey, or forms an upper storey on an existing enlargement of the original dwellinghouse, the roof pitch of the enlarged part must, so far as practicable, be the same as the roof pitch of the original dwellinghouse.
A.4—(1) The following conditions apply to development permitted by Class A which exceeds the limits in paragraph A.1(f) but is allowed by paragraph A.1(g).
(2) Before beginning the development the developer must provide the following information to the local planning authority—
(a)a written description of the proposed development including—
(i)how far the enlarged part of the dwellinghouse extends beyond the rear wall of the original dwellinghouse;
(ii)the maximum height of the enlarged part of the dwellinghouse; and
(iii)the height of the eaves of the enlarged part of the dwellinghouse;
(iv)where the enlarged part will be joined to an existing enlargement of the dwellinghouse, the information in sub-paragraphs (i) to (iii) must be provided in respect of the total enlargement (being the enlarged part together with the existing enlargement to which it will be joined);
(b)a plan indicating the site and showing the proposed development and any existing enlargement of the original dwellinghouse to which the enlarged part will be joined;
(c)the addresses of any adjoining premises;
(d)the developer's contact address; and
(e)the developer's email address if the developer is content to receive communications electronically,
together with any fee required to be paid.
(3) The local planning authority may refuse an application where, in the opinion of the authority—
(a)the proposed development does not comply with, or
(b)the developer has provided insufficient information to enable the authority to establish whether the proposed development complies with,
the conditions, limitations or restrictions applicable to development permitted by Class A which exceeds the limits in paragraph A.1(f) but is allowed by paragraph A.1(g).
(4) Sub-paragraphs (5) to (7) and (9) do not apply where a local planning authority refuses an application under sub-paragraph (3) and for the purposes of section 78 (appeals) of the Act such a refusal is to be treated as a refusal of an application for approval.
(5) The local planning authority must notify each adjoining owner or occupier about the proposed development by serving on them a notice which—
(a)describes the development by setting out the information provided to the authority by the developer under paragraph A.4(2)(a);
(b)provides the address of the proposed development;
(c)specifies the date when the information referred to in sub-paragraph (2) was received by the local planning authority and the date when the period referred to in sub-paragraph (10)(c) would expire; and
(d)specifies the date (being not less than 21 days from the date of the notice) by which representations are to be received by the local planning authority.
(6) The local planning authority must send a copy of the notice referred to in sub-paragraph (5) to the developer.
(7) Where any owner or occupier of any adjoining premises objects to the proposed development, the prior approval of the local planning authority is required as to the impact of the proposed development on the amenity of any adjoining premises.
(8) The local planning authority may require the developer to submit such further information regarding the proposed development as the authority may reasonably require in order to determine the application.
(9) The local planning authority must, when considering the impact referred to in sub-paragraph (7)—
(a)take into account any representations made as a result of the notice given under sub-paragraph (5); and
(b)consider the amenity of all adjoining premises, not just adjoining premises which are the subject of representations.
(10) The development must not begin before the occurrence of one of the following—
(a)the receipt by the developer from the local planning authority of a written notice that their prior approval is not required;
(b)the receipt by the developer from the local planning authority of a written notice giving their prior approval; or
(c)the expiry of 42 days following the date on which the information referred to in sub-paragraph (2) was received by the local planning authority without the local planning authority notifying the developer as to whether prior approval is given or refused.
(11) The development must be carried out—
(a)where prior approval is required, in accordance with the details approved by the local planning authority;
(b)where prior approval is not required, or where sub-paragraph (10)(c) applies, in accordance with the information provided under sub-paragraph (2),
unless the local planning authority and the developer agree otherwise in writing.
(12) The local planning authority may grant prior approval unconditionally or subject to conditions reasonably related to the impact of the proposed development on the amenity of any adjoining premises.
(16) When computing the number of days in sub-paragraph (5)(d), any day which is a public holiday must be disregarded.
Explore GPDO Part 1
Choose the work you are planning. These guides explain the English rules for houses.
- Part 1 overview
Start here for permitted development at a house - Class A: house extension and alteration
Rear and side extensions, windows and doors - Class AA: additional storeys
Build upwards above an existing house - Class B: roof enlargement
Dormers and hip-to-gable enlargements - Class C: other roof alteration
Rooflights and other roof alterations - Class D: porch
Porches outside an external door - Class E: outbuilding
Garden buildings, pools and other incidental development - Class F: hard surface
Driveways, paths and patios - Class G: chimney and flue
Chimneys, flues and soil and vent pipes - Class H: satellite dish
Microwave antennas on the house or in its curtilage - Paragraph I: definitions
Terms used in Part 1
Page Updated: 25th September 2026