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Rear extension rules: 3 m, 4 m and stepped rear walls
An ordinary single-storey rear extension can project up to 4 m from the original rear wall of a detached house, or 3 m for any other house, under Class A in England. Those dimensions are only part of the test.
We need to check the original walls, previous extensions, height and boundaries too. Where the house has an outrigger or stepped rear, an extension can be both a rear and a side enlargement.
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.
The ordinary rear extension limits
- Detached house: no more than 4 m beyond the relevant original rear wall
- Any other house, including a semi-detached or terraced house: no more than 3 m
- A single-storey rear enlargement: no more than 4 m in overall height
- The existing house’s roof and eaves heights remain upper limits
- If any part of the enlargement is within 2 m of a curtilage boundary, its eaves must not exceed 3 m
These rules can cover a conventional extension, conservatory or orangery. The name you give the room does not alter the footprint and height limits. The conservatory exception concerns the materials condition, not a bigger size allowance.
Measure from the original house
The original house is the house as it existed on 1st July 1948, or as first built if later. It is not the house as it looked when you bought it.
Government guidance measures the rear projection from the base of the original rear wall to the outer edge of the extension wall, excluding guttering and bargeboards. A later extension does not give you a fresh starting line. Where the new extension joins existing enlargements, the total enlargement must satisfy the relevant limits.
Stepped rear walls and outriggers
A house can have more than one original rear wall. A common example is a main rear wall with an original outrigger projecting behind it. You cannot draw one imaginary rear wall across the very back of the outrigger and use that line for every part of the proposal.
Identify each original rear wall that the proposed enlargement extends beyond. An infill beside an outrigger may be measured from the rear of the main house. Work behind the end of the outrigger may also need a separate depth check from that original wall. Government diagrams demonstrate that satisfying one rear-wall measurement does not necessarily satisfy the other.
First establish whether the outrigger is original. If it was added later, it is an existing enlargement instead, which changes the reference walls and can engage the joined-enlargement rule.
Why being at the back does not avoid the side rules
The lengthwise wall of an original outrigger can form a side elevation. An extension beyond it can therefore trigger the side extension rules, even within the overall width of the main house.
When that happens, the enlargement must be single-storey, no higher than 4 m and no wider than half the original house. Measure the width of the whole connected enlargement, not just the part beside the outrigger.
A full-width wraparound on an 8 m wide original house can therefore fail because its 8 m enlargement width exceeds the 4 m half-width limit. The larger rear-extension procedure does not waive that rule. Some narrower combinations can comply, but the whole proposal must meet every applicable test.
Existing side extensions and joined work
A.1(ja) applies the relevant limits to new work together with any existing enlargement to which it will be joined. A later rear extension can fail because it joins an existing side extension and makes the combined enlargement too wide.
Separate planning applications or construction phases do not reset the measurements. Nor should a certificate for earlier work be treated as confirmation that the next extension will be lawful.
When can you use the 6 m or 8 m allowance?
A qualifying single-storey rear enlargement can extend farther using the larger home extension procedure: more than 4 m and up to 8 m for a detached house, or more than 3 m and up to 6 m for any other house.
That route is unavailable on article 2(3) land or a Site of Special Scientific Interest. The prior approval procedure must be followed before development begins. It does not relax the other Class A restrictions.
Two-storey rear extensions
A rear enlargement with more than one storey must project no more than 3 m beyond the original rear wall and must not be within 7 m of any curtilage boundary opposite the rear wall. It cannot extend beyond an original side-elevation wall under Class A.
The roof pitch must match the original house as far as practicable. The existing roof and eaves limits, the 3 m eaves rule near a boundary and the conditions for upper-floor side windows also apply.
Adding a first floor above an existing ground-floor extension needs particular care. For example, a joined two-storey enlargement extending 4 m from the original rear wall does not meet the 3 m multi-storey limit, even if the ground-floor extension was lawful.
On article 2(3) land, a rear enlargement with more than one storey is excluded from Class A.
Other checks before you build
The main Class A guide covers the 50% ground-coverage calculation, protected-land restrictions, materials, balconies and raised platforms, planning conditions, Article 4 directions and houses excluded because of their creation route.
If every relevant requirement is satisfied, an ordinary rear extension does not normally need a separate planning application. A Lawful Development Certificate can establish that the specific proposal would be lawful. Building Regulations, listed building consent, party wall requirements, property rights and any CIL procedure remain separate checks.
A council’s 45-degree rule is not a national dimensional limit in Class A. A proposal needing a planning application, or a larger-extension amenity assessment, raises different questions from simply meeting the ordinary permitted development dimensions.
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