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Extension measurement: original walls, eaves and boundaries
An extension measurement is only useful if it starts and finishes at the correct place. Class A in England uses several different reference points: the original house, the existing house, the roof surface, the room floor and the curtilage boundary.
We explain the most common measurements here. This guide supports the main Class A checklist; it does not replace the other eligibility tests and conditions.
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.
Original house and existing house are different
The original house is the house as it stood on 1st July 1948, or as first built if later. The existing house is the house immediately before the proposed work. A later extension may be part of the existing house without being part of the original house.
Class A uses the original house for rear projections, original walls and the half-width test. It uses the existing house for the highest roof and eaves limits. Mixing those reference points can produce the wrong result.
Rear depth starts at the original rear wall
Government guidance measures from the base of the original rear wall to the outer edge of the extension wall, excluding guttering and bargeboards. Do not measure the internal room depth or start at the back of a later extension.
A stepped original house may have several rear walls. Identify every relevant rear wall that the proposal extends beyond. An original outrigger can supply both a rear wall and an additional side wall. See the rear extension guide.
Side width is compared with the original house
Where an enlargement extends beyond an original side-elevation wall, its width must not exceed half the original house’s width. Government guidance takes the original house at its widest point. This is not half the width of the plot.
Measure the whole enlargement. If the new work joins an existing enlargement, A.1(ja) brings the relevant measurements of the total enlargement into the test. A connected wraparound cannot be reduced to the narrow strip beside the outrigger for this purpose. See the side extension guide.
Where do you measure eaves height?
Government guidance measures eaves from ground level at the external wall to the point where that wall, projected upwards if necessary, meets the upper surface of the roof.
- Do not measure to the underside of a gutter
- Do not measure to the bottom of an overhanging soffit
- For a pitched roof, use the roof surface where it meets the projected outside wall
- For a flat roof, use the equivalent meeting of the roof surface and outside wall
Parapet walls and overhanging eaves are not included in the guidance’s eaves measurement. A parapet can nevertheless be the highest part of the enlargement, so it can count towards the overall height.
Which ground level counts?
Article 2(2) refers to the surface of the ground immediately adjacent to the building. If that surface is not uniform, it refers to the highest adjacent ground surface.
This does not mean measuring from a raised deck. Government guidance expressly excludes additions laid on the ground such as decking. Nor should you assume that deliberately depositing material changes the lawful reference level without other planning consequences.
On a sloping site, record existing and proposed levels clearly. Where the existing house has different eaves heights, the guidance normally uses its highest eaves, but gives a specific qualification for the elevation being extended on sloping ground. These are situations where a proper measured drawing is preferable to a tape-measure estimate.
Overall height is a separate measurement
A single-storey rear enlargement and an enlargement beyond an original side wall are subject to a 4 m overall height limit. The relevant part must also remain within the existing house’s highest roof and eaves limits.
Government guidance excludes chimneys, firewalls, parapets and other protrusions when identifying the highest part of the existing main roof. When measuring the proposed enlargement, however, its highest part can include a parapet. Do not use the top of the existing chimney as your permitted extension height.
The 2 m rule limits eaves, not the building’s distance
If any part of the enlarged part is within 2 m of a curtilage boundary, the enlargement’s eaves must not exceed 3 m. The existing house’s eaves limit still applies if lower.
Class A does not impose a universal 2 m gap beside a single-storey extension. But the boundary position must be established accurately, and other matters such as party wall requirements or rights over neighbouring land are separate.
Calculate the 50% ground coverage correctly
Start with the curtilage area, remove the footprint of the original house, and calculate half of what remains. Compare that allowance with the ground covered by buildings other than the original house after the proposed work.
In this example, 400 m² of curtilage minus a 100 m² original house leaves 300 m². Half is 150 m². An existing 30 m² extension and 20 m² detached garage use 50 m², leaving 100 m² under this test alone.
Detached buildings count even if they are old or were built alongside the original house. Later extensions granted planning permission count too. This is a footprint calculation, not a sum of all upstairs and downstairs floors. The lawful curtilage is not necessarily the entire land ownership.
Measure opening windows from the room floor
For an upper-floor side window under A.3(b), the window must be obscure-glazed. It must also be non-opening unless its opening parts are more than 1.7 m above the floor of that room.
The exception applies to the parts which open, not simply the top of the frame or the position of a handle. At exactly 1.7 m, the opening-height exception is not met. Raising the opening section does not remove the separate obscure-glazing condition.
What should your drawings show?
- The original and existing house, with the evidence distinguishing them
- Every proposed part and any existing enlargement it will join
- Relevant original front, side and rear walls
- External dimensions, roof and eaves heights, and existing and proposed ground levels
- Curtilage boundaries, relevant highways and distances to them
- Materials and any upper-floor side windows
Where a measurement or interpretation is critical, a Lawful Development Certificate can establish the position for a properly evidenced proposal before construction.
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 Created: 25th September 2026