Side extensions and the half the width test
Class A allows a side extension only if it is single storey, within 4 metres in height and no wider than half the original house. The half width test is the one that catches people.
A side extension under permitted development has to be single storey, no more than 4 metres high, and no wider than half the width of the original house. Those three limits sit in Class A of Schedule 2, Part 1 of the Town and Country Planning (General Permitted Development) (England) Order 2015, and all three have to be met together. Fail any one and the extension needs planning permission, however modest it looks from the street.
The half width test is the one that produces arguments, because almost nobody measures the original house.
What Class A allows to the side
Class A permits the enlargement, improvement or other alteration of a dwellinghouse, within a set of limits. For anything extending beyond a wall forming a side elevation of the original house, the limits tighten.
The extension must not extend beyond a wall forming the principal elevation, and it must not extend beyond a wall forming a side elevation that fronts a highway. On a corner plot that second rule usually removes the obvious place to build, since the flank wall facing the side road is a side elevation fronting a highway even where there is no access from it.
There is also a curtilage limit that catches large plots with outbuildings. The total area of ground covered by buildings within the curtilage, other than the original house, must not exceed half the area of the curtilage. Garages, sheds and garden rooms all count towards that figure.
Measuring half the width of the original house
The comparison is with the original dwellinghouse, meaning the house as it was built, or as it stood on 1 July 1948 where it is older. Not the house as you bought it, and not the house including the 1998 kitchen extension.
Width is the width of the original house, so on a house that has already gained a side addition the available allowance is measured against the original width and reduced by what has already been built. Our note on extending a house that has already been extended works through the cumulative arithmetic in more detail.
Two measuring points matter. Take the width at the widest part of the original house, measured externally, and be consistent about whether bay windows and chimney breasts are included, because the drawings have to show the same basis. And where the house is not rectangular, the width is measured along the elevation the extension sits beside rather than diagonally.
An illustrative case: a semi-detached house with an original width of 6.4 metres allows a side extension of up to 3.2 metres in width. If a 1.5 metre side addition already exists, the remaining allowance is 1.7 metres. Those figures are illustrative, and a real scheme needs the original width measured on site.
Single storey, and 4 metres
A side extension under Class A cannot have more than a single storey. That rules out the two storey side extension that most families actually want, which is why so many of them are applied for rather than built as of right.
The 4 metre height limit is measured from the highest ground level immediately next to the extension. On a sloping plot the measurement is taken from the higher side, which can cost 300 or 400 millimetres of roof and turn a comfortable hipped roof into a flat one.
Note the interaction with the rear. A single storey extension to the rear has its own depth and height limits and its own larger home extension prior approval route. Those limits do not transfer to the side, and a side extension gets no equivalent prior approval procedure.
Wrap around extensions
An extension that runs along the side and then around to the rear is the design that most often gets built without permission and most often has to be regularised afterwards.
The Order treats the enlarged part as a whole. A wrap around extends beyond both a side wall and a rear wall of the original house, and the government's technical guidance on householder permitted development is clear that this combination falls outside the class. The practical answer is that a wrap around needs planning permission, even where the side element on its own and the rear element on its own would each have been within the limits.
Builders will tell you otherwise. Ask for the clause.
Article 2(3) land and losing the right entirely
On article 2(3) land, which covers conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads, Class A does not permit any extension beyond a wall forming a side elevation of the original house.
So in a conservation area the side extension question is always a planning application. So is cladding, and the two are often proposed together, which is covered in rendering or cladding a house and when permission is needed.
The right can also be removed by an article 4 direction or by a condition on the permission that created the house. Estate houses built since the 1990s frequently carry such conditions, and the decision notice is the place to check. Where the house is listed, listed building consent is needed for works affecting its character regardless of the planning position.
The 3 metre eaves rule near a boundary
Where any part of the extension is within 2 metres of the boundary of the curtilage, the height of the eaves must not exceed 3 metres.
For a side extension this rule is usually decisive, because a side extension is almost always within 2 metres of the boundary. A 3 metre eaves height gives roughly a standard ground floor storey plus the roof build-up, which points towards a shallow pitched roof, a hipped roof pulled away from the boundary, or a flat roof with a parapet kept under the limit.
Measure the eaves to the point where the roof meets the wall, and remember that the 4 metre overall height limit applies at the same time. Both have to be satisfied.
Designing something that reads as subordinate
Meeting the limits is a legal test. Getting a good building is a separate problem, and it matters whenever the extension needs permission rather than falling within the class.
Councils assess side extensions against the same handful of concerns almost everywhere: whether the extension is subordinate to the host dwelling, whether the terracing effect of building up to a boundary erodes the gaps that give a street its character, whether the roof form follows the original, and whether the front building line is respected. A set back of the front face by half a metre or more, a roof set down below the main ridge, and matching window proportions answer most of it.
Where the house sits in a pair or a run with visible gaps between properties, expect the gap to be the main issue. Some councils have a householder design guide with a stated minimum gap to the boundary, and where one exists it is the document the officer will quote. A Design and Access Statement is the right place to show the analysis, and the policy case belongs in a short Planning Statement.
If the works are within the class, a lawful development certificate is the only formal way to confirm that they are lawful. Apply before starting, not after.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How is half the width of the original house measured?
Against the house as it was built, or as it stood on 1 July 1948 for older properties, measured externally at its widest part along the elevation the extension sits beside. Any existing side addition reduces what remains of the allowance. Keep the same basis on the drawings for bay windows and chimney breasts as you used on site.
Can a side extension be two storeys under permitted development?
No. Class A allows a single storey only where the extension goes beyond a wall forming a side elevation of the original house, with a maximum height of 4 metres measured from the highest adjoining ground level. A two storey side extension always needs planning permission, and there is no prior approval route for it.
Is a wrap around side and rear extension permitted development?
No. The enlarged part is assessed as a whole, and a wrap around extends beyond both a side wall and a rear wall of the original house, which falls outside the class. That remains the position even where the side element alone and the rear element alone would each have met the limits. It needs planning permission.
What is the 3 metre eaves rule?
Where any part of the extension is within 2 metres of the curtilage boundary, the eaves height must not exceed 3 metres. Side extensions are almost always within 2 metres of a boundary, so this limit usually decides the roof form: a shallow pitch, a hip pulled back from the boundary, or a flat roof with the parapet kept below the limit.
Can I build a side extension in a conservation area?
Not under permitted development. On article 2(3) land, which includes conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads, Class A does not permit any extension beyond a side elevation of the original house. A planning application is needed, and the case turns on subordination, roof form and the gap to the boundary.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.