Original dwellinghouse and why the baseline matters
Permitted development limits are measured from the house as first built, not the house you bought. Here is how to work out your real baseline and prove it.
The original dwellinghouse is the house as it was first built, or as it stood on 1 July 1948 if it is older than that. Anything added since is not part of it. Every permitted development limit for a house, the 3 metre and 4 metre rear extension depths, the 40 and 50 cubic metre roof volumes, the 50 per cent curtilage rule, is measured against that baseline rather than against the house you bought.
So a 1930s semi that already carries a 3 metre rear extension built by an owner in 1998 has, in most cases, spent its single-storey rear allowance. The conservatory you never saw go up still counts against you.
What original means in the Order
Article 2(1) of the Town and Country Planning (General Permitted Development) (England) Order 2015 defines it plainly. In relation to a building existing on 1 July 1948, "original" means as it existed on that date. In relation to a building built after that date, it means as so built.
That is the whole definition. There is no allowance for works that were lawful, no credit for an extension that had planning permission, and no reset when the house changes hands.
The same article also tells you that a dwellinghouse does not include a building containing one or more flats, or a flat within such a building. Owners of maisonettes and converted flats have no Part 1 rights at all, so the baseline question never arises for them.
Why the date is 1 July 1948
That was the first appointed day under the Town and Country Planning Act 1947, when the modern system of development control began. Works carried out before it were not development requiring permission in the way we now understand, so the drafters treated the house as it then stood as the starting point.
For a Victorian terrace, this matters more than people expect. The two-storey back addition on a Camberwell or Headingley terrace was almost always built with the house in the 1880s or 1890s. It is part of the original dwellinghouse. The rear wall you measure from is therefore the rear wall of that outrigger across the part of the plan it occupies, and the shallower main rear wall beside it. Councils vary in how they treat the resulting stepped line, and the government's technical guidance on householder permitted development is the place to start before you argue it. Check the position with your case officer.
Extensions by previous owners eat your allowance
A previous extension does not move the measuring line forward. Under Class A a single-storey rear extension must not extend beyond a wall forming the rear wall of the original dwellinghouse by more than 4 metres for a detached house or 3 metres for any other house. If someone has already taken 3 metres, there is nothing left to take.
Class B works the same way for roofs. The cubic content of the resulting roof space must not exceed the cubic content of the original roof space by more than 40 cubic metres for a terrace or 50 cubic metres for any other dwellinghouse. A rear dormer added in 2006 consumed part of that volume, and so did any earlier one.
Class E, covering outbuildings, uses a different but related test. The ground covered by buildings within the curtilage, other than the original dwellinghouse, must not exceed 50 per cent of the curtilage excluding the ground area of the original house. Extensions, garages, sheds and the garden office all count towards that half.
Which limits move with the baseline and which do not
| Right | Limit | Measured against |
|---|---|---|
| Class A single-storey rear extension | 4 m detached, 3 m other houses | Rear wall of the original dwellinghouse |
| Class A larger home extension (prior approval) | 8 m detached, 6 m other houses | Rear wall of the original dwellinghouse |
| Class A extension of more than one storey | 3 m beyond the rear wall, and not within 7 m of the boundary opposite | Rear wall of the original dwellinghouse |
| Class A side extension | Width no more than half the width of the original house | Original width |
| Class A and Class E curtilage cover | 50 per cent | Curtilage excluding the original house footprint |
| Class B roof enlargement | 40 m³ terrace, 50 m³ other houses | Cubic content of the original roof space |
| Class A single-storey height | 4 m overall, 3 m eaves within 2 m of a boundary | Absolute, not baseline-related |
The last row matters. Height limits, materials conditions and the rule against extending beyond a principal elevation fronting a highway are absolute. They apply to the house as it stands today.
Proving what the original footprint was
Start with the council's online planning register and search by address, not by applicant. Applications from the 1970s and 1980s are often scanned and indexed poorly, so search the street name too. Decision notices give you the permitted depth, and the approved drawings give you the built one.
Where the register is silent, the usual evidence is historic Ordnance Survey editions held by the county record office or a university map library, aerial photography, the title plan at HM Land Registry, and a measured survey of the existing house showing brick bonds, changed floor levels and roof junctions. An honest surveyor can usually read a 1970s extension off the brickwork in ten minutes.
Illustrative example. A detached 1965 house has a 2.5 metre kitchen extension across half its rear wall and a 12 m² garden studio. The owner wants another 2 metres of kitchen. Because the measurement runs from the original rear wall, the new work would sit 4.5 metres beyond it, past the 4 metre detached limit, so it needs either the larger home extension prior approval route or a full householder application. The studio does not affect the depth test at all, but it does count towards the 50 per cent curtilage figure.
Rebuilds, Article 4 and conservation areas
A house demolished and rebuilt under permission is built on the later date, so the replacement becomes its own original. That sounds like a fresh start. In practice it rarely is, because permissions for replacement dwellings very often carry a condition removing Part 1 rights, and greenfield or Green Belt replacements almost always do. Read the decision notice before assuming anything.
An Article 4 direction can remove Class A, B or E rights across a conservation area or a single street, and many councils have made them for front extensions, porches and windows. On Article 2(3) land, which covers conservation areas, National Parks, National Landscapes and World Heritage Sites, Class B is withdrawn outright and side extensions and cladding under Class A are not permitted. A neighbourhood development order can pull in the other direction and grant rights a community wants, though few householder ones exist.
Settle the baseline before you spend money on drawings
- Download every decision notice and approved plan for the address from the council's register.
- Mark up a plan of the house showing, in one colour, what was there when it was built or in 1948.
- Measure the proposal against the Class A, B and E limits from that line, not from the current back wall.
- Where the answer is within about half a metre of a limit, apply for a Lawful Development Certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990. That certificate is the only formal way to confirm that works are lawful, and it costs half a householder application fee.
- If the proposal exceeds the limits, go for a householder application and support it with a short Planning Statement that deals with scale, neighbour amenity and the design policies in the adopted local plan.
Get the baseline wrong and a refusal is the mild outcome. An extension built beyond the Class A limits is unlawful development, open to enforcement, and it will surface on the first conveyancing search when you sell.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Does an extension built with planning permission count against my permitted development allowance?
Yes. The Order measures from the original dwellinghouse, meaning the house as built or as it stood on 1 July 1948. It makes no difference whether a later extension was permitted development or granted planning permission. Either way it sits outside the original, so the rear depth, roof volume and curtilage tests all run from the older line.
How do I find out what my house looked like in 1948?
Historic Ordnance Survey editions at the county record office are the usual starting point, backed by aerial photography and the title plan held at HM Land Registry. A measured survey helps too, because changes in brick bond, floor level and roof junction usually reveal where an addition begins. Keep copies for any certificate application.
If I demolish and rebuild, do I get a fresh permitted development allowance?
In principle the replacement house is built on the later date and becomes its own original. In practice most permissions for replacement dwellings carry a condition removing Part 1 rights, and Green Belt and countryside replacements almost always do. Read the decision notice and any conditions before you assume a clean slate.
Does the original dwellinghouse rule apply to flats?
No. Article 2(1) excludes a building containing one or more flats and a flat within such a building from the meaning of dwellinghouse, so Schedule 2 Part 1 rights do not apply. Alterations to a flat need planning permission where they amount to development, and usually landlord consent under the lease as well.
What happens if I build beyond the limit by mistake?
The work is unlawful development rather than a technical slip. The council can serve an enforcement notice, and the breach will appear on a conveyancing search when you sell. A retrospective application is possible, though it is decided on planning merits with no fallback argument. Applying for a certificate first avoids all of it.
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.