What “Original House” Means in Planning

One word in the permitted development rules decides more projects than any other, and almost nobody reads it carefully. It is not a description of your house. It is a legal definition with a fixed date attached, and it means that a great many extensions are measured from a wall that no longer exists.

The definition, in one sentence

Two limbs, one date, and no discretion anywhere in it.

The General Permitted Development Order defines original as: in relation to a building existing on 1 July 1948, as it existed on that date; and in relation to a building built after that date, as it was built.

So there are only two possible answers for any house. If it was standing on 1 July 1948, the original house is whatever stood there on that day, including anything a Victorian or Edwardian owner had already added. If it went up afterwards, the original house is the house as first constructed, on the day it was finished.

Nothing that happened later changes the answer. Not an extension, not a planning permission, not a sale, not a demolition, not forty years of undisturbed occupation. The original house is a fixed thing, and every allowance in Class A is measured against it.

Why 1 July 1948

The date is not arbitrary and it is not about architecture.

It is the day the Town and Country Planning Act 1947 came into force, which is when general planning control over development began in England. Before that date most building work needed no permission from anybody, so there is no reliable public record of what was added and when. Rather than pretend otherwise, the Order draws a line at the point the records start and treats everything on the far side of it as part of the house.

The practical consequence is one of the few genuinely good pieces of news in this subject. If your house is Victorian and a previous owner tacked a scullery or a two storey rear addition onto it in 1912, that addition is part of the original house. It does not count against your allowance. On a Victorian terrace with a rear outrigger, the outrigger is almost always original in this sense, which is why an infill extension in the side return is measured from the back of the outrigger rather than from the main rear wall of the house.

The inverse is worse news for post-war housing. A 1965 semi that was extended in 1988 is measured from the 1965 footprint, and the 1988 work has already spent part of the allowance.

Original and existing are not the same word

Class A uses both, deliberately, and swapping them is how a compliant extension turns out not to be.

The Order alternates between the two, and it is not sloppy drafting. Roughly speaking, anything about how far out you go is measured from the original house, and anything about how high you go is measured against the house as it stands.

Read the two columns together and the logic comes out. The rules about spread protect the plot and the neighbours, so they are anchored to a fixed historic footprint that cannot be inflated one extension at a time. The rules about height and appearance are about how the finished thing looks and behaves, so they refer to the building actually standing there. A previous owner’s extension therefore consumes your depth but does not raise your height limit, and it sets the materials you have to match while contributing nothing to what you are allowed to build.

What has already been spent

Four different things draw on the same account, and only one of them looks like an extension.

Any previous enlargement beyond the original rear wall reduces what remains, whether it was built last year or in 1975. A conservatory counts: it is exempt from parts of the Building Regulations, not from the planning measurement, and a 3 metre lean-to on a semi has used the lot.

Outbuildings count too, though not against the depth. Every building in the curtilage other than the original house feeds the 50 per cent ground cover limit, so a detached double garage, a large summerhouse and a run of sheds can put a modest rear extension outside permitted development on an area calculation nobody thought to do.

Roof work draws on a separate account. The 40 or 50 cubic metre allowance for roof enlargement is measured against the original roof space, so a dormer added by a previous owner reduces what is left for yours, and a hip to gable enlargement uses a surprising amount of it.

What does not reset it

The allowance belongs to the house. It is not a personal entitlement and it does not renew.

Selling the property changes nothing. A new owner inherits the house as it is and the allowance as it stands, which is why buyers are routinely told by an agent that there is scope to extend when there is none. Nobody at the point of sale has any duty to work it out.

Demolishing a previous extension and rebuilding does not help either. Knock the 1990s conservatory off and the measurement is still taken from the original rear wall, so the ground you have cleared is not ground you have won back. What demolition does buy is design freedom on the same allowance, which is sometimes worth having on its own.

Nor does time. Where unauthorised building work has become immune from enforcement, it is lawful and it stays up, but it remains part of the house and the measurement is still taken from the original wall behind it. Immunity from enforcement and a restored allowance are two different things.

The one thing that can move

The house is fixed. The land around it is not.

The 50 per cent ground cover test compares buildings in the curtilage against the area of the curtilage, and the curtilage is assessed as it is at the time of the development rather than as it was in 1948. Buy a strip of your neighbour’s garden, or bring a paddock into the domestic curtilage properly, and the denominator grows.

Two cautions come with that. Whether land forms part of the curtilage is a question of fact rather than a matter of who owns it, so a field you happen to own behind the fence is not automatically in. And the ground cover rule is the only one this helps. It does not add a millimetre to your depth allowance, because that is measured from a wall.

The awkward cases

Most houses give a clean answer. A few do not, and those are the ones worth taking advice on rather than guessing.

A house demolished entirely and rebuilt under a planning permission is generally original as newly built, because that is the building that exists. A partial rebuild, where a substantial part of the old structure was retained, is a different and much less certain question.

Buildings converted to houses are harder again. A barn, a chapel or a coach house standing on 1 July 1948 was a building existing on that date, so the definition reaches back to its state then rather than to the day it became a dwelling. Conversions carried out under permitted development change of use rules often have their Part 1 rights removed as well, so the question can be academic.

Two cottages knocked into one, a house split into flats and later reinstated, and properties where the original was rebuilt after war damage all fall outside anything a rule of thumb can settle. Get the council’s view in writing rather than designing on an assumption.

How to find out what the original house was

This is a research job, and it is a couple of hours rather than a couple of weeks.

1. The council’s online planning register. Search the address, not just the postcode, and read every entry rather than the most recent. Applications from the 1970s and 1980s are often listed with little more than a reference and a one line description, and that one line is frequently enough.
2. The approved drawings behind each permission. These show the footprint before and after, which is exactly what you need. Where they have not been scanned, the council will usually retrieve them.
3. Historic Ordnance Survey mapping. For a pre-1948 house this is the primary evidence, because a large scale map surveyed in the 1930s or 1940s shows the footprint as it stood at the relevant date.
4. Aerial photography. Post-war survey photography covers most of Essex and settles arguments about whether a rear addition was there in 1948 or arrived in 1955.
5. The title plan and any transfer plans. These establish the curtilage and whether it has changed, which matters for the ground cover test.
6. The house itself. A change in brick, a straight joint running up an elevation, a different roof pitch, a damp proof course at the wrong level or a floor that steps down all mark a junction between original and added.
7. The neighbours. On a street where people have lived for decades, somebody watched it go up and can tell you which year.

Where you intend to rely on the answer, a Lawful Development Certificate turns the research into a document. The burden of proof sits with the applicant on the balance of probabilities, so the evidence you have gathered is the application rather than an accompaniment to it. There is more in Lawful Development Certificates.

What it means for the design

The reason this matters commercially is sequencing. Establish the original house first, then design, then price. Do it the other way round and you pay a technician to draw something that turns out to need an application, or worse, you build to a depth that was never available.

Where the allowance has genuinely gone, that is not the end of the project. It means the route changes to a full householder application, or in some cases to the larger home extension prior approval route, which starts from the same original wall but permits considerably more of it. Both are covered in permitted development rights for extensions.

We do this research on every job before anybody draws a line, because the answer determines the design rather than the other way round. If you have been told the house has scope to extend and want that checked properly, the survey and the quotation cost nothing.

Definitions and thresholds change over time, and how any of this applies depends on your specific property and its planning history. Treat this as an explanation of how the measurement works rather than as confirmation of what you can build.

Last reviewed August 2026. Planning and Building Regulations change, and your property may differ.