The original dwellinghouse: how past extensions use up your allowance
Every householder permitted development limit is measured from the original house, not the house you own today. Extensions built by previous owners, with or without permission, still count.
Permitted development limits for a house are measured from the original dwellinghouse, which the Town and Country Planning (General Permitted Development) (England) Order 2015 defines as the building as it stood on 1 July 1948, or as first built if it was built after that date. Every original dwellinghouse permitted development calculation starts there. A 1970s rear extension built with full planning permission still counts against your allowance, because the definition is about a date, not about consent.
That single point accounts for a large share of enforcement cases involving householders. Somebody measures 3 metres from the back of the house they bought, builds it, and discovers the back of the house was already 2.5 metres beyond the original wall.
What original dwellinghouse means in the GPDO 2015
The definition sits in the interpretation article at the front of the Order, and it applies throughout Schedule 2 Part 1.
For a house existing on 1 July 1948, the original dwellinghouse is that house as it stood on that date. For a house built later, it is the house as built. Nothing that was added afterwards is part of the original, whether it was permitted development, granted permission, or built unlawfully and later became immune from enforcement.
The date is not arbitrary. It is the day the modern planning system took effect under the Town and Country Planning Act 1947, so it marks the point from which development control has applied.
One consequence surprises people. A replacement dwelling, demolished and rebuilt under a planning permission, resets the baseline. The house as built under that permission is the original dwellinghouse for permitted development purposes from then on.
The 1 July 1948 date and houses built after it
For a Victorian or Edwardian house, establishing the original footprint means going back to mapping and photographs from the 1940s or earlier. For a house on a 1990s estate, the original is the house on the approved drawings, and the council's own planning file usually settles it in ten minutes.
Two traps sit in the middle of that range.
A house built in 1955 with a garage added by the developer before first occupation raises a question of fact about whether the garage was part of the house as built or a later addition. The approved drawings and the completion records are the evidence.
And a house that has been substantially rebuilt, rather than replaced under a permission, does not reset anything. Rebuilding a wall is not building a new dwellinghouse.
How earlier extensions count against the allowance
The Class A limits are all expressed by reference to the original.
- A single-storey rear extension is measured beyond the wall forming the original rear wall of the dwellinghouse, not beyond the current back wall.
- A side extension must not exceed half the width of the original dwellinghouse.
- Total ground cover by buildings within the curtilage, other than the original dwellinghouse, is capped as a proportion of the curtilage.
- Two-storey rear extensions are measured from the original rear wall and have their own boundary separation requirement.
So if a previous owner built a 2.5 metre single-storey rear extension, a semi-detached house with a 3 metre allowance has 0.5 metres left, and a further extension of any greater depth needs an application. The larger home extension route, with prior approval and neighbour consultation, is measured from the original rear wall too.
Outbuildings interact with this through the ground cover cap. A large garage and a garden room already built may leave no room for a further building, even a small one, and the calculation is against the curtilage of the original dwellinghouse rather than the plot as it is today.
Where the numbers are tight, a short Planning Statement setting out the measured history and the arithmetic is worth submitting even on a modest application, because it lets the officer check your working rather than reconstruct it.
Extensions built under a planning permission and whether they count
They count. This is the single most common misunderstanding, and it is worth stating plainly.
Permitted development rights and planning permissions are different sources of authority. A permission granted in 1975 authorised that extension. It did not enlarge the original dwellinghouse, because the original dwellinghouse is fixed by the 1948 date or by first construction.
The same applies to extensions built under permitted development by an earlier owner, to conservatories, and to extensions that were unlawful but have become immune from enforcement action through the passage of time. Immunity protects what was built. It does not convert it into part of the original house.
There is one honest complication. Where a permission was granted for the whole house as extended, in a form that effectively re-permitted the entire dwelling, the question of what constitutes the original building can become arguable. That is rare, and it is a question for a certificate rather than for a rule of thumb.
Finding the evidence: historic maps, deeds and applications
Work from the outside in. Public records first, private papers second.
- The council's online planning register, searched by address and by adjoining addresses, for every application since the 1970s or earlier.
- Historic Ordnance Survey mapping at successive editions, which shows footprints and outbuildings by decade.
- Aerial photography from the 1940s onwards, including wartime survey imagery where the local record office holds it.
- The Land Registry title plan and any pre-registration conveyances, which sometimes carry a plan showing the earlier footprint.
- Estate agents' particulars from previous sales, which often describe when an extension was added.
- The council's building control records, which can survive where planning records do not.
Measure the surviving original fabric on site as well. Changes in brick, a straight joint in the external wall, a step in the roofline or a different floor level usually mark where the original building stopped.
Illustrative example: a 1930s semi with a 1.8 metre 1960s kitchen extension and a 1.2 metre 2004 utility beyond it. The current rear wall is 3 metres beyond the original. There is no remaining Class A depth allowance at all, and any further rear extension needs a planning application, however modest.
What to do when the history is unclear
Some houses simply do not have a clean record. Rural properties, houses in areas where records were lost, and buildings altered before any register existed all produce genuine uncertainty.
Three things help. A measured survey that identifies the original building fabric, prepared by someone who has looked at the structure rather than the drawings. A statutory declaration from a long-standing owner or neighbour describing what was there and when it changed, dated and signed. And a schedule of the works you can evidence, with what you cannot evidence stated as such rather than glossed over.
Do not resolve the doubt in your own favour on paper. Officers read a great many of these, and an account that quietly assumes the most generous interpretation stands out.
Where the property sits in a conservation area, the position tightens further, because side extensions and larger rear extensions are removed from permitted development on designated land, and a Heritage Statement will be needed for any application.
Using a lawful development certificate to fix the position
A certificate of proposed lawful development under section 192 of the Town and Country Planning Act 1990 asks the council to confirm that what you intend to build would be lawful. It is decided on fact and law rather than on planning merits, so neighbour objections about appearance do not affect it.
Apply before you build. The application fee is lower than a householder application and is indexed annually, so check the current schedule. Submit the drawings showing the original footprint, the existing footprint and the proposal on the same sheet, with dimensions, plus the evidence bundle.
The certificate is what a buyer's solicitor will ask for in five years' time. And it is far easier to obtain before an extension exists than after somebody has complained about one.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a lawful development certificate is the only formal way to confirm that works are lawful.
Work out what your house needs before you draw it. Browse the report types by category to see what a householder application usually requires.
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Frequently asked questions
Does an extension built with planning permission count against permitted development?
Yes. The original dwellinghouse is defined by date, being the house as it stood on 1 July 1948 or as first built afterwards. A permission authorised the extension but did not make it part of the original house, so its depth still counts when you measure a new rear extension from the original rear wall.
What if my house was built after 1948?
The original dwellinghouse is the house as first built. The approved drawings on the council's planning register usually settle the footprint quickly, along with any conditions removing permitted development rights, which are common on modern estates and are easy to miss when buying.
Does demolishing and rebuilding reset the allowance?
A replacement dwelling built under a planning permission becomes the original dwellinghouse as built, so the baseline resets from that point. Substantially rebuilding parts of an existing house does not, because no new dwellinghouse has been created. The distinction matters and is worth confirming through a certificate.
How do I prove what the original house looked like?
Use the council's planning register, historic Ordnance Survey editions, aerial photography from the 1940s onwards, Land Registry plans, old sale particulars and building control records. Add a measured survey identifying original fabric on site, since changes in brickwork, straight joints and roofline steps usually mark where the original building ended.
Can permitted development rights be removed from my house?
Yes, by a condition on a planning permission, commonly on new estates and conversions, or by an Article 4 direction covering an area. Both are recorded publicly, the condition on the decision notice and the direction on the council's website, so check for each before assuming any allowance exists at all.
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.