What original dwellinghouse means in the GPDO

"Original dwellinghouse" in the permitted development rules does not mean the house as you bought it. Article 2(1) of the Town and Country Planning (General Permitted Development) (England) Order 2015 defines "original" as the building as it existed on 1 July 1948, or, for a building put up on or after that date, as it was first built. Every extension added since then, by you or by anyone before you, counts against the allowances in Schedule 2 Part 1, whether or not it had planning permission at the time.

That single definition explains most of the disappointment householders experience when they discover the rear extension they wanted is not permitted development after all.

The definition and the 1948 reference date

1 July 1948 is the date the Town and Country Planning Act 1947 took effect, which is when the modern planning system began. Anything standing on that date is treated as the starting point, because before it there was no national requirement to seek permission.

So for a Victorian terrace, the original dwellinghouse is the house as it was in the summer of 1948, including any pre-war rear addition. For a 1970s semi, the original dwellinghouse is the house as the developer built it. For a house completed last year, the original dwellinghouse is the house as completed, and the allowance is untouched.

The definition applies throughout Part 1, and the limits that depend on it are the ones people actually use:

• Class A rear and side extension depths are measured from the original rear wall or the original side wall rather than from the wall of the extension your predecessor built.
• Class A.1(a) caps the ground area covered by buildings in the curtilage, other than the original dwellinghouse, at 50 per cent of the curtilage excluding the footprint of the original house. Every extension, shed, garage and outbuilding counts towards that 50 per cent.
• Class B roof enlargements have a cumulative volume allowance of 40 m³ for a terrace house and 50 m³ otherwise, measured as an addition to the volume of the original roof space.
• Class E outbuildings draw on the same 50 per cent curtilage figure.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

Why an earlier owner's extension reduces your allowance

The allowances attach to the building rather than to the owner. Buying the house does not reset anything, and the fact that a previous extension was itself lawful makes no difference to the arithmetic. A 1930s semi that gained a 3 m single-storey rear extension in 1994 has already used its Class A rear depth for a semi-detached house, so a further rear extension of any depth needs planning permission.

Two consequences catch people out most often.

The first is the 50 per cent curtilage figure. A house with a garage conversion, a rear extension, a garden room and a large shed can be at or over 50 per cent before anyone draws a line, and the calculation excludes the original house footprint from both sides of the sum, which makes the available margin smaller than it looks. Measure it from a scaled survey rather than from the deed plan.

The second is Class B. A loft conversion carried out in 2005 with two rear dormers may have consumed most of the 40 m³ or 50 m³ allowance, and the remaining volume will not take a third dormer. Volume here means the added volume above the original roof slope, including the thickness of the construction, which is easy to underestimate.

Where an earlier enlargement was built without permission and is now immune from enforcement, it still counts. Immunity from enforcement makes the extension lawful. It does not make it part of the original dwellinghouse.

Finding evidence of the original footprint

You need a drawing of the house as it stood in 1948, or as first built, and a credible source for it. The sources that work, in rough order of usefulness:

1. The council's planning history for the address, which often includes the drawings submitted with each extension. Many authorities now publish scanned files back to the 1970s or earlier on their public register.
2. Historic Ordnance Survey mapping. The large-scale county series and early National Grid sheets show building footprints, and the survey and publication dates are printed on the sheet, which matters because a 1950s map may show a 1930s survey.
3. Wartime and immediate post-war aerial photography, including the RAF vertical survey of the late 1940s, held in national and local collections. For the 1948 question this is often the only direct evidence.
4. Deeds, conveyancing plans and old sale particulars held by the current or previous owner's solicitor.
5. For post-1948 houses, the original estate layout and house type drawings from the approval for the estate, which the council will hold.

Put the source, its date and a marked-up extract in the application or the certificate application. An officer who can see where the original wall was will accept a measurement taken from it. An officer given a bare assertion will ask for the certificate route instead.

Rebuilt and replacement houses

A replacement dwelling built under planning permission is a new building, and once it is complete it is the original dwellinghouse as built. That does reset the Part 1 allowances, which is one of the less obvious advantages of demolition and rebuild over heavy extension.

Substantial alteration short of rebuilding does not reset anything. If the permission was for extension and remodelling, even extensive remodelling, the original dwellinghouse remains the pre-existing building. The distinction turns on whether a new building was created, which is a question of fact rather than of how the application was described.

Replacement dwellings in the Green Belt, in National Landscapes and on land covered by Article 2(3) of the Order carry their own policy tests and restrictions, and a permission for a replacement house frequently comes with a condition removing Part 1 rights precisely to prevent the reset. Read the decision notice before assuming the allowance is back. The additional constraints that apply in designated areas are set out in more detail under Article 2(3) land and permitted development in protected areas.

Houses built after 1948

For a post-1948 house, "as so built" means the house as constructed and first occupied, which is the state shown on the approved drawings for the estate. Anything the first owner added under permitted development in year two counts against your allowance in year forty.

There is one practical wrinkle on new estates. Where a house was built with an integral garage that has since been converted into a habitable room, the conversion usually did not enlarge the building, so it does not consume the Class A depth allowance, but the garage footprint was always part of the original dwellinghouse and cannot be counted as an addition either. It does, though, remove a parking space, which is a planning consideration in its own right if you later apply for something else.

Flats and maisonettes sit outside all of this, because Part 1 applies only to dwellinghouses. The reasoning is explained under why flats and maisonettes have no Part 1 permitted development.

Conditions removing permitted development on new estates

On many housing estates approved in the last twenty years, the permission carries a condition removing some or all of Part 1 rights. It is common on small plots, on schemes with design codes, and where the layout depends on gaps between houses being kept open.

The condition sits on the estate permission rather than on your plot, so the place to look is the original decision notice for the whole development, not the building control file for your house. Ask the council for the decision notice and the conditions schedule by application reference, then read the schedule in full. A condition removing "Classes A to E of Part 1 of Schedule 2" means every extension, roof alteration, porch and outbuilding needs an application.

Article 4 directions do the same job across an area rather than a site. Check the address against the national planning data service, then confirm the wording and date on the council's own site.

Checking the position before you draw anything

Work in this order. Establish the original footprint from evidence. Measure what has been added since, from a survey. Subtract. Read the decision notices for the estate and for every previous extension. Check for an Article 4 direction and for a conservation area or listed building designation.

If the remaining allowance is tight, or if the evidence of the original building is thin, apply for a lawful development certificate for proposed works under section 192 of the Town and Country Planning Act 1990 before you build. It is the only formal way to confirm that works are lawful, the fee is a fraction of a full application, and a certificate is what a buyer's solicitor will ask for.

Where the allowance has gone, a full application with a short planning statement setting out the design, the neighbour amenity position and the relevant local plan policies is usually a better use of the money than an argument about volumes. Browse all 66 report types and start a draft.

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