What counts as the original house on a post-1948 build

The original dwellinghouse meaning for a post 1948 build is the house as it was first constructed, not as you bought it. The General Permitted Development Order 2015 defines original by reference to two alternative dates: for a building that existed on 1 July 1948, the building as it stood on that date, and for a building built later, the building as so built. On a 1990s estate house the baseline is therefore whatever the original planning permission and approved drawings show, and every enlargement since then counts against the allowance.

That sounds simple until you find a conservatory that the developer added, a garage that was converted by a previous owner, and a planning condition nobody mentioned at purchase.

The two alternative dates in the definition

The definition does two jobs. For older housing it fixes a historic baseline, which is why Victorian and inter-war houses are assessed against their 1948 footprint and why historic Ordnance Survey editions matter so much there. For anything built since, it fixes the baseline at first construction, which moves the evidence from maps to the planning register.

The practical difference is large. On a pre-1948 house, an extension added in 1960 counts against the allowance even though it predates the current Order. On a house built in 1996, the question is simply what the 1996 permission authorised and what has happened since.

As first built, and what proves it

As so built means the house as completed under the original permission. The evidence, in rough order of strength:

1. The original planning permission, decision notice and approved drawings from the council's planning register.
2. The building control completion certificate and any associated plans.
3. A warranty or structural guarantee document showing the original layout.
4. The first council tax banding record and its date.
5. Dated aerial photography and successive Ordnance Survey editions.
6. The title plan and the transfer plan from first sale.

Most councils now hold the original estate permission online, often as a single application covering a phase with house type drawings attached. Find the house type rather than hunting for your plot number, then check the site layout plan to confirm which type was built on your plot. That one document usually answers the whole question.

Developer-built extensions before first occupation

This is where the argument usually sits. A conservatory, a sun room or a larger kitchen built as part of the original construction, under the original permission, forms part of the original dwellinghouse and does not count against the allowance. The same structure added after completion, even by the same developer in the same month, is an addition and does count.

So the test is not who built it or when it appeared. It is whether it was part of the house as authorised and completed under the original permission. The approved drawings settle it, and where they show the conservatory, the full allowance remains available. Where a later amendment or a separate householder application appears on the register, it does not.

Show homes and optional layouts

Estates are often built with house type variants: a standard plan, a plan with a larger rear projection, and sometimes a plan with a garage conversion already done. Each variant is usually covered by the original permission or by an approved amendment, and each produces a different original footprint.

Show homes are a particular trap, because they are frequently built with the optional extras and then sold with them. If your house was the show home, check the approved drawings for your plot specifically rather than the standard house type, and check for any non-material amendment under section 96A of the Town and Country Planning Act 1990 that changed what was authorised. The evidential discipline here is the same one applied to historic agricultural use, and our note on proving a building was in agricultural use under Class Q describes how councils weigh that kind of documentary record.

Garages built under the original permission

An integral garage within the main envelope forms part of the original dwellinghouse, so converting it to habitable space is an internal alteration and not an enlargement, although a new or altered window or door in the elevation has its own rules and a dropped kerb needs separate consent.

A detached garage built at the same time as the house is a different matter. It is a building within the curtilage rather than part of the original dwellinghouse, so it does not reduce the extension allowance. It does count towards the proportion of the curtilage covered by buildings, which is the test our note on the 50 per cent curtilage rule for garden outbuildings sets out, and on a small modern plot with a garage and a shed that limit is often reached before the depth limit is.

Where the allowance has already been spent

Three situations account for most of the disappointment.

The first is a previous rear extension. It counts in full, measured from the original rear wall, and a 3 metre extension added in 2008 leaves very little on a semi-detached house.

The second is a planning condition. New estates are routinely granted with conditions removing some or all permitted development rights, particularly on small plots, in flatted blocks and where the layout depends on retained gaps between dwellings. Read the conditions on the original permission and any reserved matters approval. A condition in these terms is binding whatever the Order says, and the only route is a planning application.

The third is an Article 4 direction, which the council can make over an area to withdraw rights. Directions are published by the authority and may not appear in national data, so ask.

There is a fourth that catches people on larger plots. Where a previous owner built an outbuilding, a garden office or a substantial hard surface, those works count towards the curtilage coverage test even though they are nothing to do with the house itself. A householder who measures only the rear extension depth and finds it comfortable can still be outside Class A because the garden is already half built on. Measure the whole curtilage before you draw, taking the figures from the title plan rather than from an estate agent's floor plan.

Where the position is unclear, a certificate of lawfulness for proposed use or development under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works would be lawful. It takes a fee and a determination period, and it is the document a buyer's solicitor will want in five years. Where you decide to apply for permission instead, a Planning Statement setting the scheme against the council's householder design guidance carries a small application perfectly well.

Enter your postcode on PlanScope to see which documents a householder scheme is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A lawful development certificate is the only formal way to confirm that works are lawful.

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