Original dwellinghouse: the 1948 baseline for extensions

"Original dwellinghouse" is the house as it stood on 1 July 1948, or as it was first built if that came later. It is the fixed baseline that every permitted development allowance for a house is measured from, and it does not move when you extend. So a three metre rear extension allowance is three metres beyond the original rear wall, not beyond whatever wall happens to be at the back of the house today. Miss that and you build something the council can, and sometimes does, ask you to take down.

The term is defined in article 2(1) of the Town and Country Planning (General Permitted Development) (England) Order 2015. Two limbs, no discretion. For a building that existed on 1 July 1948, "original" means the building as it stood on that date. For a building erected after that date, it means the building as originally built.

What the Order actually defines

Schedule 2 Part 1 of the GPDO grants householder permitted development rights: enlargements under Class A, roof alterations under Class B, roof lights under Class C, porches under Class D, outbuildings under Class E, and so on down to electric vehicle charging points and heat pumps. Almost every numerical limit in Part 1 is tied back to the original dwellinghouse rather than the current one.

Class A alone measures three separate things from that baseline. The depth of a single-storey rear extension is taken beyond the rear wall of the original dwellinghouse: 3m for a terraced or semi-detached house, 4m for a detached one, doubled to 6m and 8m under the larger home extension procedure with prior approval. Ground coverage is capped at 50 per cent of the curtilage excluding the original dwellinghouse itself. And a side extension cannot exceed half the width of the original house.

Class E works the same way. The 50 per cent curtilage test for a garden room or outbuilding counts every extension and every shed built since the original house, whether it needed permission or not. Two owners before you built a double garage. That garage is still on the clock.

Why 1 July 1948 fixes the baseline

The Town and Country Planning Act 1947 came into force on 1 July 1948, and that is when development control as we know it began in England and Wales. Before that date there was no national requirement to obtain planning permission for an extension, so Parliament drew a line and treated everything standing on the appointed day as the starting position.

The practical effect for most readers is simple. A Victorian terrace, a 1930s semi, an interwar bungalow: whatever was there on 1 July 1948 is your original house, including any Edwardian rear addition that predates the Act. A 1970s estate house or a new build from 2011 is measured as constructed, from the approved drawings.

Two traps sit either side of that line. A pre-1948 house that was partly demolished and rebuilt later is still assessed against its 1948 form, which can be hard to evidence. And a replacement dwelling built under a planning permission starts a fresh baseline on completion, so the allowances reset even though the plot has been developed before.

How earlier extensions consume the allowance

Permitted development rights are not renewed by a change of ownership, a sale, or the passage of time. If a previous owner used the full 3m rear depth, there is no further rear depth available under Class A, and any additional enlargement needs planning permission.

Three points follow from that and they catch people out regularly:

1. Extensions built under a planning permission still count. A grant of permission for a rear extension does not sit outside the GPDO arithmetic. The extension is part of the current house and the original rear wall stays where it was.
2. Extensions built unlawfully also count. Immunity from enforcement makes the building lawful, but it never makes it part of the original dwellinghouse.
3. An extension that has since been demolished still counts for ground coverage only while it stands, but the original rear wall never changes either way.

The cumulative test in Class A is about the enlarged part of the dwellinghouse taken as a whole. Councils read it that way and so do inspectors on appeal. If the case officer asks for a historic plan showing the rear elevation before 1948, that is what they are testing.

Conservatories, porches and outbuildings

A conservatory is an extension. There is no separate conservatory exemption in the GPDO, and the persistent belief that conservatories are free of planning control comes from the building regulations, which exempt certain conservatories under 30m2 from most requirements. Different regime, different rules. Every square metre of conservatory counts towards the Class A depth and the 50 per cent curtilage cap.

Porches sit under Class D with their own limits: no more than 3m2 of external ground area, no higher than 3m, and at least 2m from any boundary with a highway. A porch built within those limits does not consume Class A allowance, because Class D is a separate grant.

Outbuildings under Class E are the reverse case. They do not affect the rear extension depth, but they are counted in the 50 per cent curtilage calculation alongside every extension, so a large garden office can quietly close off a future extension. Ordering the works matters more than most people expect.

Proving the original footprint

The burden of showing that works are permitted development sits with the owner. Councils rarely hold usable pre-1948 records for an individual house, so the evidence usually has to be assembled from elsewhere.

Two dated sources that agree with each other are usually enough for a case officer. One undated photograph is not. Where the plot carries restrictions recorded against the title, they may also appear as local land charges, which is worth checking at the same time as the planning history.

The only formal way to settle the question is a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 for proposed works, or section 191 for works already carried out. It costs a fee, it takes eight weeks, and it gives you a document a buyer's solicitor will accept.

An illustrative twice-extended semi

This example is illustrative. A semi-detached house built in 1936 has a 2m deep kitchen addition put on in 1988 under a planning permission, and a 3.5m by 3m conservatory added in 2004 under permitted development.

The owner now wants a 3m rear extension and assumes the allowance is untouched because the conservatory "was only a conservatory". It is not. The original rear wall is the 1936 one. The 1988 addition already used 2m of the 3m available to a semi-detached house, so at most 1m of further depth is available in that position, and the conservatory has taken depth as well where it projects beyond the original wall. Ground coverage from both additions counts towards the 50 per cent curtilage cap.

The realistic options are a householder planning application, the larger home extension prior approval route if the proposal is a single-storey rear extension within the 6m limit, or a redesign that works within what is left. A householder application needs drawings, a completed form and, on most validation lists, a short written justification. PlanScope can draft the supporting documents once the constraints are known. Remove the conservatory first and the arithmetic changes again, which is why the sequence of works is worth planning before anyone orders materials.

Local policy sits on top of all of this. Article 4 directions, conservation area status and conditions on the original permission can all withdraw rights that the GPDO would otherwise grant, and those sit in the development plan and the council's own records rather than in the Order.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

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