Rear extension depth from the original rear wall

The 3 m and 4 m rear extension limits are measured from the original rear wall, not from whatever is standing there today. If the house already has a 2 m kitchen extension built in 1995, a semi-detached property has 1 m of the 3 m allowance left, and a further 3 m extension from the current back wall needs planning permission. The word "original" is doing all the work in that sentence, and it has a fixed legal meaning.

Article 2(1) of the General Permitted Development Order 2015 defines original in relation to a building as it was built, or, if it was built before 1 July 1948, as it existed on that date. So for a Victorian terrace, the datum is the back wall as it stood on 1 July 1948, including any Edwardian or interwar addition, but excluding everything built since. For a house built in 1972, the datum is the 1972 footprint.

Why the datum is the original wall

The purpose is to stop incremental growth. Without it, a house could be extended 3 m every few years and the cumulative effect on the neighbours would never be assessed. Fixing the measurement to the original wall means the total permitted enlargement across the life of the building is the same whether it happens in one go or in five.

The same principle runs through the rest of Class A. The 50 per cent limit on how much of the curtilage can be covered by buildings is measured against the curtilage of the original dwellinghouse, and it counts every outbuilding and extension added since, as our note on converting an outbuilding into habitable accommodation explains.

One consequence people find counterintuitive: it makes no difference whether the earlier extension was permitted development or had planning permission. Both count against the allowance. A permission granted for a 4 m extension on a semi does not reset the datum.

The dates that fix what original means

Two dates matter and they are easy to mix up.

1 July 1948 is the appointed day when the modern planning system came into force. A house standing before that date has its original form frozen at that date, whatever was done to it in the previous century.

The date of construction applies to everything built afterwards. A 1965 semi extended in 1980 has its original rear wall where the 1965 builders left it.

There is no rolling window. An extension built forty years ago still counts. The only thing that changes over time is the difficulty of proving what was there.

Single storey depth limits

For a single storey rear extension under Class A, the limits on how far it can extend beyond the rear wall of the original dwellinghouse are:

Alongside the depth, the height limits apply: 4 m overall for a single storey rear extension, and eaves no higher than 3 m where any part is within 2 m of a boundary of the curtilage.

For an extension of more than one storey, the rear projection limit is 3 m regardless of house type, and no part may come within 7 m of the boundary opposite the rear wall. Two-storey rear extensions on ordinary suburban plots often fail on that 7 m rule rather than on the depth.

On article 2(3) land, which includes conservation areas, National Landscapes, National Parks and World Heritage Sites, rear extensions are limited to a single storey, side extensions are excluded altogether and the larger home extension scheme does not apply.

The prior approval route for deeper extensions

Between the standard limit and the larger limit sits the neighbour consultation scheme in paragraph A.4 of Class A. It is not planning permission and it is not a free pass.

The procedure runs like this:

1. Submit a written description of the proposed development, including how far it extends beyond the original rear wall, the maximum height and the eaves height, together with a plan of the site and the addresses of adjoining premises.
2. The council notifies the owners and occupiers of adjoining premises, meaning those sharing a boundary, and allows 21 days for representations.
3. If nobody objects within that period, the council cannot refuse on amenity grounds and prior approval is not required.
4. If an objection is received, the council considers the impact on the amenity of adjoining premises, and only that.
5. The council has 42 days from receipt to notify its decision. If it does not, the development may proceed as described.
6. The developer must notify the council of completion as soon as reasonably practicable.

Three practical notes. Only adjoining premises are consulted, so a house across the road has no standing. The council's consideration is limited to amenity, so objections about parking or drainage carry no weight. And the description you submit is what you are permitted to build, so get the dimensions right first time.

Cumulative effect of earlier additions

The arithmetic is simple once the datum is right, but the survey usually is not.

Take a semi-detached house with an original rear wall, a 1.5 m single storey kitchen extension added in 1988, and a proposed further extension. The remaining standard allowance is 1.5 m measured from the original wall, which means only 1.5 m beyond the 1988 extension's rear face would be permitted development. Under the larger scheme the total from the original wall could reach 6 m, leaving 4.5 m of new projection.

Two refinements. A conservatory counts, even though people assume it does not, because it is an enlargement of the dwellinghouse. And a detached garage or outbuilding in the garden does not count towards the rear projection limit, though it does count towards the 50 per cent curtilage figure.

Small projections need care as well. A bay window forming part of the original house sets the datum at the main wall behind it in most readings, and the treatment of bays, canopies and porches is set out in our note on bay windows, canopies and small projections.

Where the extension turns the corner, the side element brings in a different set of limits, covered in our note on wrap around extensions and the Class A limits.

Proving the original footprint

The council will ask, and the burden is on you. Useful evidence, in roughly descending order of usefulness:

• Historic Ordnance Survey maps at 1:1250 or 1:2500, which show building footprints and are held by county record offices and major libraries.
• The council's own planning history for the property, which is often online back to the 1970s and available as a file request before that.
• Aerial photography, including postwar survey imagery for the 1948 baseline.
• Title deeds and the original conveyance plan, particularly for interwar estates.
• Building control records for the later extensions.
• A statutory declaration from a long-standing owner or neighbour, used where documentary evidence is missing.

Where the position is unclear, or where the calculation is close to the limit, apply for a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 before building. It costs half the planning application fee for a proposed use, it produces a formal statement that the works are lawful, and it is the document a buyer's solicitor will ask for in ten years. A refused certificate also tells you the position without the risk of an enforcement notice.

Set the evidence out in a short Planning Statement with the historic maps attached and the measurements annotated on the existing plan. Case officers assessing a certificate application are looking for a clear chain from the original footprint to the proposal, and an unexplained set of drawings makes that harder than it needs to be.

This is general guidance, not legal advice. Permitted development rights turn on the precise wording of the Order and on whether rights have been removed by condition or by an Article 4 direction, and a Lawful Development Certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional.

Enter your postcode on PlanScope to see which documents your extension is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.