Rear extension depth: the 3 and 4 metre lines

A single-storey rear extension can be built under permitted development to a depth of 3 metres beyond the rear wall of the original house if the house is semi-detached or terraced, or 4 metres if it is detached. Those are the depths in Class A of the householder permitted development rights, and they apply with no notification to the council at all. Go deeper, up to 6 metres or 8 metres on the same split, and you enter the larger home extension procedure, which means telling the council first and waiting.

The two sets of numbers get muddled constantly, usually because the larger depths were introduced as a temporary measure and are now part of the order. Both still sit inside the same class, with the same height limits and the same disqualifications.

The depth limits that need no notification

The relevant restriction is paragraph A.1(f) of Class A, Schedule 2 Part 1 to the Town and Country Planning (General Permitted Development) (England) Order 2015. A single-storey rear extension is outside permitted development if it extends beyond the rear wall of the original dwellinghouse by more than 4 metres for a detached house, or more than 3 metres for any other house.

Semi-detached and terraced houses sit in that second category. So does an end of terrace, and so does a house attached to a garage that forms part of the original dwelling.

For an extension of more than one storey, the depth drops to 3 metres whatever the house type, and the extension must also sit no closer than 7 metres to the rear boundary opposite the rear wall. On a shallow 1960s plot that 7 metre figure is what kills a two-storey rear extension long before the depth does.

Height limits that apply at the same depth

Depth is only one of the tests, and a design that clears A.1(f) often fails on height.

A single-storey rear extension cannot exceed 4 metres in height overall. Where any part of the extension comes within 2 metres of the boundary of the curtilage, the height of the eaves cannot exceed 3 metres. That second limit is measured to the eaves, not the ridge, and it governs most narrow terraced plots where the extension runs up to the party boundary.

Heights are measured from the highest part of the adjoining ground surface. On a sloping garden, measure from the higher side. Builders tend to measure from the finished internal floor, which is not what the order says, and a 2.6 metre internal height with a shallow pitch can land above 3 metres at the eaves once the levels are taken properly.

Two further conditions attach to everything built under Class A. The materials must be similar in appearance to those of the existing house, which is where rendering or cladding decisions start to matter, and any upper-floor window in a side elevation has to be obscure-glazed and non-opening below 1.7 metres above floor level. Roof terraces, balconies and verandas are excluded from Class A altogether, so the choices set out in our piece on Juliet balconies and overlooking apply to the rear elevation too.

The larger home extension route

Paragraph A.4 of Class A allows a single-storey rear extension up to 8 metres for a detached house and up to 6 metres for any other house, subject to a prior approval procedure. The height limits above still apply, unchanged.

1. Send the council a written description of the proposed works including how far the extension projects beyond the original rear wall, its maximum height and its eaves height.
2. Include a plan of the site showing the proposed development, and the addresses of all adjoining premises, meaning any property sharing a boundary including to the rear.
3. The council notifies the adjoining owners and occupiers and allows 21 days for them to object.
4. If an adjoining occupier objects within that period, the council must consider the effect of the extension on the amenity of adjoining premises, and only that.
5. Development cannot begin until the council tells you prior approval is not required, or grants it, or 42 days pass from receipt of a valid notification without a decision.

The important feature of the procedure is how narrow the test is. The council is not assessing design, materials, overdevelopment of the plot or the loss of garden. Amenity of adjoining premises, on objection, is the whole of it. One objection from a neighbour whose kitchen window sits beside the boundary therefore matters far more than ten objections from across the street.

Work must also be completed by the deadline set in the order and the council told when it is finished. Keep the dated acknowledgement.

Measuring from the original rear wall

Everything turns on the rear wall of the original dwellinghouse. Original means the house as it was first built, or as it stood on 1 July 1948 if it is older than that.

A house extended by 3 metres in 1998 has already used the allowance. The new work is measured from where the 1948 or as-built rear wall stood, not from the back of the existing kitchen. Conservatories count. So do earlier outriggers added after the original construction, and the fact that the previous owner never told the council makes no difference to the arithmetic.

Two other allowances are consumed in the background. The total area of ground covered by buildings within the curtilage, excluding the original house, cannot exceed half the curtilage, which brings garages, sheds and garden rooms into the calculation. And the extension cannot extend beyond a side elevation that fronts a highway, which is the rule that makes front and side extensions harder than rear ones.

Find the original footprint before you draw anything. Old sale particulars, a 1970s mortgage valuation plan, historic Ordnance Survey extracts and the council's own online planning history all help. Where nothing survives, say so in the submission rather than assuming.

Designated land and Article 4 directions

The larger home extension route is not available on article 2(3) land. That means conservation areas, National Landscapes, National Parks, the Broads and World Heritage Sites, where the 6 and 8 metre depths simply do not exist and the 3 and 4 metre limits are the ceiling.

On the same land, rear extensions of more than one storey fall outside Class A entirely, cladding is excluded, and side extensions are not permitted.

Separately, an Article 4 direction can withdraw householder rights street by street. Many councils have made directions covering Victorian terraces outside conservation areas. A direction does not make the work unlawful, it makes it an application, so a modest rear extension in an Article 4 area needs a householder application with a short Planning Statement rather than a notification.

Getting the position confirmed

Permitted development is self-assessed, which is comfortable until a buyer's solicitor asks for proof. A lawful development certificate for proposed use or development under section 192 of the Town and Country Planning Act 1990 is the only formal way to establish that works would be lawful, and it is worth the fee where the depth is close to a limit or the original rear wall is uncertain.

Measure the eaves at the boundary, fix the original rear wall on a drawing, and check the council's online map for an Article 4 direction before the builder orders steel. Enter your postcode on PlanScope to see which documents an application would need if the extension turns out to fall outside Class A.

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.