Two-storey rear extensions: the 3m and 7m rules

Two-storey extension permitted development is governed by two numbers that do not appear anywhere in the single-storey rules. The extension must not project more than 3 metres beyond the rear wall of the original house, and no part of it may sit within 7 metres of the boundary opposite that rear wall. Both apply to every house type. A detached house gets no extra depth at two storeys, which is the point most people find surprising.

Fail either and the extension is not permitted development, and there is no prior approval route to rescue it as there is for deeper single-storey extensions.

The 3 metre cap and the 7 metre rule

The depth is measured from the rear wall of the original dwellinghouse, meaning the house as built or as it stood on 1 July 1948. If a previous owner added a single-storey rear extension in 1990, your 3 metres is measured from the 1948 wall, not from the back of that extension.

The 7 metre rule is the one that quietly removes the rights from most modern estates. It measures from the extension to the boundary of the curtilage that lies opposite the rear wall of the house, which in ordinary language means the rear boundary. A garden 20 metres deep leaves plenty of room. A garden 9 metres deep, which is common on housing built since the 1980s, leaves 2 metres of usable projection before the 7 metre zone is reached, and a garden under 7 metres deep leaves none at all.

Measure the garden before drawing anything. It takes five minutes and it settles the question.

That last row matters. On land in a conservation area, a National Park, a National Landscape, the Broads or a World Heritage Site, rear extensions of more than one storey fall outside Class A entirely, as do side extensions and most cladding.

Roof pitch, eaves and the conditions that bind you afterwards

Class A attaches conditions as well as limits, and two apply specifically to two-storey work.

The roof pitch of the enlarged part must, so far as practicable, match the roof pitch of the original house. This is not a design aspiration but a condition of the permission, and an extension built with a shallow pitch on a house with a steep one may not be lawful even where every dimension is right.

Materials must be of similar appearance to those of the existing house. Similar rather than identical, which gives some latitude, though matching brick from a different batch remains the usual expectation and a rendered extension on a brick house will be argued about.

The eaves condition is a limit rather than a condition, but it has the same effect. Where any part of the extension comes within 2 metres of a boundary, the eaves height must not exceed 3 metres. At two storeys that is a hard constraint, because a first floor with a normal ceiling height and a floor zone puts the eaves above 3 metres in almost every case. The practical result is that a two-storey extension within 2 metres of a flank boundary is rarely permitted development.

Windows in side elevations at first floor

Any window installed in a wall or roof slope forming a side elevation of the house, at first floor level or above, must be obscure glazed, and non-opening unless the opening part is more than 1.7 metres above the floor of the room in which it sits.

This is a condition attached to the permission, so it binds the completed building. Replacing the obscure glazing with clear glass later is a breach, and it is one that neighbours report.

The 1.7 metre measurement is from the internal floor level, not from the ground. On a landing or a bathroom that is straightforward. On a bedroom with a low cill it usually means the window has to be fixed shut, which has consequences for ventilation that need resolving under the Building Regulations rather than ignored.

Why detached houses get no extra depth

Under the single-storey rules, a detached house gets a deeper allowance than a semi or a terrace, and deeper still through the prior approval procedure. None of that carries over. The two-storey limit is 3 metres for every house type, and there is no prior approval route for it.

The reasoning is about impact rather than about fairness. A single-storey extension mostly affects the ground floor outlook and the garden. A two-storey extension affects first floor windows, casts a longer shadow and reads as a change to the form of the house from a much wider area. The government drew the line at 3 metres across the board and left everything deeper to be judged on its merits.

That means a scheme for a 4 metre two-storey rear extension is a planning application, not a certificate application, and it should be designed as one from the start rather than value engineered down to 3 metres and back up again.

Making the design argument on a full application

Where the limits bite, the application turns on subordination and on impact to neighbours. Officers look for three things.

The extension should read as an addition rather than as a doubling of the house. Setting the ridge down below the main ridge, setting the flank in from the original wall by a modest amount, and keeping the eaves line consistent all achieve that. A two-storey extension the full width of the rear elevation with a matching ridge height is the hardest version to argue.

Neighbour impact is assessed against the council's adopted design guidance, which usually contains a 45-degree test taken from the nearest habitable room window and a separation standard for facing windows. Draw those tests on the submitted plans rather than waiting for the officer to draw them.

Overlooking from new first floor windows should be addressed on the drawings: which windows serve which rooms, what they face, and where obscure glazing or a change of position resolves the issue.

A planning statement that works through those three points in order, and a design and access statement that explains the massing decisions, do more than any amount of description of the internal layout. On a listed building or in a conservation area, a heritage statement is needed as well, and the starting position is harder because permitted development has already been removed.

Deciding which route to take

Work through it in order. Measure the garden depth to the rear boundary. Measure the projection you want from the original rear wall. Check whether any part comes within 2 metres of a flank boundary. Check the designation status of the land.

If all four answers are inside the limits, apply for a Lawful Development Certificate for proposed development, since it is the only formal way to confirm that works are lawful and it costs half the fee of a full application. If any answer is outside them, go straight to a householder planning application and spend the design effort on subordination instead of on trimming dimensions. The single-storey rear extension depth limits apply alongside these, because many schemes end up as a two-storey element with a deeper single-storey element beside it, and each part is assessed against its own rules.

This is general guidance, not legal advice. Permitted development rights vary by site and can be removed by condition or Article 4 direction. Check the position with your local planning authority or a qualified professional before relying on it.

Enter your postcode on PlanScope to see which documents a two-storey extension application 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.