Roof pitch and eaves height on a two storey side extension

A two storey side extension always needs planning permission. Class A of Part 1 of the General Permitted Development Order allows side extensions only where the enlarged part has a single storey, so the moment a first floor extends beyond a side elevation of the original house the permitted development route closes. Which means the roof is a matter of design judgement rather than a set of dimensions, and the two numbers officers argue about most are the eaves height and the ridge height relative to the existing house.

Where permitted development stops

Class A is precise about this. The rights do not extend to an enlarged part that has more than a single storey and extends beyond a wall forming a side elevation of the original dwellinghouse. Separately, where any part of the extension sits within two metres of the boundary of the curtilage, the eaves height of the enlarged part cannot exceed three metres.

Both limits push householders towards a full application for anything two storey at the side. That is not bad news. An application lets you set the roof properly instead of contorting the design around a rule written for single storey additions, and a householder application carries an eight week determination period.

Design guidance sits in the council's householder or residential extensions supplementary planning document. Find yours before drawing anything. The guidance is not law, and departing from it is possible with a reason, though an officer who has to justify the departure needs that reason in writing.

Why a set down ridge is asked for

Most councils ask for the ridge of a two storey side extension to sit below the ridge of the host dwelling, commonly by around half a metre, though the figure varies by authority.

The reasoning is about hierarchy rather than arithmetic. A side extension that matches the ridge exactly reads as a second house grafted onto the first, and in a pair of semis it destroys the symmetry the pair was designed around. Dropping the ridge tells the eye which part came first. It also keeps the extension subordinate, which is the word every householder design guide uses and the test an officer will apply.

There is a practical consequence upstairs. A set down ridge with a matched pitch reduces the width available at first floor, or forces the roof to start lower, and on a narrow plot that can cost a bedroom. Work the section before you commit to a layout.

Where the host roof is shallow, a half metre drop can leave almost no first floor headroom, and the honest answer is sometimes a different roof form rather than a squeezed version of the same one.

Eaves alignment and the street scene

Eaves lines carry more visual weight than ridge lines from ground level, because that is where the roof meets the wall and where the horizontal rhythm of a street is read.

An extension whose eaves align exactly with the host dwelling reads as part of the same building even where the ridge is set down. An extension with eaves at a different height reads as a bolt-on, whatever the materials do. On a street of similar houses the eaves line is often continuous across several properties, and breaking it is the change neighbours notice first.

That gives a simple design rule for most cases. Align the eaves, drop the ridge, keep the pitch. Where the plot falls away and level alignment is impossible, say so on the section and explain the response.

The set back at first floor

The second common requirement is that the front wall of the extension sits behind the front elevation of the house, often by around a metre.

This does two things. It stops the extension competing with the original frontage, and it keeps the original house legible as the principal form. On a semi it also preserves some difference between the pair, which matters where the neighbouring house is unaltered.

The set back is easier to accept than the set down, because it costs less floorspace, and it can be turned to advantage. A recessed entrance under the set back gives a covered threshold and somewhere for bins that is not the front garden.

Where the house sits on a corner plot, the council may also ask for a set back from the flank boundary to keep a gap between the extension and the pavement. Corner plots attract more scrutiny generally, because the flank elevation becomes a frontage.

Matching the pitch or changing it deliberately

Matching the existing pitch is the default and it is usually right. A 1930s semi with a 40 degree hipped roof extended with a 30 degree pitch will look wrong from the street even if every other dimension is correct.

There are cases for changing it. A flat roofed or shallow pitched link between the house and the extension can express the join and let the extension read as a separate, lower element, which sometimes works better on a wide plot than a continuous roof. A contemporary addition with a clearly different geometry can be a good answer where the host building has no strong character worth matching.

The test is whether the change is deliberate and explained, or accidental and careless. Officers can tell the difference. So can the neighbours who will comment on it.

In a conservation area, the tolerance narrows sharply and the roof form of the original building carries more weight, so the design case belongs in the supporting statement rather than in a phone call.

Gable, hip and how the roof reads from the highway

Three roof forms cover almost every two storey side extension.

A hip to match a hipped host roof, which is the quietest option and usually the safest.

A gable end, which gives more usable first floor space and reads as a bolder addition, and which councils often resist on a hipped street because it changes the roofscape.

A half hip or hip to gable arrangement, which is a compromise that can look considered or awkward depending on how it is detailed.

Look along the street before choosing. If every house has a hipped roof and one has been gabled, the gabled one will be visible from a hundred metres away, and that observation will appear in the officer's report.

Drawings that make the relationship obvious

Officers assess this from elevations, and a poor drawing set causes more delay than a poor design.

1. Draw existing and proposed front elevations at the same scale, side by side, with the ridge and eaves heights dimensioned on both.
2. Include the neighbouring house on the street elevation, because the relationship to it is the point.
3. Show a section through the junction where the new roof meets the old.
4. Dimension the set back at first floor and the set down at ridge as figures, not as notes.
5. Add a roof plan where the geometry is anything other than a simple continuation.

Where an application is finely balanced, a short design note in the Design and Access Statement explaining the subordination strategy carries real weight, and on a scheme with policy issues a Planning Statement does the wider job. Owners of houses in an area covered by an Article 4 direction should first check which permitted development rights have been withdrawn, because the baseline changes. And if a previous scheme has already been turned down, reworking the design against the stated amenity reasons is a better use of time than resubmitting the same roof.

Enter your postcode on PlanScope to see which reports a householder application on your property 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. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.