Two-storey side and rear extension on a semi-detached house

A two storey side extension needs planning permission. There is no route round it. Schedule 2 Part 1 Class A of the General Permitted Development Order 2015 allows a side extension only if it is single storey, no more than four metres high and no wider than half the width of the original house, so the moment you add a first floor to the side you are making a householder application. Wrap that side extension round to the rear and the rear element has its own limits to fail as well.

Which is not bad news. A householder application on a semi is eight weeks, a modest fee and a decision made by the case officer in most cases. The reason these schemes get refused is rarely the principle. It is the width, the gap to the boundary and what the first floor does to the neighbour.

Why the two storey form falls outside permitted development

Class A splits side and rear extensions. Side extensions: single storey only, four metre maximum height, half the width of the original dwelling. Rear extensions: a two storey rear extension is allowed on a semi only where it projects no more than three metres beyond the original rear wall and sits at least seven metres from the rear boundary. The seven metre rule is the one that kills most two storey rear extensions on ordinary suburban plots, because typical rear gardens on interwar semis do not leave seven metres once you have projected three.

"Original" means as the house stood on 1 July 1948, or as first built if later. A 1970s rear addition already built by a previous owner counts against your allowance. Check the planning history before you measure anything.

Where an Article 4 direction removes householder rights, or the house is in a conservation area, the calculation changes again. And if the house is listed, listed building consent is a separate application running alongside the planning one.

Width, gap and set back

Most adopted householder design guides converge on the same three moves for a first floor side extension on a semi:

• A set in of at least one metre from the side boundary at first floor level, so the gap between the pair of semis survives.
• A width of no more than half the width of the original house, borrowed from the permitted development test and applied as a proportionality check.
• A front elevation set back of around one metre from the main front wall, with the extension roof set down below the main ridge.

None of that is statute. It is design guidance, applied through the adopted plan's design policy and whatever householder supplementary planning document the council has published. Which means it is arguable, and the argument you win is the one about the gap.

The one metre gap does real work. On a pair of semis the space between the flank walls reads as rhythm along the street, and two neighbouring owners each building flush to the boundary produces a continuous two storey wall. Councils call that the terracing effect and they refuse it on character grounds. A first floor set in of a metre, carried up to the eaves, keeps the pair reading as a pair even after both are extended.

The first floor and the neighbour

A single storey rear extension affects a neighbour's ground floor windows and garden. A two storey one affects bedrooms, and that changes the tone of the consultation responses.

The tests case officers apply are loss of light, loss of outlook and overlooking, in that order of seriousness. For light, many councils use a 45 degree line drawn in plan and in elevation from the centre of the nearest habitable room window on the neighbouring property. If your two storey rear projection crosses that line, expect a request to reduce the depth or step the first floor back from the boundary.

Overlooking is easier to fix than light. A flank wall with no windows, or only obscure glazed and high level openings to bathrooms and landings, removes the objection. Councils will condition that glazing so it stays obscure. Where the impact is contested and the scheme is worth defending, a Daylight and Sunlight Assessment run against BRE 209 gives you numbers rather than assertions, though on a single householder scheme the cost is usually only justified when a refusal is likely.

Roof form and the terracing effect

The roof is where a competent scheme separates itself from a refused one. Three forms recur:

1. A hipped roof to the side extension, set down from the main ridge, which reads as subordinate and keeps the gap legible at roof level.
2. A continuation of the main ridge across the extension, which produces the widest and most dominant result and is the form most often resisted on a semi.
3. A flat or shallow mono pitch behind a parapet, which suits a rear element but looks alien on a side elevation in a pitched roof street.

Match the pitch and the materials to the host dwelling. Reclaimed or closely matched brick on the side elevation, the same tile profile, the same eaves and verge details. A case officer who can see the extension is an extension but cannot see a clash of materials will write a supportive report.

Parking and access

The overlooked issue. A two storey side extension usually goes over the driveway, and if the resulting frontage cannot hold the number of off street spaces the adopted parking standard requires for the new bedroom count, the highway officer will object.

Work it out properly before you draw. Count bedrooms after the extension, find the council's adopted parking standard for that number in that accessibility zone, and measure whether the remaining frontage takes the spaces at 2.4 metres by 4.8 metres each without blocking the front door or requiring a reversing manoeuvre onto a classified road. Where the driveway is lost entirely and the street is already under pressure, the case for the extension gets much harder.

A new or widened dropped kerb on a classified road is a separate consent from the highway authority under section 184 of the Highways Act 1980, and on a classified road it may also need planning permission in its own right.

The documents to submit

For a householder application on a semi outside a designated area:

1. The householder application form and the correct fee, taken from the current fees schedule rather than an old figure, because application fees are indexed annually from April.
2. A site location plan at 1:1250 with the application site edged red and any other land in the same ownership edged blue.
3. Existing and proposed floor plans and elevations at 1:100 or 1:50, plus a block plan showing the extension, boundaries and parking.
4. A street scene elevation showing your house and both neighbours, which is the single most useful drawing for a terracing argument.
5. A short Planning Statement addressing the adopted design policy, the gap, the parking and the neighbour impact.

A Design and Access Statement is required for householder development in a conservation area or to a listed building, and is optional elsewhere. In a conservation area you will also need a Heritage Statement setting out the significance of the area and how the extension affects it.

As an illustrative example: a 1930s semi with a 6.8 metre frontage, extending 2.6 metres to the side at both levels with the first floor set in one metre from the boundary, plus a 3 metre two storey rear projection stepped in from the party boundary and a single storey element beyond. Four drawings, a street scene, a two page planning statement. That is a scheme a case officer can recommend.

When to reduce the scheme

If the officer's early view is negative, the reductions that usually rescue the application, in order of how much floor area they cost you:

• Set the first floor side element in a further half metre and keep the ground floor where it is.
• Drop the two storey rear projection to 2 metres and take the extra depth at ground floor only, where Class A limits are more generous.
• Hip the side roof rather than gabling it.
• Retain one off street parking space by narrowing the ground floor side element at the front.

Ask for a pre-application view before you spend on the full drawing set where the scheme is finely balanced. It costs a published fee and it tells you which of those reductions the officer actually wants.

This is general guidance, not legal advice. Permitted development limits turn on the exact measurements and planning history of your property, and a Lawful Development Certificate is the only formal way to confirm that works are lawful, so check the position with your local planning authority or a qualified professional before relying on it.

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