Set back on a two storey side extension from the flank
The set back at first floor exists to stop a street of semis reading as a terrace. Here is where the dimension comes from, when it is waived, and how to design the return without losing space.
Most councils' householder design guidance asks for a two storey side extension to be set back from the front wall of the house at first floor, usually by about a metre, and set down below the main ridge. The set back is there to stop a street of semi-detached houses reading as a continuous terrace once several owners have extended sideways. It is design guidance rather than law, which means it can be argued with, but the argument has to be about the character of your particular street.
Start with a point that surprises people. A two storey side extension is not permitted development in any circumstances. Class A allows side extensions at single storey only, no more than four metres high and no wider than half the width of the original house, so anything at first floor needs planning permission from the start.
Why the set back exists
Look along a street of 1930s semis. The rhythm comes from the gaps: pairs of houses, then a gap, then another pair. Those gaps let you see sky and greenery between the buildings, and they are what makes a suburban street read as houses rather than as a wall.
Fill the gaps at first floor, on both sides of each pair, and the street becomes a terrace. That is the terracing effect, and it is the single most common reason a two storey side extension is refused. It is a cumulative harm, which is why an officer may refuse yours even though it looks modest on its own, and why the same officer may point to a neighbour's extension that was approved before the gaps ran out.
Setting the first floor back from the front wall keeps a visible break in the frontage. Setting the roof down below the main ridge keeps the original house reading as the dominant form.
The usual dimensions, and where they come from
The figures that appear most often in householder design guides are a set back of about one metre at first floor, a set down of the ridge below the original ridge, and retention of a gap of around one metre to the side boundary at first floor.
None of these are national rules. They come from supplementary planning documents adopted by individual authorities, so the first thing to do is find your council's householder design guide and read the section on side extensions. Some authorities specify the set back precisely, some describe it qualitatively, and a few apply different figures in different character areas.
Where the guide is silent, the general design policies in the local plan apply, and the argument runs on character and subservience rather than on a number. The principle of keeping an extension visually secondary to the host dwelling is set out in more detail in our note on subservience in side extension design.
The three moves that make it work
A well designed two storey side extension usually does all of the following.
- Sets the first floor front wall back from the main front elevation, so the original house line stays legible.
- Drops the ridge below the original ridge, and often the eaves as well, so the roof reads as an addition.
- Keeps a gap to the boundary at first floor, or at least maintains the visual gap where a neighbour's flank is close.
The ground floor can usually come forward to the main front wall and can often extend closer to the boundary, because the effect at ground level is much less visible from the street. That is the compromise most officers will accept: full width at ground floor for the kitchen or utility, set back above.
Where a hipped roof exists, matching the hip on the extension almost always looks better than a gable, and it also reduces the apparent bulk from the street.
Where the set back gets waived
There are streets where the set back adds nothing, and an officer will say so if you make the case.
A house at the end of a terrace, where there is no gap to protect and the extension continues an established building line. A corner plot where the flank faces a side road, so the extension is read against a different street entirely. A street that already has continuous frontage, where the rhythm being protected does not exist. A plot with a wide side garden, where the gap to the boundary remains generous even with a full width first floor.
In each case the argument is evidential. Photograph the street in both directions, mark the existing pattern on a street scene drawing, and show that the character your council's guidance is protecting is either absent or unaffected. An officer who can see the point from the drawings does not need to visit twice.
Designing the return without losing rooms
The complaint from householders is that a metre of set back costs a metre of bedroom. It does not have to.
Use the set back as an architectural feature rather than a leftover. A recessed entrance with a canopy at ground level, with the first floor above it, uses the space. A stair or landing window in the returned wall brings light into the middle of the plan, which is usually the darkest part of an extended semi. A small projecting bay at ground floor under the set back gives back floor area where it is most useful.
And check the internal planning before you fix the elevations. A bedroom that loses a metre at the front can often gain it at the rear, where the extension meets a rear projection, and a wraparound arrangement can recover the area entirely. Wraparound schemes do bring their own permitted development complications, covered in wraparound extensions and the permitted development trap.
What to show on the drawings
Validation officers and case officers both look for the same things, and applications that show them get through faster.
- existing and proposed front, side and rear elevations at a consistent scale, with the set back and set down dimensioned
- a street scene elevation showing your house and at least the two properties either side, which is the drawing that answers the terracing question directly
- a block plan showing the gap to the side boundary at both ground and first floor
- a roof plan where the roof form is doing work in the design
- materials annotated, matching the host dwelling unless there is a reason not to
For anything beyond a straightforward case, a short Design and Access Statement explaining the design response to the street pattern is worth writing, and where the property sits in a conservation area a heritage assessment will be needed as well.
Illustrative example: a 1930s semi with a 2.4 metre side gap. The proposal takes the ground floor to within 0.5 metres of the boundary and sets the first floor back one metre from the front wall with the ridge 0.5 metres below the original. A stair window sits in the set back return. The street scene drawing shows the neighbouring pair unextended, so the gap in the street rhythm survives. Illustrative only, and your council's guidance may ask for different figures.
One last practical point that has nothing to do with planning. Building up to or close to a boundary usually engages the Party Wall etc. Act 1996, which requires notice to the adjoining owner. That is a separate legal process from the planning application, and starting it early avoids a delay at the point you want to build.
This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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Frequently asked questions
Is a two storey side extension ever permitted development?
No. Class A of the householder permitted development rights allows side extensions at single storey only, up to four metres high and no wider than half the width of the original house. Anything at first floor on the side of a house needs planning permission, and on conservation area land side extensions are not permitted development at all.
Where does the one metre set back come from?
From householder design guidance adopted by individual councils as supplementary planning documents, not from national rules. The figure of about a metre at first floor, with the ridge set down below the original, appears in many such guides. Find your council's householder design guide and read its section on side extensions before fixing the drawings.
What is the terracing effect?
The loss of the gaps between pairs of houses, so that a street of semi-detached properties starts to read as a continuous terrace. It is a cumulative harm, which is why an extension that looks modest on its own can still be refused, and why the pattern of existing extensions along the street is part of the assessment.
When will a council accept no set back?
Where there is no gap in the street rhythm to protect: an end of terrace continuing an established building line, a corner plot whose flank is read against a side road, a street that already has continuous frontage, or a plot wide enough that a generous gap survives. Make the case with photographs and a street scene drawing.
Which drawings should I submit?
Existing and proposed elevations at a consistent scale with the set back and set down dimensioned, a street scene elevation including the neighbouring properties either side, a block plan showing the boundary gap at ground and first floor, a roof plan where the roof form matters, and annotated materials.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.