The corner plot myth
Corner plots often have a lot of open space to the side, and the natural assumption is that more space means more freedom to build. It’s usually the opposite. That extra space exists precisely because corner plots face two roads — and it’s that second road frontage, not the space itself, that changes everything.
Most side extensions can be built under permitted development, without a full planning application. Corner plots very often can’t. If your side extension would face that second road, it doesn’t comply with the standard permitted development rules at all, regardless of size — it needs a full household planning application instead.
What permitted development allows, for comparison
So you can see exactly what a corner plot loses access to, here’s what a standard side extension is normally allowed under permitted development:
– Up to 50% of the width of the original house — so a house 7 metres wide could take a side extension up to 3.5 metres wide
– Up to 4 metres in height with a pitched roof, or 3 metres with a flat roof
– Built to the same depth as the house, and even extending past the rear by 3 metres on a semi-detached or terraced property, or 4 metres on a detached property
– Built in line with the front wall of the house
– Must not wrap around the corner of the house — this is the specific rule that rules most corner plots out
A standard side plot, with only one road frontage, can generally use these rules without a planning application at all. A corner plot facing a second road almost always can’t, no matter how much space is available.
The “build line” restriction
Even once you’re in household application territory, there’s a further constraint specific to corner plots: the build line of the road behind you. This is an imaginary line drawn across the front of all the houses along that second road.
A side extension generally can’t breach it, even where it would otherwise fit within the standard width and depth limits above. This can end up restricting the usable width of your extension more than you’d expect, and it’s easy to miss if you’re only looking at the rules for your own road.
It’s also worth knowing that a neighbour further down the road may have built a larger side extension in the past. That doesn’t mean the same is available to you — plot position, the second frontage, and the build line all vary house by house, so what one neighbour achieved isn’t necessarily transferable to yours.
What this means for you
A corner plot doesn’t mean no side extension — it means a different route to get there. Assume you’ll need a household planning application rather than permitted development, and check the build line on your specific plot before you settle on a design, because it can shape the extension’s width more than the standard rules alone would suggest.
