Single storey side extensions are often permitted development. Two-storey side extensions never are. In conservation areas and on semi-detached properties, the restrictions tighten further than most people expect. The half-width rule is also calculated against the original house — not the house as it currently stands.
Single storey: usually permitted development within strict limits. Two storey: always needs full planning permission. No exceptions.
A single-storey side extension is permitted development under Class A of the GPDO if it is no wider than half the width of the original house, does not exceed 4 metres in height, does not extend beyond the front wall of the original house, does not take garden coverage over 50%, the property is not listed and not in a conservation area, and no Article 4 Direction or planning condition applies.
There are no exceptions to this rule. A two-storey side extension requires a full planning application regardless of its size or the type of property. This is one of the clearest rules in permitted development and one of the most commonly misunderstood.
The half-width rule is measured against the original house — not the house as it currently stands. If a previous owner already built a single-storey side extension, that may have used up the half-width allowance. Another side extension would then require full planning permission. The planning history reveals what previous owners built.
In a conservation area, side extensions that would be visible from a highway require planning permission regardless of size. Some estates also carry conditions requiring gaps between semi-detached houses to be maintained — these show in the planning history for the original development.
Standard searches check the public register. We go further — querying live portals, blocked legacy systems, pre-merger authority databases, committee PDF archives, Land Registry title constraints, and comparable decisions across your postcode cluster. What we retrieve determines what you know before you build, buy, or appeal.