Do I Need Planning Permission for a Two-Storey Side Extension? (2026 UK)
Short answer: almost always yes. Permitted Development only covers single-storey side extensions that are no more than half the width of the original house and under 4m high. Add a second storey and you are outside PD, so a full householder planning application is normally required — typically £258 and 8 weeks for a decision. Get the design and neighbour impact right first time and approval is very achievable.
Two-storey side extension — the short answer
- Planning permission: normally required — PD does not allow two-storey side extensions.
- Application type: Householder planning application, fee £258, ~8 weeks.
- Building Regs: always required, separate from planning (£500–£900 typical).
- Party Wall Act: almost always triggered when building on or near the boundary.
- Key constraint: a 1m set-in from the boundary at first-floor level and matching materials greatly improve approval odds.
A side extension is the one extension type Permitted Development treats most strictly: even single-storey side additions must be no more than half the width of the original house and single-storey. There is no PD allowance for a second storey on the side, so unless you have an existing lawful two-storey side structure, you will be making a full application. The good news is that two-storey side extensions are routinely approved where the design respects the street scene, keeps a set-back at first floor, and protects neighbours’ light and privacy.
When You Need Permission vs When You Don’t
How the route is decided for side extensions in England in 2026.
Always confirm with a Lawful Development Certificate (£129) if you believe a single-storey element is PD — it is the only document that proves it. For the two-storey element, budget for the £258 householder application and an architect or architectural technician to prepare drawings.
What Makes a Two-Storey Side Extension Get Approved
Set back at first floor
Keeping the first floor set in ~1m from the side boundary and slightly back from the front avoids a ‘terracing’ effect — the single most common approval condition.
Match the house
Matching brick, roof pitch and window proportions keeps it subordinate to the original — officers reject side extensions that dominate the street scene.
Protect neighbour light
Apply the 45° and 25° daylight tests to neighbouring windows. Obscure-glaze and fix any flank windows that would overlook to avoid privacy objections.
Sort the party wall early
Serve Party Wall notices 1–2 months before work. It runs separately from planning but stalls builds if left late where you excavate or build near the boundary.
Common Questions
Related Guides
More planning and cost guides for your extension project.
Planning for a single-storey side extension
When a side return slips through as Permitted Development.
Read Guide →Get 3 Free Side Extension Quotes
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