How to Plan a Side Extension Step-by-Step (2026 UK)
A side extension fills the gap down the side of your house to widen a kitchen or hallway. Done right it’s one of the best value-adding extensions — but the boundary, party wall and planning route trip up most homeowners. This 8-step framework takes you from feasibility check to Building Control sign-off, with realistic 2026 costs and timings, so you brief builders with confidence and avoid the expensive mistakes.
Planning a side extension — the short version
- Feasibility first: measure the side gap — PD limits single-storey side extensions to half the house width.
- Planning route: single-storey often PD; two-storey needs full permission (£258).
- Always needed: Building Regulations and, near the boundary, a Party Wall agreement.
- Typical timeline: 8–14 weeks on site for a single-storey side return.
- Get 3 like-for-like quotes on one written brief before you commit.
The order matters: confirm what you can build before you fall in love with a design, and sort the party wall before the builder starts. Skip a step and you risk a stop notice, a neighbour dispute, or a redesign. Follow the eight steps below and the build runs to plan.
The 8 Steps to Plan a Side Extension
1. Check feasibility & boundary width
Measure the side passage and confirm ownership of the boundary. PD caps a single-storey side extension at half the original house width and 4m high — if your gap is narrow, the usable internal width after walls may be tight.
2. Confirm the planning route
Single-storey side returns are often Permitted Development — get a Lawful Development Certificate (£129) to prove it. Two-storey side extensions, conservation areas and Article 4 properties need full planning permission (£258).
3. Commission structural & Building Regs
A structural engineer specifies foundations, the steel beam where you remove the old external wall, and the roof tie-in. Side extensions always require a Building Regulations application (£500–£900).
4. Serve Party Wall notices
Building on or near the boundary, or excavating within 3m of next door, triggers the Party Wall Act. Serve notice 1–2 months early; if dissented, each side appoints a surveyor and an award is agreed before work starts.
5. Vet and hire a builder
Get 3 fixed-price quotes on the same scope: foundations, structure, roof, glazing, electrics, heating, plaster and decoration. Check FMB or TrustMark membership, £2m+ public liability and recent references.
6. Sign a written contract
Agree a fixed price, a milestone payment schedule, a written variations process and a completion date. Hold a 5% retention for snagging for up to 3 months after handover.
7. Manage the build
A single-storey side return typically runs 8–14 weeks. Keep a signed variations log, take progress photos, and book Building Control inspections at foundations, damp-proof course, steel and completion.
8. Sign-off and update records
Collect the Building Regulations completion certificate, update buildings insurance and your EPC, and keep the LDC, party wall award and certificate with your deeds for resale. Release the retention once snags are closed.
Common Questions
Related Guides
More how-to, cost and planning guides for your extension.
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