The right extension for your home
The best extension isn't the biggest — it's the one that fits your house, your plot and how you actually live. London's housing stock rewards different approaches: a Victorian terrace wants a side-return, a 1930s semi suits a double-storey rear, a detached home can take a wrap-around. We start by reading the property, not by selling you a standard box.
Types of extension we build
Single-storey rear extension
The classic: extending into the garden to create an open-plan kitchen-diner with bifold or sliding doors and roof lights. Usually permitted development on a house, so often no full planning application. The most requested project we do.
Side-return extension
Filling the narrow alley alongside a Victorian or Edwardian terrace — typically 1–1.5m wide, but it transforms a cramped galley kitchen into a wide, light-filled space. Small footprint, huge impact, and usually permitted development.
Wrap-around extension
Combining a rear and a side-return into an L-shape for the maximum ground-floor space. Ideal where you want a genuinely large kitchen-living-dining room. More often needs planning permission because of the footprint.
Double-storey extension
Adding downstairs living space and an extra bedroom or bathroom above in one project. It's the best value per square metre — the expensive elements (foundations and roof) are shared across two floors, so you get roughly double the space for 50–65% more cost than a single-storey.
What an extension costs
| Type | Typical London cost | Build time |
|---|---|---|
| Side-return | £55,000–£90,000 | 8–12 weeks |
| Single-storey rear (20–30 m²) | £45,000–£95,000 | 10–14 weeks |
| Wrap-around | £90,000–£150,000 | 14–18 weeks |
| Double-storey | £80,000–£170,000 | 14–20 weeks |
Ranges are for a completed, mid-specification project in London — a starting point, not a fixed quote. For the full breakdown by size, spec and cost-per-m², read our house extension cost guide.
How we run the project
A good extension is 90% preparation. Here's how we take it from idea to finished room:
- Free survey & brief — we visit, measure, and give an honest view on what's possible and what it'll cost.
- Design & drawings — architectural plans and a structural engineer's calculations for the steelwork.
- Planning or permitted development — we submit the application or secure a Lawful Development Certificate.
- Party wall — where you share walls, we coordinate the party-wall awards with neighbours.
- Fixed-price build — groundwork, structure, roof, glazing, first and second fix, plastering and finishes.
- Building control & handover — final inspection, completion certificate, snagging and clean handover.
Planning permission — do you need it?
Many single-storey rear and side-return extensions are permitted development and need no planning application, within size and height limits. But conservation areas, flats and maisonettes, and homes that have already been extended usually do need to apply. We check your exact address first — and always secure a Lawful Development Certificate even when permission isn't required, because your future buyer's solicitor will ask for it. Read our guide to permitted development in London for the full rules.
Why build with Delta
- Director-led — you deal with the person responsible for the job, not a call centre.
- Fixed-price quotes — itemised and honest, so you know the number before we start.
- Everything under one roof — design, planning, structure, party wall and build, all coordinated.
- Guaranteed — 1-year workmanship guarantee, £2M public liability insurance, building-regs sign-off as standard.
- Local — based in Hounslow, working across West, South West, North, South East, East and Central London.
Frequently asked questions
How much does a house extension cost in London?
A single-storey rear extension typically costs £45,000–£95,000, a side-return £55,000–£90,000, and a double-storey £80,000–£170,000 — roughly £2,200–£4,500 per m². We give a fixed-price quote after a free survey. See our extension cost guide.
Do I need planning permission for an extension?
Many single-storey rear and side-return extensions are permitted development and need no application, within size limits. Conservation areas, flats and previously-extended homes usually need to apply. We check and handle it either way.
How long does an extension take?
Around 10–14 weeks on site for a single-storey rear, 8–12 for a side-return, 14–20 for a double-storey — plus 6–10 weeks beforehand for design and approvals.
Do you handle planning, structure and party wall?
Yes — drawings, structural calculations, planning/permitted-development, party-wall awards and building-regs sign-off are all managed under one roof.
Are you insured and guaranteed?
Yes — 1-year workmanship guarantee, £2M public liability insurance, and building-regulations completion sign-off as standard.
Related
Get a fixed-price quote for your extension
Send us your address and a rough idea of what you want — a director will reply within one working day with an honest view on cost, timeline and whether you'll need planning. No obligation.
Request a free quote