House Extension Costs UK 2026: Single & Double Storey Guide

A single-storey rear extension typically costs £1,800–£3,000 per square metre in the UK. For a common 4×5m (20m²) extension, expect to pay £36,000–£60,000 fully finished.

Double-storey extensions cost more per m² to design and plan but less per m² to build than single-storey (the foundations and roof are shared). A 20m² footprint double-storey extension typically costs £55,000–£100,000.

Location matters significantly: London and the South East add 20–40% to national averages. Northern England and Wales are typically 10–15% below the national average.

Single-storey rear extension

• Cost: £30,000–£70,000 (20–30m²)
• Most popular type. Usually covered by Permitted Development rights if under 4m depth for detached houses (3m for semi-detached/terraced).

Double-storey rear extension

• Cost: £55,000–£110,000 (same footprint as above but two floors)
• Almost always requires full planning permission. Adds significant living space and property value.

Side return extension

• Cost: £25,000–£50,000
• Common in Victorian terrace houses. Fills in the alley beside the kitchen to create open-plan space. Often combined with a rear extension.

Wrap-around extension

• Cost: £50,000–£120,000+
• Combines rear and side return into an L-shaped or wrap-around footprint. Creates dramatic open-plan kitchen-diners.

Two-storey side extension

• Cost: £60,000–£130,000
• Effectively widens the entire house. Requires planning permission and is subject to neighbour consultation.

A fully finished extension cost includes:

• Foundations and groundwork: £5,000–£15,000
• Structure (walls, RSJ beams if needed): £8,000–£25,000
• Roof: £5,000–£15,000 (flat roof on single-storey is less; pitched roof to match the house is more)
• Windows, doors, and glazing: £3,000–£15,000+ (bifold/sliding doors are a major cost driver)
• Insulation and air tightness: £1,500–£4,000
• Internal fit-out (plastering, flooring, decorating): £4,000–£12,000
• Electrics and plumbing: £2,000–£8,000

Fees and extras (not always included in builder quotes):

• Architect/designer fees: £1,500–£5,000
• Structural engineer: £500–£1,500
• Planning application: £258 in England
• Building regulations application: £500–£900
• Party wall surveyor (if required): £700–£1,000 per surveyor

Many extensions are permitted under Permitted Development (PD) rights, meaning you don't need full planning permission. However, PD rights have strict limits:

Permitted Development rules for a rear extension (England):

• Detached houses: up to 4m depth (or 8m under the Neighbour Consultation Scheme)
• Semi-detached/terraced: up to 3m depth (or 6m under the Scheme)
• Maximum height of 4m
• No more than half the original garden can be covered

You will need planning permission if:

• Your property is in a Conservation Area or AONB
• It's a listed building
• You've already used PD rights
• The extension exceeds PD limits
• It includes a raised platform or balcony

Always get a Certificate of Lawful Development even if PD applies — it protects you when you sell. Cost: £103–£206 in England.

A typical single-storey extension takes 8–16 weeks on site from groundwork to completion:

• Groundwork and foundations: 1–2 weeks
• Walls and structure: 2–4 weeks
• Roof: 1–2 weeks
• Windows and doors: 1 week (lead times can be 4–8 weeks, order early)
• First fix services (electrics, plumbing): 1 week
• Plasterboard, insulation, plastering: 1–2 weeks
• Second fix and fit-out: 2–3 weeks

Add 4–12 weeks for the design and planning phase before work starts on site.

• Get 3 quotes from local builders and extension specialists. Prices can vary by 30–50% for identical specs.
• Appoint an architect before approaching builders: A proper set of drawings means every builder is quoting the same spec, making comparisons meaningful.
• Watch out for low quotes: A builder who quotes £15,000 less than others is likely leaving something out. Ask for itemised quotes.
• Manage cash flow carefully: Most builders work on staged payments. Never pay more than 10–15% upfront and agree a payment schedule tied to completion milestones.
• Consider a project manager: For extensions over £50,000, a project manager (1–3% of contract value) can save money and stress.