Extension Costs UK 2026: Real Prices, Calculator & Scenarios

Extension costs UK in 2026 typically land between £1,900 and £2,700 per m² for a good-quality single-storey extension at UK-average rates, before glazing, fees and contingency. That is roughly £1,750–£2,500 per m² in the North of England and Wales and £2,400–£3,400 per m² in London. A 20m² single-storey rear extension with a bi-fold and two rooflights comes to about £51,000–£78,000 all-in at UK-average rates, and £64,000–£98,000 in London, before a kitchen.

Want a figure for your own project? Use our free extension cost calculator: set the size, storeys, finish and region and it gives an all-in range including fees, VAT and contingency. The per-m² rates in this guide are the same ones the calculator uses.

Below we break down what drives the figures, give you full worked scenarios for the most common UK property types, and cover the hidden costs that catch homeowners out. The scenarios are built from quotes we see week to week across the UK, not website averages.

How Much Does a House Extension Cost Per Square Metre in 2026?

The most reliable way to estimate extension costs is by square metre (m²). These are the rates our extension cost calculator uses for the works with a good-quality finish and normal side access, VAT included, before glazing, rooflights, fees and contingency:

Single-Storey Extension (per m²):

• London: £2,400 – £3,400
• South East: £2,150 – £3,000
• South West & East of England: £1,950 – £2,800
• Midlands: £1,850 – £2,600
• North of England & Wales: £1,750 – £2,500
• Scotland: £1,800 – £2,550
• Northern Ireland: £1,600 – £2,300

Two-Storey Extension (per m² of floor area, both floors added together):

• London: £2,050 – £2,950
• South East: £1,850 – £2,650
• South West & East of England: £1,700 – £2,400
• Midlands: £1,600 – £2,300
• North of England & Wales: £1,500 – £2,150
• Scotland: £1,550 – £2,250
• Northern Ireland: £1,400 – £2,000

Wrap-Around Extension (per m²): £2,100 – £3,000 at UK-average rates, about £2,600 – £3,750 in London, because there is more external wall and roof for every m² of floor.

Two-storey extensions are roughly 10–15% cheaper per m² because you pay for the foundations and roof once and get two floors out of them. The total is still higher because you are building twice the floor area. Finish matters too: a builder's standard finish is about 12% below these rates and a high specification about 28% above. Where everything has to come through the house, as on many London terraces, add roughly another 12%.

Quick Cost Formula: Estimate Your Extension

The extension cost calculator does the sums for you. For a quick ballpark by hand, use this formula:

Extension area (m²) × cost per m² for your region = base build cost

Then add:

• Professional fees (drawings, structural calculations, Building Control and any planning application): 9–14% of build cost
• Contingency: 10% minimum on a straightforward site, 15% on a pre-war property or where the ground is unknown
• Kitchen supply and fit (if applicable): £9,000 – £22,000 for mid-range units, worktops and appliances
• Making good the garden: £1,500 – £6,000

Example: A 30m² single-storey extension in the Midlands:

• Base build: 30 × £2,300 = £69,000
• Professional fees (12%): £8,280
• Contingency (10%): £6,900
• Kitchen fit-out: £12,000
• Making good the garden: £3,500
• Total estimated cost: £99,680

UK Extension Cost Scenarios by Property Type

These are realistic, all-in 2026 figures based on quotes we review every week. Each includes build cost, professional fees, Building Control, contingency and a mid-range kitchen or bathroom fit-out where relevant. Building Control is shown as a typical £1,000 allowance: councils set their own fees, so yours may be lower or higher.

Scenario 1: 4m × 5m (20m²) Single-Storey Kitchen Extension, Victorian Terrace, North London

• Base build: 20m² × £3,200 = £64,000
• Steel for opening up to existing kitchen: £4,500
• Bi-fold doors (3m): £3,800
• Professional fees + party wall surveyors (two neighbours): £12,500
• Building Control + planning (in Conservation Area): £1,548 (householder planning fee £548 + Building Control allowance £1,000)
• Contingency (12%): £8,400
• Mid-range kitchen supply & fit: £18,000
• All-in total: £112,748

Scenario 2: 5m × 6m (30m²) Wrap-Around Extension, Edwardian Semi, Manchester

• Base build: 30m² × £2,200 = £66,000
• Side return groundworks & drain diversion: £6,500
• Roof lantern + bi-folds: £6,800
• Professional fees (10%): £7,900
• Building Control + LDC: £1,274 (lawful development certificate £274 + Building Control allowance £1,000)
• Contingency (10%): £6,600
• Kitchen + utility refit: £14,000
• All-in total: £109,074

Scenario 3: 6m × 4m (24m²) Two-Storey Side Extension, 1930s Semi, Birmingham

• Base build (two floors): 48m² × £1,900 = £91,200
• Matching brickwork to existing: £3,500
• Bedroom + en-suite fit-out: £11,000
• Professional fees + structural engineer: £11,500
• Planning permission + Building Control: £1,548 (householder planning fee £548 + Building Control allowance £1,000)
• Contingency (10%): £9,100
• All-in total: £127,848

Scenario 4: 3m × 6m (18m²) Rear Extension, 1970s Detached, Leeds

• Base build: 18m² × £1,900 = £34,200
• Patio doors (standard 2.4m): £1,400
• Professional fees (12%): £4,100
• Building Control + LDC: £1,274 (lawful development certificate £274 + Building Control allowance £1,000)
• Contingency (10%): £3,420
• Kitchen refresh (keeping existing units): £6,500
• All-in total: £50,894

Scenario 5: 4m × 7m (28m²) Side Return + Roof Lantern, Georgian Townhouse, Bristol

• Base build: 28m² × £2,600 = £72,800
• Underpinning existing wall: £8,500
• Glazed roof lantern (4m × 2m): £5,200
• Professional fees + party wall: £11,500
• Planning (in Conservation Area) + Building Control: £1,548 (householder planning fee £548 + Building Control allowance £1,000)
• Contingency (15%): £10,900
• High-spec kitchen: £24,000
• All-in total: £134,448

Scenario 6: 30m² Single-Storey Extension, Sandstone Tenement Ground Floor, Glasgow

• Base build: 30m² × £2,100 = £63,000
• Stone-matching repairs to existing facade: £4,200
• Bi-fold doors: £3,500
• Professional fees + Scottish Building Warrant: £9,200
• Contingency (10%): £6,300
• Kitchen supply & fit: £12,500
• All-in total: £98,700

Extension Costs UK: Quick Reference Table

Project	Region	Typical all-in cost
15m² single-storey, no kitchen	North / Wales	£36,000 – £56,000
20m² single-storey, no kitchen	Midlands	£49,000 – £76,000
20m² kitchen extension (kitchen + steel)	Midlands	£62,000 – £110,000
20m² kitchen extension (kitchen + steel)	London	£80,000 – £142,000
30m² wrap-around, no kitchen	North / Midlands	£74,000 – £119,000
40m² two-storey (20m² footprint)	UK average	£80,000 – £120,000
50m² high-spec kitchen extension	London	£222,000 – £373,000

Every row comes from the extension cost calculator with a good-quality finish (high specification on the last row), normal side access, 9–14% fees and a 10% contingency. Single-storey and wrap-around rows assume a 3-panel bi-fold and two rooflights; the two-storey row assumes French doors and no rooflights; the last row assumes a slim-frame slider, a roof lantern, a kitchen and steel.

VAT note: most domestic extension work is charged at the standard 20% VAT rate and the figures above include it. Listed building alteration work can sometimes qualify for reduced or zero rating — ask your builder to confirm in writing.

Side Return, Wrap-Around and Basement Extensions

Side Return Extension

Common in Victorian and Edwardian terraces, a side return extension fills in the narrow alley down the side of the house and turns a dark, narrow kitchen into a bright, open one. Price it as a single-storey extension: £1,900 – £2,700 per m² at UK-average rates, about £2,400 – £3,400 in London. Side returns usually need a party wall agreement with the neighbour, often need steel to open up the old side wall, and where everything has to come through the house the calculator adds roughly 12%. Scenario 5 above is a worked side return budget.

Wrap-Around Extension

A wrap-around combines a rear extension and a side return to make the most of the ground floor. There is more external wall and roof for every m² of floor, so it costs more per m²: £2,100 – £3,000 at UK-average rates and about £2,600 – £3,750 in London. Scenario 2 above is a worked example, and you can choose Wrap-around in the calculator to price your own.

Basement Extension

Basements are the most expensive way to add space and are mainly built in high-value areas such as London, where there is no room to go out or up. A typical 40m² basement extension can cost £100,000 – £160,000 before fitting out. Our calculator does not price basements, because the cost depends so heavily on ground conditions and the structural work involved.

What's Included in the Per-Square-Metre Cost?

A typical per-m² price from a builder should include:

• Groundworks and foundations — Excavation, concrete, drainage connections
• Structural shell — Walls, steelwork, floor slab, roof structure
• Roofing — Tiles or flat roof covering, insulation, fascias and soffits
• Windows and doors — Including bi-fold or patio doors
• First fix — Plumbing, electrics, plastering
• Second fix — Sockets, switches, radiators, sanitary ware
• Decoration — Basic painting and finishing

What's usually NOT included:

• Kitchen or bathroom units and appliances
• Flooring (beyond screed)
• Landscaping and external works
• Furniture and furnishings
• Party wall surveyor fees

Always clarify exactly what's in and out of the quote. Our guide to builder quote red flags explains what to watch for.

The Hidden Costs Nobody Warns You About

These are the costs that catch homeowners off guard. Budget for them from the start:

1. Party Wall Agreements (£700 – £2,000+ per neighbour)

If your extension is on or near a shared boundary, or involves work on a shared wall, you'll need to serve a Party Wall Notice under the Party Wall etc. Act 1996. Serve it at least two months before work on a shared wall starts, or one month before digging foundations close to a neighbour's building; leaving it late is a common cause of delays. Each adjoining neighbour can agree in writing. If they don't, you'll need party wall surveyors, either one agreed surveyor or one each, and the cost normally falls on you as the person building.

2. Drainage Diversions (£3,000 – £15,000)

If a public sewer runs under your proposed extension, you'll need to divert it — or redesign. Thames Water and other utilities can take months to approve diversions, so check early.

3. Asbestos Removal (£1,500 – £5,000+)

If your home was built before 2000, there's a chance of asbestos in roof tiles, floor tiles, textured coatings, or insulation. Removal must be done by licensed contractors.

4. Ground Conditions

• Clay soil — May need deeper foundations (trench fill rather than strip). Can add £5,000 – £15,000.
• High water table — Additional waterproofing and drainage. £3,000 – £8,000.
• Trees nearby — Root protection zones can dictate foundation depth and design.

5. Temporary Accommodation

For major extensions, especially those involving the kitchen, you may need to live elsewhere for 2-4 weeks. Budget £2,000 – £5,000 for temporary rental or hotel.

Planning Permission vs Permitted Development

Not every extension needs planning permission. Under Permitted Development (PD) rights, you can build:

• Single-storey rear extension: Up to 3m (attached) or 4m (detached) — or 6m/8m under prior approval
• No more than 50% of the garden area covered by extensions
• Maximum height: 4m for single-storey, matching the existing ridgeline for two-storey
• Side extensions: Maximum 3m wide and single storey

You WILL need planning permission if:

• You live in a Conservation Area, National Park, or AONB
• The property is a listed building
• Your PD rights have been removed (check Article 4 directions)
• The extension exceeds PD limits

Cost of planning permission: in England a householder application currently costs £548, with a rise to £575 planned from 8 December 2026 (Planning Portal). If refused, an appeal is free but time-consuming.

Our advice: Even if you think you're within PD rights, get a Lawful Development Certificate (LDC), which currently costs £274 (half the householder application fee). This provides legal proof your extension is permitted, which is invaluable when you sell. Read more about regulations in our loft conversion planning guide.

How to Reduce Extension Costs (Without Cutting Corners)

Choose simple geometry

Flat roofs are cheaper than pitched. Rectangular footprints are cheaper than L-shapes. Every corner, angle, and junction adds cost.

Standardise openings

Standard window and door sizes are significantly cheaper than bespoke. A 2.4m bi-fold door costs half the price of a 4.8m set.

Manage the spec yourself

Supply your own kitchen, bathroom, flooring, and lighting. Builders mark up supply-and-fit significantly. Buy direct and have the builder install.

Time your build wisely

Builders are busiest from March to September. Starting in autumn or winter can be 5-10% cheaper and you'll have more choice of contractors.

Get a builder's input BEFORE the architect

Architects design what looks good. Builders know what costs money. Getting a builder involved early can save thousands by avoiding expensive details that add little value. This is exactly what Chat to Your Builder consultations are designed for.

How Long Does a House Extension Take?

Typical timelines from start on site to completion:

• Small single-storey (15-20m²): 8-12 weeks
• Large single-storey (25-40m²): 12-16 weeks
• Two-storey extension: 14-20 weeks
• Side return extension: 8-12 weeks
• Wrap-around extension: 16-24 weeks

Add 4-8 weeks for planning permission (if needed) and 2-4 weeks for Building Control sign-off.

Do Extensions Add Value?

In most cases, yes — but not always pound-for-pound. Here are the typical returns:

• Single-storey kitchen extension: Adds 5-10% to property value
• Two-storey extension (extra bedroom + bathroom): Adds 10-20% to property value
• Loft conversion: Adds 10-15% to property value

The maths matter. If your home is worth £300,000 and a £100,000 extension adds £60,000 in value, you've technically "lost" £40,000 in pure financial terms. But if it means you stay in your home rather than spending £40,000+ on stamp duty, legal fees, and moving costs, it makes perfect sense.

How to Finance an Extension

Most homeowners fund an extension through one of these routes:

• Remortgaging to release equity: the most common approach. Factor in any early repayment charges on your existing deal.
• Home improvement loans: unsecured personal loans, usually for smaller projects
• Second charge mortgage: secured against your home, often at a higher rate than a remortgage
• Savings: no borrowing costs at all

Getting independent mortgage advice before you commit is well worth the time. Whichever route you choose, budget on the all-in figure, including fees and contingency, not the headline per-m² price.

How Chat to Your Builder Can Help

Before committing to an extension, you can put your plans and quotes in front of a working builder with nearly 30 years on site. We can help you:

• Get a realistic budget based on your specific project and location
• Identify potential hidden costs before they become nasty surprises
• Review builder quotes to check they're fair and complete with a builder quote review — see our guide to checking builders
• Understand what's realistic for your budget and timeline
• Avoid common mistakes that add cost without adding value

Whether you're extending in London, Manchester, Bristol, or Edinburgh, a written review and a call within 24 hours costs £59 and could save you thousands. Book a review.