Extension Costs UK 2025: Real Prices, Calculator & Scenarios
Extension costs UK in 2025 typically land between £1,600 and £3,800 per m² for a single-storey rear extension, depending on region, spec and property type. A standard 20m² kitchen extension on a Midlands semi will usually cost £45,000–£70,000 all-in; the same footprint in inner London is closer to £75,000–£110,000.
Those are the headline figures. Below we break down what drives them, give you full worked scenarios for the most common UK property types, and cover the hidden costs that catch homeowners out. All numbers are from quotes our builders see week-to-week across the UK — not website averages.
How Much Does a House Extension Cost Per Square Metre in 2025?
The most reliable way to estimate extension costs is by square metre (m²). Here are the current average ranges across the UK:
Single-Storey Rear Extension:
- London & South East: £2,400 – £3,800 per m²
- Midlands & East: £1,800 – £2,800 per m²
- North of England: £1,600 – £2,400 per m²
- Scotland: £1,700 – £2,600 per m²
- Wales: £1,600 – £2,300 per m²
Two-Storey Extension:
- London & South East: £2,000 – £3,200 per m²
- Midlands & East: £1,500 – £2,400 per m²
- North of England: £1,300 – £2,000 per m²
- Scotland: £1,400 – £2,200 per m²
- Wales: £1,300 – £1,900 per m²
Two-storey extensions are cheaper per m² because you're sharing foundations and roof structure across two floors. You're effectively getting double the space for roughly 50-60% more cost.
Quick Cost Calculator: Estimate Your Extension
To get a ballpark figure, use this simple formula:
Extension area (m²) × Cost per m² for your region = Base build cost
Then add:
- Professional fees (architect, structural engineer, planning): 10-15% of build cost
- Building Control fees: £500 – £1,500
- Contingency: 10-15% of build cost
- Kitchen/bathroom fit-out (if applicable): £5,000 – £30,000+
- Landscaping and making good: £2,000 – £8,000
Example: A 30m² single-storey extension in the Midlands:
- Base build: 30 × £2,300 = £69,000
- Professional fees (12%): £8,280
- Building Control: £1,000
- Contingency (10%): £6,900
- Kitchen fit-out: £12,000
- Landscaping: £3,500
- Total estimated cost: £100,680
UK Extension Cost Scenarios by Property Type
These are realistic, all-in 2025 figures based on quotes our builders review every week. Each includes build cost, professional fees, Building Control, contingency and a mid-range kitchen or bathroom fit-out where relevant.
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): £1,800
- Contingency (12%): £8,400
- Mid-range kitchen supply & fit: £18,000
- All-in total: £113,000
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: £1,400
- Contingency (10%): £6,600
- Kitchen + utility refit: £14,000
- All-in total: £109,200
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: £1,300
- Contingency (10%): £9,100
- All-in total: £127,600
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: £1,200
- Contingency (10%): £3,420
- Kitchen refresh (keeping existing units): £6,500
- All-in total: £50,820
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
- Conservation Area planning: £1,800
- Contingency (15%): £10,900
- High-spec kitchen: £24,000
- All-in total: £134,700
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 small single-storey | North / Wales | £30,000 – £45,000 |
| 20m² kitchen extension | Midlands | £45,000 – £70,000 |
| 20m² kitchen extension | London / SE | £75,000 – £110,000 |
| 30m² wrap-around | Midlands / North | £80,000 – £120,000 |
| 40m² two-storey | UK average | £90,000 – £150,000 |
| 50m²+ luxury extension | London / SE | £180,000 – £300,000+ |
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.
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, you'll need a Party Wall Agreement under the Party Wall etc. Act 1996. Each adjoining neighbour needs to agree — and if they don't, you'll both need party wall surveyors.
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: £258 for a householder application in England (2025). 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) for £129. 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 Chat to Your Builder Can Help
Before committing to an extension, a 30-minute consultation with one of our experienced builders 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 — 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, our consultations cost just 59 and could save you thousands. Book your consultation today.
Frequently asked questions
How much do extension costs in the UK really start from in 2025?
Realistically, a small (15–20m²) single-storey rear extension in the North of England or Wales starts around £30,000–£45,000 all-in. In London and the South East the same footprint usually starts at £55,000–£75,000 because of higher labour rates, party wall surveyors and more complex access.
What is the average cost per m² for a house extension in the UK?
For 2025, expect £1,600–£2,400 per m² in the North, £1,800–£2,800 per m² in the Midlands, and £2,400–£3,800 per m² in London and the South East for a single-storey extension. Two-storey extensions are 15–25% cheaper per m² because you share foundations and roof.
Why are extension costs in London so much higher than the rest of the UK?
Three reasons: London day rates for skilled trades are 30–50% higher; access is harder (narrow streets, no skip space, restricted parking); and most properties trigger party wall agreements with at least one neighbour. Conservation Areas and Article 4 directions also push up planning and material costs.
Do I need planning permission for my extension or can I use Permitted Development?
Most single-storey rear extensions up to 4m (detached) or 3m (semi/terraced) fall under Permitted Development, with extensions up to 6m/8m allowed under prior approval. You'll need full planning permission if you're in a Conservation Area, National Park, AONB, listed building, or if an Article 4 direction has removed your PD rights. Always apply for a £129 Lawful Development Certificate even when PD applies — it protects you when you sell.
What hidden costs should I budget for on top of the headline build price?
Add 25–35% on top of the builder's quote for: party wall surveyors (£700–£2,000 per neighbour), structural engineer (£600–£1,500), Building Control fees (£500–£1,500), planning fees (£258), drainage diversions if a sewer runs under the build (£3,000–£15,000), and a 10–15% contingency. Most homeowners also under-budget for kitchen/bathroom fit-out and landscaping at the end.
How long does a UK house extension take from start to finish?
Build time on site is typically 8–12 weeks for a small single-storey, 12–16 weeks for a larger single-storey, and 14–20 weeks for a two-storey. Add 8–12 weeks before that for planning permission (if required) and 4–8 weeks for design and tendering. Realistically, plan for 6–9 months from first idea to moving back in.
Will an extension add more value than it costs?
Not always pound-for-pound. A well-designed kitchen extension typically adds 5–10% to the value of the home, and a two-storey extension adding a bedroom and bathroom adds 10–20%. In high-value areas like London, the value uplift usually exceeds the build cost. In lower-value areas, the financial case is weaker — but extending is often still cheaper than moving once you factor in stamp duty, legal fees and moving costs.
Can I get a more accurate extension cost estimate for my specific home?
Yes — a 30-minute phone consultation with one of our builders (£59) will give you a realistic build cost range for your specific property, layout and region, plus a sanity-check on any quotes you've already received. We've reviewed thousands of extension projects across the UK.
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