Budget £40,000 to £56,000 for a mid‑spec 20m² single‑storey rear extension shell in most of the North West, before fit‑out, professional fees or VAT are added. Add fit‑out, design and Building Control costs, and a realistic finished total for the same project usually lands closer to £55,000 to £75,000. What pushes that figure up? Glazing choice, ground conditions, and how far you go beyond a basic shell.
TL;DR:
- Mid-spec rear extension costs typically range from £55,000 to £75,000 for a 20m² shell, including proper fit-out and finishes.
- Ground conditions can significantly impact costs, with soft or unstable ground requiring more expensive foundations and risking budget overruns.
- Regional variation affects prices, with London projects generally costing more than those in the North West due to higher labor and material rates.
- A fixed-price quote from a single contractor covering design, planning, structural work, and build reduces the risk of unexpected costs during construction.
Table of Contents
- Rear extension cost: budget, mid-spec, and high-spec ranges
- What does a rear extension cost per square metre?
- What makes a rear extension more expensive?
- What’s included in a builder’s quote, and what isn’t?
- Do you need planning permission for a rear extension?
- How long does a rear extension take, and how does labour affect cost?
- How to budget and compare quotes properly
- A practitioner’s view on where rear extension budgets go wrong
- Get a fixed-price quote for your rear extension
- Sources
Rear extension cost: budget, mid-spec, and high-spec ranges
Most quotes fall into three clear bands, and knowing which one you’re shopping in saves a lot of confused comparisons later.
- Budget band: a bare shell with standard brick, basic uPVC windows, and minimal glazing typically runs £30,000 to £40,000 for 20m², with a basic fit‑out pushing the total to around £45,000.
- Mid-spec band: this is where most homeowners land, with aluminium bifold or sliding doors, underfloor heating, a plastered and decorated finish, and a proper kitchen or living space fit‑out. Expect £55,000 to £75,000 all in.
- High-spec band: structural glazing, glass roof lanterns, and premium finishes push totals to £80,000 or beyond. Inner London projects command a significant premium over the same specification in the North West.
Regional variation is real and worth planning around before you get quotes.
What does a rear extension cost per square metre?
Per‑square‑metre pricing gives you a fast sanity check on any quote, provided you know which specification band it reflects. RenoCalc’s cost data puts standard specification shells at roughly £1,500 to £2,000/m², good specification at £2,000 to £2,800/m², and high specification at £2,800 to £4,000 or more, all before VAT, design fees, or fit‑out.
| Size | Standard spec | Mid spec | High spec |
|---|---|---|---|
| 20m² | £30,000–£40,000 | £40,000–£56,000 | £56,000–£80,000 |
| 25m² | £30,000–£40,000 | £55,000–£75,000 | £80,000 or more |
| 30m² | £45,000 | £55,000–£75,000 | £80,000 or more |
These are shell figures, before fees and VAT. Large glazed spans and structural openings change the economics fast: a wall of glass replacing brick doesn’t just add the door cost, it adds structural steelwork to carry the load above, which is why two extensions of identical footprint can price thousands apart.
What makes a rear extension more expensive?
Ground conditions are the biggest wildcard. A straightforward strip foundation on stable clay costs far less than a raft or piled foundation needed on soft ground or near trees, and you often can’t know which you need until a structural engineer has assessed the site.
- Groundworks and foundations: variable, but this is where budgets most commonly overrun.
- Structural openings and RSJs: knocking through to the existing house or spanning a wide bifold opening needs steel beams, adding cost according to span and load.
- Glazing choice: uPVC bifolds cost less than aluminium; specialist structural glass costs considerably more again.
- Site access: a terraced property with no side access means manual handling of materials, which adds labour time and cost compared with a detached home with driveway access.
Regional labour rates matter too. Trades in Greater Manchester and Merseyside typically charge less than London rates, which is part of why TradeMatch’s per‑m² data shows such a wide spread nationally.
Pro Tip: Ask your builder to confirm foundation type in writing before signing anything. A verbal assumption of a standard strip foundation that turns into a piled solution on day one of digging is one of the most common causes of budget blowouts we see.
What’s included in a builder’s quote, and what isn’t?
A typical shell quote covers foundations, external walls, roof structure, and windows and doors. It usually excludes several things homeowners assume are covered.
- Professional fees: architect fees and structural engineer calculations are usually quoted separately, and it’s worth budgeting a meaningful percentage of build cost for design and structural work.
- VAT: most contractors are VAT‑registered, so expect 20% VAT on labour and materials unless a specific exemption applies.
- Fit-out: kitchen units, flooring, electrics, plumbing, and decoration are separate cost categories, often the single biggest variable in your final bill.
- Building Control fees: charged by your local authority or an approved inspector, separate from planning fees.
Read any quote line by line before comparing it against another. A £45,000 quote that excludes fit‑out and a £58,000 quote that includes it aren’t actually £13,000 apart.
Do you need planning permission for a rear extension?
Many single‑storey rear extensions fall under Permitted Development, meaning no planning application is needed, but the rules have real limits worth checking before you commit to a design.
- Standard Permitted Development allows depths of 3m for terraced or semi‑detached homes, and 4m for detached houses.
- The Larger Home Extension prior approval scheme allows deeper single‑storey extensions, up to 6m or 8m, subject to a neighbour consultation process run by your local council.
- Article 4 directions and designated land (conservation areas, national parks) can remove Permitted Development rights entirely, so check your council’s planning portal first.
- Even under Permitted Development, Building Regulations approval is almost always required, and a completion certificate matters when you come to sell.
How long does a rear extension take, and how does labour affect cost?
Design and permits typically take 6 to 12 weeks, groundworks and shell build 8 to 12 weeks, and fit‑out a further 4 to 6 weeks. Weather, specialist glazing lead times, and underfloor heating installers can all cause slippage, and every week of delay adds site costs.
How to budget and compare quotes properly
A good quote should read like a specification, not a single number.
- Check the scope includes preliminaries: scaffolding, skip hire, site welfare, and insurance, not just materials and labour.
- Allow a contingency of at least 10% to 15% on top of the quoted price, more if ground conditions are unknown or a build‑over agreement with the water company is likely near a sewer.
- Ask who manages subcontractors, what the payment milestones are, and whether a completion certificate and workmanship warranty are included.
- Watch for red flags: vague scope descriptions, a low deposit paired with an unclear payment schedule, or a builder who can’t show proof of insurance or past references.
Pro Tip: If a quote seems significantly cheaper than every other one you’ve received, ask exactly what’s excluded before you get excited. It’s almost always fit‑out, fees, or a foundation assumption that will surface as an extra later.
A practitioner’s view on where rear extension budgets go wrong
Most cost overruns I see don’t come from bad luck, they come from gaps in scope that only surface once digging starts. A single accountable design‑and‑build contractor closes that gap by pricing the ground, the structure, and the fit‑out together from day one. Across Warrington, St Helens, Wigan and the wider North West, that’s the difference between a fixed price and a moving target.
— Gareth
Get a fixed-price quote for your rear extension
Complete-Property-Solutions is the option for homeowners who want one number, not three separate invoices from an architect, a builder, and a fit‑out contractor. As a family‑run business working across Warrington, St Helens, Wigan, Great Sankey, Penketh, Culcheth, Lowton, Rainhill and Haydock. We manage design, planning support, structural work including RSJ installations, and fixed‑price building through to completion, all under one contract with one point of contact.
A first site visit covers your existing layout, likely foundation type, and the specification level you’re picturing, and leads to a written, itemised quotation rather than a ballpark figure. If you’re weighing up a rear extension against other options, our guide to planning a house extension is a useful next step, or you can look at what a rear kitchen extension involves if the kitchen is driving the project. When you’re ready to see numbers against your own property, visit our house extensions page and get in touch for a site visit.
Sources
For the underlying rules and data behind this guide, see gov.uk’s permitted development guidance, planning application fees, and electrical fit‑out context from Smart Home Technical Ltd.
- UK Extension Cost Per m² 2026 — From £1,820 in Liverpool to £3,640 in Inner London | TradeMatch UK
- Single storey extension cost UK — RenoCalc
- Single-storey extensions guidance — Planning Portal / gov.uk
- Gov
