Expect to pay between £1,850 and £3,500 per m² for a two storey extension in 2026. A typical project cost depends on size and specification. The exact figure depends on specification, ground conditions and whether you need full planning permission. Your first move should be checking your permitted development status and booking a fixed-price site survey with a local builder.
TL;DR:
- Ground conditions significantly impact costs, with difficult soil or sloping sites adding up to considerably in foundation expenses.
- Most two-storey extensions require full planning permission, especially if close to property boundaries or on designated land, with strict limits on size and coverage.
- Building regulations mandate staged inspections for foundations, drainage, and structure, making early approval essential to avoid costly rework.
- Budgeting should include statutory fees, professional charges, and potential delays, with site surveys and early planning advice reducing unexpected expenses.
- Opting for a fixed-price, design-and-build service with a local firm can minimize scope creep, guarantee costs, and streamline project management.
Table of Contents
- What does a two storey extension cost per m²?
- What drives the price of a two storey extension?
- Do I need planning permission for a two storey extension?
- Building Regulations: what inspections do you need?
- What fees and timescales should you budget for?
- Is a two storey extension better value than moving?
- Why choose a managed design-and-build service?
- Quick, practical tips to reduce unexpected costs
- Get a fixed-price quote for your extension
- Sources
What does a two storey extension cost per m²?
Specification drives price more than almost anything else. A basic-spec build with standard fittings runs around £1,850 per m², a good-quality mid-range finish sits closer to £2,200 per m², and a premium specification with underfloor heating, bespoke joinery and high-end glazing can reach £2,600 to £3,500 per m², according to Ideal Home’s expert extension guide.
Those ranges already factor in the contingency buffer, but they will shift depending on where you live. London and the South East typically have higher labour and material costs than many other areas, including Warrington, St Helens and Wigan, and this regional gap can be worth £200 to £400 per m² on its own. Homeowners in Warrington, St Helens and Wigan are typically working from the lower end of these bands compared with the national averages quoted by many guides.
What drives the price of a two storey extension?
Groundworks are often the biggest surprise. Standard strip foundations work fine on stable ground, but clay soil, sloping sites or proximity to trees can force mini-piles or a raft foundation, and difficult ground conditions can add £12,000 to £22,000 to the foundation bill alone.
Beyond the ground, several other factors push costs up or down:
- Structural steels for two openings, plus any needle propping and scaffolding needed to tie into the existing roof
- Glazing and joinery specification, particularly bi-folding or full-height windows
- Kitchen and bathroom fit-outs, which often mean rerouting drainage and service pipes
- Tight site access, bespoke roof geometry, or matching an unusual brick
Pro Tip: Commission a ground investigation before you get quotes, not after. A borehole survey costing a few hundred pounds can save you thousands in foundation surprises once digging starts.
Do I need planning permission for a two storey extension?
Many two storey extensions need full planning permission. Permitted development rights for double-storey work are tightly limited, and around 70% of two storey extensions fall outside them entirely.
Where PD does apply, the conditions are strict:
- The extension cannot extend more than 3m beyond the original rear wall
- It must stay at least 7m from the rear boundary
- Total ground coverage cannot exceed 50% of the original garden
- PD rights vanish on designated land, in Conservation Areas, or where an Article 4 direction removes them
If you’re close to these limits, apply for a Lawful Development Certificate to get formal written proof your project qualifies. It matters far more at resale than most homeowners expect. Side extensions of two storeys generally require planning permission regardless of size, a mistake architects flag repeatedly. Check your local Article 4 status and speak to your planning authority before you commit to a design, and budget for pre-application advice if your case is borderline.
Building Regulations: what inspections do you need?
Every two storey extension must comply with Building Regulations regardless of its planning status, and that means staged inspections at foundations, drainage, structure and completion.
- Notify Building Control before work starts, via full plans or a building notice
- Never cover foundations, drainage or structural steel before the inspector has signed them off
- A final completion certificate is only issued once every required stage has passed
Covering up work early to save time is one of the costliest mistakes we see. If work fails a later inspection, opening it back up can cost far more than the delay would have. Party wall notices may apply if building close to a neighbour’s boundary; our structural alterations guide covers this in more depth.
What fees and timescales should you budget for?
Statutory and professional fees add up fast, so build them into your budget from day one rather than treating them as extras:
- Householder planning application fee applies
- Lawful Development Certificate has a fee
- Pre-application advice can cost a few hundred pounds
- Architect fees vary, often into several thousands
- Structural engineer fees vary, often into hundreds or over a thousand
Two storey builds typically take several months on site once groundworks begin, though weather delays, planning conditions and material lead times can push this out further. Set aside a contingency on top of your build cost, and agree a staged payment schedule tied to project milestones rather than a fixed calendar. It protects both you and your builder if timescales shift.
Is a two storey extension better value than moving?
Two storey extensions usually deliver more square metres per pound than single-storey builds, because the foundations, groundworks and roof costs are shared across both floors rather than duplicated. That’s the practical reason many homeowners in Great Sankey, Penketh and Rainhill choose to extend upward and outward rather than build two separate single-storey additions.
Resale uplift varies by region and finish, but a well-executed extension with full Building Regulations sign-off and a completion certificate typically supports a stronger valuation than an uncertified equivalent, something lenders and buyers’ solicitors check closely. See our guide on extensions that add value for more detail. Moving still has its place if you can’t face months of disruption or a longer build programme, but for many homeowners the numbers favour staying put and building.
Why choose a managed design-and-build service?
Complete-Property-Solutions has more than 35 years of experience delivering two storey extensions across Warrington, St Helens, Wigan and the surrounding towns, and we’re proud members of the Guild of Master Craftsmen.
A managed, fixed-price service typically includes:
- One contract covering design, planning liaison, and construction
- Dedicated project management from survey through to completion certificate
- Approved installer partnerships for glazing and roofing, keeping specification consistent
| What you get | Why it matters |
|---|---|
| Single fixed-price contract | No surprise invoices mid-build |
| Local site surveys | Accurate quotes, fewer assumptions |
| Guild of Master Craftsmen membership | Independently recognised workmanship |
A site visit in Culcheth, Lowton or Haydock lets us quote against your actual ground conditions and access rather than national averages.
Quick, practical tips to reduce unexpected costs
We’ve seen the same avoidable overspends crop up on countless projects. Commission a ground investigation before you tender, not after digging starts, and lock in your specification choices early rather than changing tack mid-build. Complex roof geometry always costs more than it looks like on a drawing.
Choosing a single design-and-build contract cuts down on scope creep and disputes between separate trades. Talking to Building Control and neighbours early, particularly around party wall matters, heads off delays that cost far more than the conversation itself.
— Gareth
Get a fixed-price quote for your extension
If you’re weighing up costs and want numbers you can actually rely on rather than a national average, a site visit changes everything. Complete-Property-Solutions offers homeowners across Warrington, Wigan, Great Sankey, Penketh, Rainhill and Haydock a genuinely fixed-price quotation, agreed before any work starts, so there are no surprise invoices halfway through your build.
Before your survey, it helps to have rough sketches of what you want, any existing floor plans, and a sense of your budget ceiling. Our team will walk your site, talk through foundation and structural options given your ground conditions, and explain exactly what’s included in your house extension quote, from groundworks to final finishes. Our design-and-build process covers planning liaison and Building Control coordination throughout, so you’re not managing multiple contractors yourself.
Get in touch to arrange your site survey, or give us a call for an initial chat about your project.
Sources
- Gov
- Two-storey extensions: the expert guide | Ideal Home
- How much does a double-storey extension cost in 2026? – BestBuilders
