Mid Cornwall · PL25 · Cornwall Council Mid
Two-storey extension costs in St Austell — what PL25 homeowners really pay
A two-storey extension in St Austell sits at £2,000–£2,500/m² built for standard finishes, rising to £2,800/m² when the existing gable needs opening up. On Georgian townhouses the biggest cost driver is almost never the shell — it's rooflines, matching stone or render, and how the new stair lands upstairs. Extensions are the bread and butter of Cornish homes — adding the kitchen-diner the original layout never had, the bedroom for a growing family, or the light and views the back of the house should always have had. The way we approach extension in St Austell starts with a measured walk-round — St Austell is the principal town of mid-Cornwall, historically the centre of the china clay industry and now a substantial market town with the Eden Project on its doorstep, with a building stock that leans toward Georgian townhouses and Edwardian villas.
St Austell sits in Mid Cornwall — just off the A390; with Truro the closest city; covering PL25 from Mevagissey, Carlyon Bay outward.
- Conservation Area
- ✓ Two-storey side: £90k–£150k build
- ✓ Two-storey rear: £110k–£180k build
- ✓ £2,000–£2,500/m² typical rate
- ✓ Full planning route mapped before design fee
Local watch-list
The PL25 constraints that shape a extension brief.
Watch #1
China clay legacy ground conditions on north and west fringes
Watch #2
Charlestown World Heritage Site sensitivities for harbour-adjacent plots
Watch #3
Mid-century estate stock with limited PD remaining after past extensions
Watch #4
Steep contour sites around Mount Charles
Who this is for
In St Austell the extension brief is almost always a private homeowner improving a forever home — so we lead with feasibility and long-term value, not show-home rhetoric.
Local context
Why St Austell is its own job.
Two things shape a St Austell application: parish character and policy. On policy — conservation Area covers the historic centre. The china clay legacy shapes the surrounding landscape and creates substantial brownfield redevelopment opportunities. For extension specifically, parts of St Austell sit within a designated Conservation Area, which means materials, fenestration and roof pitches all need to read sympathetically with the existing streetscape. Get that local reading right and the rest of the St Austell programme tends to run on time. On Georgian townhouses in particular — the kind you'll also find toward Charlestown — the extension brief always has to read the existing fabric first.
Planning note
Most extensions in Cornwall are either permitted development or a straightforward householder application — but Conservation Area and AONB sites need a more careful design conversation upfront.
What we focus on
Extensions considerations specific to St Austell.
01
Drainage on older Cornish properties is rarely on a clean modern map; CCTV survey before design is often money well spent.
02
Permitted development for rear extensions runs to four metres on a detached house, three on a semi or terrace — but Article 4 areas remove this in some parishes.
03
Extensions over a certain proportion of the original house trigger full Part L upgrade obligations to the existing building — worth knowing before brief is set.
04
Cornish granite and slate-hung walls react differently to new openings than modern brickwork — lintel choice and structural sequencing matter.
Recent work nearby
Recent Charlestown courtyard extension threaded under a Grade II listed roofline with a glazed link.
See more recent Mid Cornwall work →Our process
How a St Austell extension project runs.
Step 1
Brief
We meet on site, talk through how you live now and what's missing from the current layout.
Step 2
Design
Two or three sketch directions with rough budgets, then refinement of the chosen route.
Step 3
Approvals
Planning or Cert of Lawfulness, then a full building regs package.
Step 4
Build
Either through your own builder with our drawings, or as a full build by our team.
Step 5
Handover
Snag, certify, hand over the keys to your new space.
Typical single-storey rear extensions run twelve to twenty weeks on site; two-storey and wraparound projects sixteen to thirty weeks.
FAQs
St Austell Extensions — local questions answered.
- Is a two-storey extension worth it in St Austell?
- On average, yes — value uplift on PL25 stock typically covers the build cost within 3–5 years. The exception is where the existing bedroom layout won't take a new stair without losing a room.
- How much does a two-storey side extension cost in St Austell?
- £90k–£150k for a 30–45m² double-storey side, including matching render/stone, new stair and one extra bedroom + en-suite upstairs.
- Does a two-storey extension need full planning in St Austell?
- Yes — Conservation Area removes two-storey PD entirely. Full householder application, 8-week determination.
- Can you handle the build as well as the design?
- Yes — that's the whole point of the studio. One contract, one point of contact, no finger-pointing between architect and builder when something needs a decision on site. In St Austell specifically, we'd start by checking the Conservation Area boundary before committing to a direction.
- What about the Party Wall Act?
- If you share a wall with a neighbour or build close to a boundary, the Act applies. We flag it early, recommend a surveyor and keep the programme aligned with the notice period.
- How much does an extension cost in Cornwall?
- Build costs in Cornwall typically run from around £2,200 to £3,200 per square metre for a good-quality single-storey extension, more for kitchen-grade fit-out or complex glazing. We give a realistic budget before drawings start, not after.
St Austell is the hub for these neighbourhoods
We run extensions across St Austell and the surrounding PL25 neighbourhoods — same studio, same site team.
Local proof — Recent extension enquiries from St Austell have clustered around Georgian townhouses — we know the route through Cornwall Council on these.
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A well-scoped two-storey extension in St Austell nearly always beats moving house. The value is in getting the upstairs plan right in the first sketch, not the last.
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