Mid Cornwall · PL25 · Cornwall Council Mid
Single storey extension costs in St Austell, and what actually moves the number
A single storey rear or side extension is the most common brief we take in St Austell, and the cost gap between two apparently identical projects usually comes down to four things: how the ground behaves once it is opened up, how much glazing is in the rear elevation, whether the kitchen is being reworked at the same time, and how the new roof meets the existing one, which is where the awkward detailing lives. 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
- ✓Cost banded feasibility before any design fee is committed
- ✓St Austell ground and access assessed on the first visit
- ✓Drawings and build from one studio, so the detail is priced not guessed
- ✓Free first consultation across Mid Cornwall
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Page purpose
Drawings only, or drawings through to handover in St Austell?
Both routes are open on a PL25 project, and the honest answer depends on whether you already have a builder you trust. Plenty of St Austell clients take the drawings, tender them locally, then come back to us for the build if the numbers do not stack up.
- Route one: feasibility, planning and a tender-ready technical pack
- Route two: the same pack, then the build under one contract
- Either way the survey and feasibility work is identical, so the decision can wait
- Fees quoted per stage against the PL25 scope, not as one opaque figure
Scope
Scope options for St Austell clients
These stages can be bought separately. Most PL25 projects use three of them.
Heritage and design statements
Written justification where designation makes the design case as important as the drawing.
Building regulation drawings
Full Plans sets with sections, construction notes and coordinated structural information.
Structural coordination
Calculations coordinated with the drawings so building control sees one consistent story.
Construction and technical drawings
1:10 and 1:20 details for the parts of the job builders normally have to guess at.
Full design and build
One contract from first sketch to handover, with the same team on paper and on site.
Local watch-list
The PL25 constraints that shape an 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
Straight facts about working with us here
- Commercial and mixed-use work in St Austell tends to sit around Holmbush retail park.
- Design and build are offered from the same studio, which means the person answering your St Austell questions has seen the detail built.
- Applications for St Austell are handled through Cornwall Council, with Truro the nearest planning office we deal with regularly.
- Designated-area work is routine here, so heritage and landscape statements are produced alongside the drawings rather than outsourced.
Planning rules change and every site is different. For the final position on your address, check with Cornwall Council as the local planning authority or the Planning Portal.
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.
Quick check
Want a straight answer on whether your St Austell project is worth pursuing?
Send a few details — we'll come back within 1 working day with a plain-English view on planning risk, likely cost band and whether it's a project we'd take on.
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For projects in St Austell and the PL25 area.
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. On older stock, the practical constraint is usually joinery, render and roof line rather than the extension itself. We draw those details out early so an extension submission is not sent back for missing information. Confirm listing status with the local planning authority before assuming permitted development. Exposure is the real design driver near the water: wind-driven rain, salt and driving damp push detailing towards better weathering, sheltered openings and robust fixings. Where an AONB or coastal policy applies, visual impact usually carries more weight than size.
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.
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.
- What drives the cost of a single storey extension in St Austell?
- Ground conditions, glazing area, structural openings into the existing house, drainage runs and access for materials. In St Austell (PL25) access is often the quiet one, because a site a lorry cannot reach is a site where everything is handled twice.
- Is a rate per square metre reliable?
- Only as a sanity check. Two extensions of the same size can differ by a third once you change the glazing spec, the roof form and the amount of existing structure being altered. We price from drawings, not from a rate.
- Does using one studio for drawings and build change the cost?
- It changes where the cost is certain. Because we draw the scheme and then build it, the detail that a separate builder would price as a risk is already resolved on paper, so there is less contingency priced into the unknowns.
- What is the cheapest way to add space in St Austell?
- Usually reworking what already exists before extending: a garage conversion, a rethought internal layout, or a smaller extension that unlocks a better plan. We will say so if that is the better answer for your house, even though it is a smaller job for us.
- 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.
- We live in the house. Can an extension in St Austell be done while we stay put?
- Usually yes for single-storey and rear work, with a temporary kitchen or bathroom arrangement agreed before the start. Whole-house remodels and roof work in St Austell are often easier if you move out for a few weeks. We set that expectation at feasibility stage rather than halfway through.
- 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.
St Austell is the hub for these neighbourhoods
We run extensions across St Austell and the surrounding PL25 neighbourhoods — same studio, same site team.
- Charlestown
PL25
Keep reading
Where to go next from St Austell
St Austell works as a hub for us: the same extension team covers Charlestown and the surrounding PL25 villages, so a site visit here often takes in more than one address.
Other services in St Austell
The honest answer for St Austell is that a rate per square metre is a starting point, not a price. We give you a cost band at feasibility stage, then a real figure once the drawings exist and the work can be priced properly.
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