Lizard Peninsula · TR12 · Cornwall Council West
Single storey extension costs in Mullion, and what actually moves the number
A single storey rear or side extension is the most common brief we take in Mullion, 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 exposed the elevation is to weather coming off the sea, which drives the weather-tightness detailing. 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 Mullion starts with a measured walk-round — Mullion is the largest village on the Lizard Peninsula, AONB-designated, with a fifteenth-century church, a working cove and the highest concentration of period housing on the peninsula, with a building stock that leans toward modern AONB-sensitive replacement dwellings and Edwardian guesthouses.
Mullion sits in Lizard Peninsula — just off the A394; covering TR12 from Mawgan, Ruan Minor, The Lizard outward.
- Conservation Area
- Cornwall AONB
- Coastal exposure zone
- Rural / open-countryside policy area
- ✓Cost banded feasibility before any design fee is committed
- ✓Mullion 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 Lizard Peninsula
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Page purpose
Drawings only, or drawings through to handover in Mullion?
Both routes are open on a TR12 project, and the honest answer depends on whether you already have a builder you trust. Plenty of Mullion 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 TR12 scope, not as one opaque figure
Scope
Scope options for Mullion clients
Not every project needs every stage. On a Mullion job we scope to what the site and the consent route actually require.
Heritage and design statements
Written justification where designation makes the design case as important as the drawing.
Structural coordination
Calculations coordinated with the drawings so building control sees one consistent story.
Full design and build
One contract from first sketch to handover, with the same team on paper and on site.
Construction and technical drawings
1:10 and 1:20 details for the parts of the job builders normally have to guess at.
Building regulation drawings
Full Plans sets with sections, construction notes and coordinated structural information.
Local watch-list
Local snags worth knowing before drawing a Mullion extension.
Watch #1
Conservation Area controls on materials, window proportions and roof form through central Mullion
Watch #2
AONB landscape-impact testing on anything read in long views across Lizard Peninsula
Watch #3
Exposure detailing on granite cottages around the church — fixings, render systems and joinery specified for salt-laden air
Watch #4
Local Plan tests on isolated dwellings, plus lane access for plant and deliveries around Mullion
Who this is for
Straight facts about working with us here
- Applications for Mullion 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.
- Drawings, planning and building regulations are all produced in-house, so an extension enquiry does not get passed between firms.
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 Mullion.
01
Cornish granite and slate-hung walls react differently to new openings than modern brickwork — lintel choice and structural sequencing matter.
02
Wind and sea-spray exposure can drive material choices on west-facing extensions; we detail accordingly.
03
Drainage on older Cornish properties is rarely on a clean modern map; CCTV survey before design is often money well spent.
04
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.
Quick check
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For projects in Mullion and the TR12 area.
Local context
Why Mullion is its own job.
In Mullion the planning picture is specific: mullion Conservation Area covers the village centre; the wider parish is entirely within the AONB and includes Heritage Coast designation. Cliff-edge and coastal margin sites face the strictest controls in West Cornwall. For extension specifically, parts of Mullion sit within a designated Conservation Area, which means materials, fenestration and roof pitches all need to read sympathetically with the existing streetscape; the surrounding landscape falls inside the Cornwall Area of Outstanding Natural Beauty, so massing, height and landscape impact carry extra weight in any planning decision; coastal salt-laden air around Mullion drives detailing choices — fixings, render systems and timber treatments all need to be specified for exposure; Cornwall Council's Local Plan applies tighter tests to isolated rural dwellings here, so design rationale and policy fit need to be set out clearly from the outset. That local reading is what makes a Mullion (TR12) project different from a generic Cornwall scheme — and is the whole reason we work this way. On modern AONB-sensitive replacement dwellings in particular — the kind you'll also find toward The Lizard — the extension brief always has to read the existing fabric first. Historic fabric changes the sequence: survey first, then design around what the building can take. Consent routes for listed buildings sit separately from ordinary planning, so treat any general guidance here as a starting point and check with Cornwall Council. Coastal sites tend to raise questions about roof form, glazing exposure and how a scheme reads from public viewpoints. Those are policy judgements rather than fixed rules, so the local planning authority or the Planning Portal should confirm the position for a given plot.
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.
Our process
How a Mullion 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
Mullion Extensions — local questions answered.
- What drives the cost of a single storey extension in Mullion?
- Ground conditions, glazing area, structural openings into the existing house, drainage runs and access for materials. In Mullion (TR12) 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 Mullion?
- 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.
- How much does an extension cost in Mullion?
- 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. In Mullion specifically, we'd start by checking the Conservation Area boundary before committing to a direction.
- We live in the house. Can an extension in Mullion 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 Mullion are often easier if you move out for a few weeks. We set that expectation at feasibility stage rather than halfway through.
- 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.
Keep reading
Where to go next from Mullion
We cover Mullion and the wider TR12 district directly. Extensions enquiries here are common enough that we know the constraints before the first visit.
Other services in Mullion
Nearby places we cover
The honest answer for Mullion 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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