Penwith · TR19
House Extensions in Treen
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. Reading Treen on the ground is half of the extension job — Treen is a coastal village in the TR19 area, where sea exposure, views and seasonal pressure shape most building decisions, with a building stock that leans toward rendered coastal houses and bungalows.
Treen sits in Penwith — covering TR19 from St Buryan, Truro, St Austell outward.
- Cornwall AONB
- Coastal exposure zone
- ✓ Fixed-fee planning packages, no surprise invoices
- ✓ Measured-survey accuracy from day one
- ✓ One studio — design, planning and build under one roof
- ✓ Local to Penwith — not a national franchise
Local watch-list
Common Treen pitfalls we plan around.
Watch #1
AONB landscape-impact scrutiny on visible elevations
Watch #2
Coastal exposure driving fixing, render and joinery spec
Who this is for
Treen runs the full mix — owner-occupier, holiday-let, commercial and the occasional smallholding — so we scope every extension enquiry from the use-class up.
Local context
Why Treen is its own job.
Coastal setting and landscape sensitivity mean rooflines, glazing, drainage and external materials need careful handling from the first sketch. For extension specifically, 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 Treen drives detailing choices — fixings, render systems and timber treatments all need to be specified for exposure. So every Treen job runs as a TR19-specific piece of work — local policy, local fabric, local builders. Most of our extension work in Treen lands on rendered coastal houses, with detailing that has to nod to the wider Truro streetscape.
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 Treen.
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
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
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.
Our process
How a Treen 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
Treen Extensions — local questions answered.
- How much does an extension cost in Treen?
- 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 Treen specifically, we'd start by checking AONB landscape sensitivity before committing to a direction.
- 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.
- 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 long does the whole process take?
- Allow roughly three months for design and approvals, then twelve to twenty weeks on site for a typical single-storey extension. Wraparounds and two-storey add-ons take longer, mostly through approval and groundworks.
- Do I need planning permission for an extension?
- Often no — single-storey rear extensions, side extensions and modest two-storey additions can sit inside permitted development on a typical detached house. Conservation Areas, AONB and Article 4 zones remove some of those rights, so we always check the address first.
Treen is part of St Buryan
Treen sits inside the St Buryan catchment — we cover both as one extension territory.
See Extensions in St Buryan →Local proof — We typically have one or two extension jobs live in the TR19 area at any time, so the local planning officers know our drawings on sight.
Get a free feasibility viewOther services in Treen
Nearby places we cover
On a Treen site the success of a extension is decided in week one — by reading the constraints right, not by drawing them away.