Mid Cornwall · TR3
House Extensions in Threemilestone
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. In Threemilestone, that work is shaped by the place itself — Threemilestone is the western residential and retail-park suburb of Truro, with the West Cornwall Hospital site nearby and significant new estate development on the Langarth Garden Village land, with a building stock that leans toward 1970s and 1980s estates and modern Persimmon, Bellway and Wainhomes estates.
Local context
Why Threemilestone is its own job.
Outside Conservation Area and AONB. Langarth Garden Village (3,500+ homes) is reshaping the west of Truro with strict design code expectations on adjacent sites. For extension specifically, Threemilestone sits outside the headline designations, which usually gives a slightly more flexible starting point — but parish-level character still matters. That's why we treat every Threemilestone project as a TR3-area job first — not a generic Cornwall job with a postcode bolted on.
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 Threemilestone.
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
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 Threemilestone 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
Threemilestone Extensions — common questions.
- 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 Threemilestone specifically, we'd start by checking the latest parish-level planning history 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.
- Will my house be liveable during the build?
- For most rear and side extensions, yes — we sequence the works so the kitchen and one bathroom stay functional until the new build is watertight and connected.
Other services in Threemilestone
Nearby places we cover