Mid Cornwall · TR1 · Cornwall Council Central
Granny annexes in Truro — self-contained living, done well
A granny annexe in Truro is either an attached extension with a separate entrance, a garden building with independent services, or a garage conversion. Cornwall Council treats all three under the "ancillary residential" test — meaning they must remain connected to the main dwelling's planning use. Get the planning wording right and the design opens up. 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 Truro version of this work has its own character — Truro is Cornwall's only city, the county town and home to Cornwall Council itself, with a Georgian core, three-spired cathedral and Lemon Quay at its centre, with a building stock that leans toward modern development at West Langarth and Victorian terraces in Hendra and Highertown.
Truro sits in Mid Cornwall — just off the A390; covering TR1 from Threemilestone, Shortlanesend, Kea outward.
- Conservation Area
- ✓Attached annexe: £75k–£110k built
- ✓Detached garden annexe: £95k–£140k
- ✓Ancillary use planning included
- ✓Self-contained services throughout
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Local watch-list
Truro-specific issues we screen on the first visit.
Watch #1
Cathedral views safeguarded across central wards
Watch #2
Steep slopes around Lemon Street complicating basement and lower-ground options
Watch #3
Article 4 in the central Conservation Area
Watch #4
Flood Zone catchment along the river corridor
Who this is for
In Truro 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.
Scope
The parts of the job we can take on in Truro
These stages can be bought separately. Most TR1 projects use three of them.
Measured survey
An accurate record of what is actually there, which is where most avoidable cost is removed.
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.
Planning application drawings
Existing and proposed plans, elevations, site and block plans drawn to validation standard.
Commercial drawings
Change of use, fit-out and access documentation for Class E and mixed-use premises.
Who this is for
What we can genuinely back up in Truro
- Commercial and mixed-use work in Truro tends to sit around Threemilestone Industrial Estate.
- Applications for Truro are handled through Cornwall Council, with Truro the nearest planning office we deal with regularly.
- Design and build are offered from the same studio, which means the person answering your Truro questions has seen the detail built.
- 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.
Keep reading
Other work we do around Truro
We cover Truro and the wider TR1 district directly. Extensions enquiries here are common enough that we know the constraints before the first visit.
Page purpose
Drawings only, or drawings through to handover in Truro?
Both routes are open on a TR1 project, and the honest answer depends on whether you already have a builder you trust. Plenty of Truro 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 TR1 scope, not as one opaque figure
Local context
Why Truro is its own job.
The Truro Conservation Area covers a wide central zone including Lemon Street, the cathedral precinct and the river frontage. As the home of Cornwall Council planning, it tends to set the tone for design expectations across the county. For extension specifically, parts of Truro sit within a designated Conservation Area, which means materials, fenestration and roof pitches all need to read sympathetically with the existing streetscape. So every Truro job runs as a TR1-specific piece of work — local policy, local fabric, local builders. Most of our extension work in Truro lands on modern development at West Langarth, with detailing that has to nod to the wider Shortlanesend streetscape. 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.
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 Truro.
01
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.
02
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.
03
Wind and sea-spray exposure can drive material choices on west-facing extensions; we detail accordingly.
04
Drainage on older Cornish properties is rarely on a clean modern map; CCTV survey before design is often money well spent.
Quick check
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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 Truro and the TR1 area.
Recent work nearby
Recent professional-services HQ retrofit on Lemon Street ran as a Conservation Area application with internal-only listed sign-off.
See more recent Mid Cornwall work →Our process
How a Truro 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
Truro Extensions — local questions answered.
- Do I need planning permission for a granny annexe in Truro?
- Yes — annexes always need planning consent to establish ancillary residential use. Attached annexes go through householder; detached garden annexes need full planning.
- How much does a granny annexe cost in Truro?
- £75k–£140k for a 30–55m² annexe with kitchenette, bedroom, bathroom and living area. Detached garden annexes cost 10–20% more than attached (utility runs and separate roof).
- Can we let out the annexe as a holiday let later?
- Only with a further change-of-use application. Ancillary consent explicitly ties the annexe to the main dwelling — subletting breaches that condition.
- How much does an extension cost in Truro?
- 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 Truro specifically, we'd start by checking the Conservation Area boundary 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.
- 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.
Local proof — We typically have one or two extension jobs live in the TR1 area at any time, so the local planning officers know our drawings on sight.
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Nearby places we cover
A Truro granny annexe is 50% design, 50% planning wording — get the ancillary-use case right and everything downstream is straightforward.
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