Two-storey annexe extensions in Cornwall
Cornwall has a wide mix of house types, and most can take a well-designed two-storey annexe extension. Here's what's involved, the rules in England and typical costs.
Cornwall at a glance
- Nation
- England
- Price region
- South West England
- Postcode areas
- PL, TR
- Towns covered
- 10 listed below, plus surrounding villages
- Approval route
- Planning permission is needed: in England, permitted development only allows single-storey extensions to the side.
- Site visits
- Free, no obligation
What a two-storey annexe extension involves
This service suits you if a parent, grown-up child or carer is moving in and you want them to have their own space while staying connected to the house. You get a layout that can be self-contained or shared, the approvals your nation needs with the annexe use made clear, accessible design where needed and the build, carried out by vetted teams working under S&K.
Cornwall towns we list: Redruth (TR5), Falmouth (TR10), St Austell (PL23), Truro (TR1), Camborne (TR14) and Newquay (PL28). Every other address in Cornwall is covered as well.
Guide price in South West England
Most of the cost of a two-storey annexe extension is set by its size and specification. For Cornwall and the rest of South West England, our guide is £2,110–£2,830 per m² including VAT, so about £88,500–£119,000 (plus kitchenette and shower room fittings, priced to your choice) for a 3 m × 7 m footprint on two floors (42 m² of new floor space). Design fees, structural calculations and approval fees are separate and listed on our prices page.
| Spec | Per m² | Typical project |
|---|---|---|
| Essential | £1,940–£2,610 per m² | £81,500–£109,500 |
| Standard | £2,110–£2,830 per m² | £88,500–£119,000 |
| Premium | £2,530–£3,400 per m² | £106,500–£143,000 |
Rules in England
In England, an annexe on two floors is assessed like any two-storey extension: at the rear it can be permitted development if it goes no more than 3 m beyond the original rear wall and stays 7 m from the rear boundary, but at the side it needs planning permission, costing £548 (or £575 for applications made from 8 December 2026). Councils often add a condition that the annexe stays ancillary to the main house and is not let separately.
Approval in England: Householder planning application: £548 (or £575 for applications made from 8 December 2026). Lawful development certificate: £274 (about £288 for applications from 8 December 2026). Councils should decide householder applications within 8 weeks of validation.
Building regulations: Building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end.
The Party Wall etc. Act 1996 applies, so notices may be needed for work on or near a shared wall or boundary.
Getting the design right
Services and metering
An annexe usually shares the main house's water, gas and electricity, with a separate circuit or sub-meter if you want to split costs. We plan heating, hot water and drainage so the annexe works well without needing new supplies.
Ground-floor living for the future
Many annexes are planned so the occupant could live on the ground floor if stairs become difficult. A ground-floor bedroom space, a level-access shower and wider doorways cost little at the design stage and are expensive to add later.
How we work in Cornwall
Free site visit
We visit your home in Cornwall, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Get it approved
Once you're happy with the design we submit it to the council for Cornwall and deal with any questions from the planning officer.
Detail and price
Construction drawings, engineer's calculations and building control sign-off, then a written fixed price for the build.
Finish and certify
Second fix, plastering and making good, then building control's final inspection and the completion paperwork.
Towns we cover in Cornwall
Two-storey annexe extensions in Cornwall: questions
How much does a two-storey annexe extension cost in Cornwall?
In South West England our guide is £2,110–£2,830 per m² including VAT, so about £88,500–£119,000 (plus kitchenette and shower room fittings, priced to your choice) for a 3 m × 7 m footprint on two floors (42 m² of new floor space). Essential and Premium specs move that by roughly 8% down or 20% up.
Can I rent out an annexe later?
Not always. Many annexe permissions include a condition that it is only used by members of the household, so renting it out may need a fresh planning application. If you think you might want to let it, tell us at the start so we can explain the options.
Does a two-storey annexe need planning permission?
It depends on where it goes and your nation. In England, a rear annexe extension can be permitted development within the 3 m and 7 m limits, but a side one always needs planning permission. If the annexe could work as a separate home, the council may treat it as a new dwelling, which needs full planning permission.
Can an annexe be designed for someone with limited mobility?
Yes. We can plan a ground-floor bedroom space, a level-access shower, wider doorways, lever handles and space for a stairlift later. These features are much cheaper to build in at the start than to add afterwards, and they keep the annexe useful as needs change.
Will I need planning permission in Cornwall?
Cornwall is in England. Planning permission is needed: in England, permitted development only allows single-storey extensions to the side.
Neighbouring areas
- Two-storey annexe extensions in Devon
- Two-storey annexe extensions in Swansea and Neath Port Talbot
- Two-storey annexe extensions in Dyfed
- Two-storey annexe extensions in Cardiff and the Vale of Glamorgan
All extension services in Cornwall · About two-storey annexe extensions
Ready to see what a second storey could do for your home?
Book a free site visit. We'll look at your house, talk through options and explain the approvals you'd need, then price the design stage in writing.