In Devon, a single-storey extension typically costs £35,000 to £80,000 and a two-storey extension £80,000 to £150,000, before drawings and fees. The size, the ground conditions and how you finish the inside make the biggest difference.
Updated October 2026 · Terbium Home, builders in Exmouth, Devon
| Job | Typical range |
|---|---|
| Single-storey extension | £35,000 – £80,000 |
| Two-storey extension | £80,000 – £150,000 |
| Loft conversion | £40,000 – £75,000 |
Ranges are for typical Devon homes, from groundworks to a finished, decorated room. Drawings, structural calculations and council fees are extra. No VAT to add: Terbium Home isn't VAT registered.
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Many single-storey rear extensions are allowed under permitted development, without a planning application, if they meet the national limits:
In a conservation area, side extensions and two-storey rear extensions always need planning permission. Some properties, including many barn conversions, have had permitted development rights removed by a planning condition, so always check your planning documents. We'll go through this with you at the site visit.
Allow time for drawings and approvals before anything starts on site: often a couple of months, longer if you need full planning permission (councils aim to decide householder applications within 8 weeks). Build time depends on size and spec, and we give you a programme with the quote so you know what happens when.
A single-storey extension of that size will usually sit within our £35,000–£80,000 range, depending on the ground, glazing and what goes inside. Our estimator gives a ballpark by size and finish.
Not always. Many rear extensions up to 3m (4m detached) and 4m high are permitted development. Conservation areas, listed buildings and properties with planning conditions are different, so check first.
Yes. Every extension needs Building Regulations approval, whether or not it needs planning permission.
Sources: Planning Portal: extensions · Planning Portal: application fees
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