Garden Annex Builders in South West

What it costs to build a garden annex in South West

A typical fully insulated garden annex in South West costs between £45,000 and £95,000 in 2026, fully installed and ready to use. Below £45,000 you're usually looking at uninsulated summer houses or kit builds with thin (under 70mm) insulation that won't perform year-round.

The price range is wide because four variables drive most of the cost: floor area (typically £1,500–£2,500 per m² installed), cladding choice (cedar and larch add £1,000–£3,500 over composite), glazing package, and groundworks. Sites in South West with easy vehicle access and level ground sit at the lower end; sloped or restricted-access sites can add £2,000–£5,000.

Plumbing, drainage and full building-regulations compliance push an annex well above the cost of any other garden build of the same size.

Designing a garden annex: what to prioritise

A garden annex is self-contained living accommodation — a granny annex with a kitchenette, bathroom and sleeping area. It's the most involved garden build: full insulation, hot and cold plumbing, foul drainage and heating designed for permanent occupation.

• Hot/cold plumbing and compliant foul drainage
• Kitchenette and a Part G/H bathroom
• Habitable-standard insulation and heating
• Full planning + building-regulations sign-off

What's typically included in a South West garden annex quote

• Foundations (concrete pad or steel-screw piles)
• SIPs or insulated timber-frame structure
• Cladding, fascias and EPDM/fibreglass roof
• 100–150mm insulation in walls, floor and roof
• Double or triple glazing
• First-fix electrics: lighting, sockets, consumer unit
• Internal finish (typically painted plasterboard)
• Delivery and install

For an annex, plumbing and drainage are central — hot/cold supply, a Part G/H-compliant bathroom, a kitchenette and a heating system sized for daily living — all built to habitable-room building-regulations standards.

Planning permission for a garden annex in South West

Most garden annexs in South West fall under permitted development and don't require planning permission, provided the build is single-storey, no taller than 2.5m at the eaves (or 4m to a pitched ridge if more than 2m from any boundary), and doesn't cover more than half your garden.

Devon, Cornwall and Somerset councils have generally clear permitted-development policies, but many properties sit in National Parks or AONBs that override standard rules.

Unlike other garden buildings, an annex used as ancillary living accommodation almost always needs full planning permission and building-regulations sign-off, and you should check the council-tax position before committing.

Local context: building in South West

Higher rainfall and exposed coastal sites mean specifying EPDM or fibreglass roofing membranes and stainless fixings is well worth the small uplift.

Strong demand from remote workers relocating from London — installers report 8–12 week lead times in summer months.

How to choose a garden annex installer in South West

When comparing quotes, look beyond headline prices. The four quality markers that matter most are: insulation depth (aim for 100mm minimum), structural warranty (10 years is standard, 25 is excellent), build approach (bespoke vs modular vs kit), and whether they handle planning and groundworks themselves or sub-contract them.

Ask to visit a previous garden annex build in South West before signing — most reputable installers will arrange this. Check that the company has been trading for at least 3–5 years and look for consistent independent reviews on Trustpilot, Google and Houzz.

Always get at least three quotes, with itemised pricing for foundations, structure, glazing and electrics so you can compare apples-to-apples. Be wary of any quote significantly cheaper than the others — corners are usually being cut on insulation, glazing or warranty.