Two-storey side extensions in Dorset

What a two-storey side extension involves

A two-storey side extension is a new two-floor addition built into the space beside your house, often over a driveway or side passage. It typically adds one or two bedrooms upstairs and a utility, study or larger kitchen below, without giving up rear garden.

Dorset towns we list: Bournemouth (BH1), Poole (BH15), Weymouth (DT3), Christchurch (BH6), Ferndown (BH22) and Dorchester (DT1). Every other address in Dorset is covered as well.

Guide price in South West England

Our guide for South West England is £2,110–£2,830 per m² including VAT. On a project with a 3 m × 7 m footprint on two floors (42 m² of new floor space), that puts a two-storey side extension in Dorset at roughly £88,500–£119,000. Professional fees for drawings and the structural engineer are listed separately on our prices page.

Rules in England

In England, a two-storey side extension always needs householder planning permission, because permitted development only allows single-storey extensions beyond a side wall. The fee is £548 (or £575 for applications made from 8 December 2026), and councils should decide within 8 weeks of validation. Expect them to look at set-back, roof height, gaps to the boundary and the effect on neighbours.

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

Set-down and set-back

Councils usually want a two-storey side extension to look subordinate to the house. Setting the front wall back from the main front and dropping the ridge slightly below the main roof helps it read as an addition, and is often expected in local design guidance.

Avoiding a terracing effect

Where semi-detached houses sit close together, side extensions on neighbouring houses can join up and turn a row of pairs into a terrace. Many councils ask for a gap to the boundary at first floor to stop this, so we check local guidance before fixing the width.

How we work in Dorset

1. Look at the house

   We start in Dorset with a free visit: measurements, photos, a look at the loft and drains, and a frank chat about budget.

2. Drawings, then consent

   We draw the scheme, check it against the rules in England, and take it through the right consent with the council.

3. Fixed price in writing

   With approved drawings and calculations, we give you a fixed build price and a programme you can plan around.

4. Build and hand over

   We manage the build from groundworks to decoration-ready finish and hand over with the completion paperwork.

Towns we cover in Dorset

• Bournemouth
• Poole
• Weymouth
• Christchurch
• Ferndown
• Dorchester
• Wimborne Minster
• Canford Heath
• Verwood
• Portland
• Bridport
• Blandford Forum
• Swanage
• Broadstone
• Parkstone
• Sherborne

Neighbouring areas

• Two-storey side extensions in the Isle of Wight
• Two-storey side extensions in Wiltshire
• Two-storey side extensions in Hampshire
• Two-storey side extensions in Somerset

All extension services in Dorset · About two-storey side extensions