Two-storey extensions for detached houses in Wiltshire

Wiltshire at a glance

Nation: England
Price region: South West England
Postcode areas: BA, SN, SP
Towns covered: 14 listed below, plus surrounding villages
Approval route: Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); a two-storey side extension needs planning permission.
Site visits: Free, no obligation

What a two-storey extension for a detached house involves

A two-storey extension for a detached house is a two-floor addition to the side, rear or both of a house that stands on its own plot. Detached houses often have room on more than one side, so there is more choice of layout and fewer shared-wall issues. The design still needs to respect boundaries, neighbours' windows and the look of the house.

The main places we list in Wiltshire are Swindon (SN1), Salisbury (SP1), Trowbridge (BA14), Chippenham (SN15), Melksham (SN12) and Warminster (BA12). We cover the villages in between too.

Guide price in South West England

A guide price for Wiltshire: around £88,500–£119,000 for a 3 m × 7 m footprint on two floors (42 m² of new floor space). That works out at £2,110–£2,830 per m² of new floor space, including VAT at 20%. Kitchens, bathroom fittings and flooring are priced separately so you can compare like with like.

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, a two-storey rear extension on a detached house can be permitted development within 3 m of the original rear wall and at least 7 m from the rear boundary, with matching roof pitch and materials, outside protected areas. Any two-storey side or wraparound extension needs planning permission, costing £548 (or £575 for applications made from 8 December 2026), with a decision due within 8 weeks of validation.

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

Keeping it subordinate

Large detached houses can take a large extension, but councils still expect it to look secondary to the main house. A lower ridge, a modest set-back at the front and matching materials keep the proportions right, even on a wide plot.

Choosing the best side

On a detached house you can often extend on either side or the rear. We look at orientation for daylight, where the stairs and landing are, the distance to each boundary, drains and trees, so the extension goes where it adds the most useful space.

How we work in Wiltshire

Visit and first ideas
One of our team visits your house in Wiltshire, checks the walls, roof and drains, and sketches out options with you.
Design and approvals
We draw the extension and check it against the rules for England, then apply for planning permission or a certificate of lawfulness, whichever fits.
Engineer it
Foundations sized for two floors, steels for any openings, and building control approval, all before anyone starts on site.
Finish and certify
Second fix, plastering and making good, then building control's final inspection and the completion paperwork.

Towns we cover in Wiltshire

Two-storey extensions for detached houses in Wiltshire: questions

How much does a two-storey extension for a detached house cost in Wiltshire?

In South West England our guide is £2,110–£2,830 per m² including VAT, so about £88,500–£119,000 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.

How big an extension can a detached house take?

It depends on the plot, the boundaries and local design policy rather than a fixed limit. Under permitted development in England, extensions must not cover more than 50% of the land around the original house. With planning permission, the council judges size on proportion, neighbours and the character of the area.

Can I extend a detached house on both sides?

Sometimes. With planning permission, a council may accept extensions on both sides if the house stays well proportioned and keeps gaps to the boundaries. It is more common to extend on one side and the rear, or with a wraparound. We look at the plot as a whole before suggesting more than one extension.

Is a detached house extension a better investment than moving?

It often costs less than the gap between your house and a larger one nearby, once property tax, legal fees and removal costs are counted, but that depends on your area. Extending also has limits: every street has a ceiling price. We give you build costs so you can compare them with local asking prices.

Will I need planning permission in Wiltshire?

Wiltshire is in England. Depends on position: a two-storey rear extension may be permitted development (3 m deep, 7 m from the rear boundary); a two-storey side extension needs planning permission.

Neighbouring areas

All extension services in Wiltshire · About two-storey extensions for detached houses