Detached House Two-Storey Extensions

What you get

This service suits you if you own a detached house and want a substantial amount of extra space, whether a side extension over a drive, a rear extension into the garden or a wraparound. You get a survey of the whole plot, options that compare layouts and costs, the approvals your nation needs, structural design and the build, carried out by vetted teams working under S&K.

It suits you if…

You own a detached house with space to the side, the rear or both
You want to add two or more bedrooms, or a main bedroom suite
You want to compare side, rear and wraparound options before choosing
Your house sits close to one boundary and has more room on the other side

Line drawing: existing house in outline, new work in brick, blue dashed line at first-floor level.

Design points that matter

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.

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.

Gaps to boundaries

Detached houses are often defined by the space around them. Keeping a gap to the side boundary at first floor maintains that character, helps planning and leaves access to the rear garden. In Wales the 10.5 m side distance decides whether permitted development applies.

Larger spans and steelwork

Bigger extensions often mean wider openings and longer spans, which need more steel and sometimes a steel frame. The engineer designs these early, because the size of the steel affects ceiling heights, floor depths and the cost.

Planning and approvals by nation

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.

Wales

In Wales, detached houses are more likely to meet the permitted development tests for a two-storey side extension: 10.5 m to the boundary opposite the side wall, a 1 m set-back from the front and no more than 50% added width. Rear permitted development allows 4 m on the ground floor and 3 m on the first floor. Otherwise the fee is £585.

Approval in Wales: Householder planning application: £585. Councils normally decide householder applications within 8 weeks.

Building regulations: Building regulations approval under the Welsh Approved Documents, which differ from England's in places (notably energy efficiency).

Scotland

In Scotland, a detached house on a large plot may be able to use Class 1B permitted development if no part of the two-storey extension is within 10 m of any boundary, it is no higher than the house and is not in a conservation area. Otherwise a householder application costs £371. A building warrant is needed either way.

Approval in Scotland: Householder planning application: £371. Councils have two months to decide a local planning application.

Building standards: A building warrant from the council's building standards service before work starts, and a completion certificate at the end. Scotland uses its own Technical Handbooks.

Northern Ireland

In Northern Ireland, a two-storey rear extension on a detached house 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, with height, materials and roof pitch conditions. Side and wraparound designs normally need a planning application, which costs £347 per dwelling.

Approval in Northern Ireland: Planning application for works to a house: £347. Councils aim to decide local applications within an average of 15 weeks; many take longer.

Building control: Building control approval from the local council under the Building Regulations (Northern Ireland), using the Technical Booklets.

Guide price

As a guide, a two-storey side extension to a detached house costs £86,000–£115,500 including VAT at 20% for a 3 m × 7 m footprint on two floors (42 m² of new floor space), based on West Midlands prices and our Standard spec; London and the South East are higher.

Guide prices by region
Region	Per m² of new floor	Typical project
London	£2,460–£3,300 per m²	£103,500–£138,500
South East England	£2,260–£3,030 per m²	£94,500–£127,000
East of England	£2,110–£2,830 per m²	£88,500–£119,000
South West England	£2,110–£2,830 per m²	£88,500–£119,000
West Midlands	£2,050–£2,750 per m²	£86,000–£115,500
East Midlands	£2,010–£2,700 per m²	£84,500–£113,000
North West England	£1,950–£2,610 per m²	£82,000–£109,500
Yorkshire and the Humber	£1,890–£2,530 per m²	£79,000–£106,500
North East England	£1,840–£2,480 per m²	£77,500–£104,000
Wales	£1,890–£2,530 per m²	£79,000–£106,500
Scotland	£1,930–£2,580 per m²	£81,000–£108,500
Northern Ireland	£1,800–£2,420 per m²	£76,000–£101,500

Typical project: a 3 m × 7 m footprint on two floors (42 m² of new floor space). Including VAT at 20%, Standard spec. Essential spec is about 8% less, Premium about 20% more. See all prices and fees or work out your own guide price.

What's included

Free site visit and survey of the whole plot
Options comparing side, rear and wraparound layouts
Permitted development check or planning application
Structural engineering for larger spans
Building regulations drawings and approval
The build, carried out by vetted teams working under S&K

How it works

Free site visit
We look at your house, the boundaries, access and drains, and talk through what you need the new space to do.
Design and approval
Measured survey and drawings, then planning permission or a lawful development certificate, depending on the rules where you live.
Engineering and fixed price
Structural calculations and building regulations approval, then a fixed build price in writing.
Build and sign-off
Vetted teams working under S&K build it, and we arrange the completion certificate.

Questions about two-storey extensions for detached houses

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 two-storey extension on a detached house more likely to be permitted development?

It can be, because detached houses often sit further from boundaries. In Wales a side extension can qualify if it is 10.5 m from the opposite boundary. In Scotland, Class 1B needs 10 m to every boundary, which some larger plots meet. In England, side extensions on two floors always need permission.

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.

Do I need a party wall notice on a detached house?

Possibly. Even with no shared wall, in England and Wales the Party Wall etc. Act 1996 can apply if you build a wall on the boundary line, or dig foundations within 3 m of a neighbour's building and deeper than theirs. Notices for that work need at least 1 month. Scotland and Northern Ireland have no Party Wall Act.

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.

Related services

Two-storey side extensions

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.

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Two-storey wraparound extensions

A two-storey wraparound extension is an L-shaped addition that extends both the side and the rear of your house, on two floors.

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Two-storey annexe extensions

A two-storey annexe extension is an attached extension laid out as self-contained living space on two floors, usually with a living area and kitchenette below and a bedroom and shower room above, for a relative living with the family.