Two-Storey Side Extensions: Plans, Approval & Build

What you get

A two-storey side extension suits a detached or semi-detached house with a side garden, driveway or garage wide enough to build on. You get a design that sits comfortably on the street, a planning application or permitted development check depending on your nation, structural design, building regulations approval and the build itself. We manage the whole process, and the work is carried out by vetted teams working under S&K.

It suits you if…

You have a side drive or garden wide enough for a room, and want to keep the rear garden
Your house is semi-detached or detached with space to the side
You need a fourth bedroom or a larger main bedroom with an en-suite
You want a utility room, study or downstairs toilet as well as space upstairs
Other houses on your street already have side extensions, which helps show what the council accepts

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

Design points that matter

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.

Matching the ridge and roof

A hipped house usually needs a hipped extension roof, and a gabled house a gable. Matching the pitch, tiles, ridge detail and brick bond is what makes a side extension look original rather than added, and councils look closely at it on a front-facing addition.

Side windows, boundaries and gutters

Upstairs windows in the side wall face the neighbour, so they are usually obscure-glazed and fixed, or left out. Building close to the boundary also raises questions about foundations, overhanging gutters and access for maintenance, so we design the eaves and gutter to stay within your land.

Access and services

Side extensions often cover the only route to the back garden, a gas meter, soil pipes or a drain run. We plan bin and bike access, move meters and pipes where needed and check drains early, because these items affect both cost and layout.

Planning and approvals by nation

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.

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, a two-storey side extension can be permitted development if it is at least 10.5 m from the boundary opposite the side wall, set back at least 1 m from the front of the house and does not increase the house's widest width by more than 50%. If it fails any test, or the house is in a protected area, a householder application costs £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 two-storey side extension is only permitted development under Class 1B if no part is within 10 m of any boundary and it is not on an elevation fronting a road. Side extensions rarely meet this, so plan for a householder application (£371, decided within two months) and a building warrant from the council before work starts.

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, permitted development for two-storey work is set out for rear extensions, so a two-storey side extension normally needs a planning application. The fee is £347 per dwelling, and councils aim for an average of 15 weeks on local applications. You also need building control approval from the council under the Technical Booklets.

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 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
Measured survey, priced in writing with the design stage
Design options that suit the house and street
Planning drawings and application, or a permitted development check in Wales
Structural engineering and foundation design
Building regulations drawings and approval
Party wall notices in England and Wales where needed
Moving meters, pipes and drains as part of the design
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 side extensions

How close to the boundary can a two-storey side extension be?

There is no single UK rule. In England it needs planning permission, and many councils ask for a gap at first floor to avoid a terracing effect. In Wales, permitted development needs at least 10.5 m to the opposite boundary. Building right up to the line may also mean Party Wall Act notices in England and Wales.

Can a two-storey side extension be wider than half my house?

In Wales, a two-storey side extension under permitted development must not increase the house's widest width by more than 50%. In England, Scotland and Northern Ireland it normally needs planning permission anyway, so the council judges width on design. As a rule, a narrower extension that is set back looks more like part of the original house.

Do I need my neighbour's permission to build a side extension?

You do not need your neighbour's permission for planning, but they can comment on the application. In England and Wales, if you build on or near the boundary or dig close to their foundations, you must serve notices under the Party Wall etc. Act 1996. In Scotland and Northern Ireland there is no Party Wall Act, so title deeds and common law apply.

Can I build a two-storey side extension over my driveway?

Yes, if the drive is wide enough for a useful room and the council accepts the design. You may lose off-road parking, which some councils weigh up, and the foundations need to suit the ground below. We check parking, drains and meter positions at the free site visit before any drawings are prepared.

Do upstairs side windows have to be obscure glass?

Usually, yes. Side-facing upper windows look towards a neighbour, so they are normally obscure-glazed and fixed below 1.7 m above the floor. That is a permitted development rule in England and Wales and a common planning condition elsewhere. We place main bedroom windows at the front or rear and use roof lights for extra light.

Related services

Two-storey extensions for detached houses

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.

Learn more

Extensions over a garage

An extension over a garage is a new first-floor room built on top of an existing attached garage, usually a bedroom, en-suite or study.

Learn more

Party wall notices for extensions

A party wall notice is a formal written notice that you must give your neighbours under the Party Wall etc. Act 1996 before certain work on or near a shared wall or boundary in England and Wales.

Two-storey side extensions