Building Regulations for Two-Storey Extensions

What you get

This service suits every two-storey extension, because each one needs building regulations approval wherever you live in the UK. You get technical drawings and a specification coordinated with the structural engineer, a full plans application, site inspections arranged at each stage, and the completion certificate you will need when you sell. We manage the process, and the work is carried out by vetted teams working under S&K.

It suits you if…

You have planning permission or permitted development confirmed and need technical approval
You want full plans approval before work starts, not a building notice
You want one team responsible for design, approval and the build
You need a completion certificate for a future sale

Design points that matter

Fire escape from the first floor

In England, first-floor habitable rooms need an escape window, with a clear opening of at least 0.33 m², at least 450 mm wide and high and its bottom no more than 1,100 mm above the floor, or a protected stair. Adding a storey above 4.5 m means a protected stairway or an alternative.

Insulation and U-values

In England, Part L sets limiting U-values for extensions: 0.18 W/m²K for walls, 0.15 for roofs, 0.18 for floors and 1.4 for windows. Glazing is normally limited to 25% of the new floor area plus windows removed, unless SAP or compensating calculations are used.

Ventilation, electrics and drainage

In England and Wales, Part F covers ventilation, including extract fans in kitchens and bathrooms. Part P covers electrical work, which must be certified. Part H covers drainage, and building over or near a public sewer may need a build-over agreement with the water company.

Full plans and inspections

Full plans approval checks the design before work starts, which we recommend for two-storey extensions. Building control then inspects key stages, such as foundations, damp-proofing, drainage and insulation, before issuing the completion certificate.

Dutyholders in England

Since October 2023, where more than one contractor is involved, a principal designer and principal contractor must be appointed. For domestic clients these duties normally pass to the contractor and designer, so on our projects we take them on.

Planning and approvals by nation

England

In England, building regulations approval comes from council building control or a registered building control approver, using the Approved Documents. Full plans are recommended for two-storey work. New editions of Approved Documents L and F take effect generally from 24 March 2027 and are not yet in force, so current projects follow the 2021 standards.

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, building regulations approval follows the Welsh Approved Documents, which differ from England's in places, notably Part L for energy efficiency. Approval comes from the council's building control service or a registered approver. The structure, fire, ventilation and drainage checks are broadly similar, with a completion certificate at the end.

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, you need a building warrant from the council's building standards service before work starts, assessed against the Scottish Building Standards Technical Handbooks rather than Approved Documents. At the end you submit a completion certificate, which the council accepts after its checks. No work should start until the warrant is granted.

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, building control approval comes from the local council under the Building Regulations (Northern Ireland), with the Technical Booklets setting out how to comply. A full plans application is usual for two-storey extensions, and the council inspects the work and issues a completion certificate.

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

Building regulations drawings and specification are a guide of £1,500–£4,000, and the building control or building warrant fee £800–£1,900, priced in writing before the design stage starts.

Item	Typical cost
Planning drawings (survey and design)	£2,000–£3,500
Building regulations drawings and specification	£1,500–£4,000
Structural engineer (calculations and beam design)	£750–£2,200
Building control or building warrant fee	£800–£1,900
Party wall surveyor, one agreed surveyor (England and Wales only)	£1,300–£1,500 plus VAT

What's included

Building regulations drawings and specification
Coordination with the structural engineer
Full plans or building warrant application
Fire escape, insulation and ventilation design
Build-over agreement with the water company where needed
Site inspections arranged at each stage
Completion certificate at the end

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 building regulations for two-storey extensions

Is a building notice or full plans better for a two-storey extension?

Full plans is better for most two-storey extensions. The design is checked and approved before work starts, so problems with structure or fire escape are found on paper rather than on site. A building notice relies on inspections during the build and suits only simple work. Scotland always needs a building warrant.

What is a completion certificate and why do I need one?

A completion certificate is the document from building control confirming that the work met the regulations. A buyer's solicitor will ask for it when you sell, and missing certificates can delay a sale. In Scotland, the council accepts your completion certificate after its own checks.

Do I need building regulations if I have planning permission?

Yes. Planning permission and building regulations are separate. Planning covers what the extension looks like and how it affects neighbours; building regulations cover how it is built and whether it is safe, warm and healthy. A two-storey extension needs both, unless permitted development replaces the planning part.

What is a build-over agreement?

A build-over agreement is permission from your water company to build over or near a public sewer. Many rear extensions sit near shared drains, and the water company will want access protected and the pipe checked. We check sewer records early so the agreement does not delay the start.

Are the new 2027 building regulations going to affect my extension?

New editions of Approved Documents L and F take effect generally from 24 March 2027 in England, and are not in force yet. Projects approved under the current rules follow the 2021 standards. If your build will start after that date, we check whether transitional arrangements apply.

Related services

Structural engineering for two-storey extensions

Structural engineering for a two-storey extension involves calculating how the new floors, walls and roof carry their loads down to the ground, and designing the beams, lintels, foundations and ties into the existing house to suit.

Learn more

Two-storey extension plans and drawings

Two-storey extension plans involve a measured survey of your house, design options, and a set of scaled drawings showing the existing and proposed floor plans, elevations, a site plan and a location plan.

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.

Building regulations for two-storey extensions