First-floor extensions in Thornaby-on-Tees

Useful details for planning a first-floor extension in Thornaby-on-Tees.

Postcode districts: TS17
County / area: County Durham, England
Price region: North East England
Guide price: £22,500–£33,000 (£1,620–£2,340 per m²)
Approval route: It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission.
Nearest towns: Stockton-on-Tees (2.5 miles), Yarm (3.1 miles), Billingham (3.9 miles)
Distance to Darlington: about 10.1 miles
First visit: Free, no obligation

About a first-floor extension

A first-floor extension is a new upstairs room built on top of an existing single-storey part of your house, such as a rear kitchen extension, a utility or a side annexe. It adds a bedroom, bathroom or study without enlarging the footprint. Whether it works depends mainly on whether the walls and foundations below can carry the extra load.

We cover Thornaby-on-Tees (TS17) and the places around it, including Stockton-on-Tees (2.5 miles), Yarm (3.1 miles) and Billingham.

What it costs in Thornaby-on-Tees

A guide price for Thornaby-on-Tees: around £22,500–£33,000 for 4 m × 3.5 m of new first-floor space (14 m²). That works out at £1,620–£2,340 per m² of new floor space, including VAT at 20%. These are guides, not quotes. Ground conditions, steelwork and access can move the figure either way.

Spec	Per m²	4 m × 3.5 m of new first-floor space (14 m²)
Essential	£1,490–£2,150 per m²	£21,000–£30,000
Standard	£1,620–£2,340 per m²	£22,500–£33,000
Premium	£1,940–£2,810 per m²	£27,000–£39,500

Planning and building regulations in Thornaby-on-Tees

In England, adding a first floor turns a single-storey extension into one of more than one storey. To be permitted development, the extension as a whole must go no more than 3 m beyond the rear wall of the original house and stay at least 7 m from the rear boundary, with matching materials and roof pitch. Over a side extension it always needs planning permission, costing £548 (or £575 for applications made from 8 December 2026).

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.

Design points for Thornaby-on-Tees homes

Replacing the flat roof

The existing roof usually comes off and is replaced by a new first-floor structure. The room below is exposed for a short time, so we plan temporary weatherproofing and sequence the work to keep the kitchen or living room usable for as long as possible.

Escape and fire safety

A new first-floor bedroom needs an escape window with a clear opening of at least 0.33 m², at least 450 mm wide and 450 mm high, with the bottom no more than 1,100 mm above the floor, unless there is a protected route to the stairs.

How we'd run your project

Start at your house
We come to your home in Thornaby-on-Tees, measure up, check the ground floor walls and roof structure, and note anything near the boundaries.
Choose the approval route
Some designs fit permitted development, most need a householder application. We decide which applies in England before drawing in detail.
Engineer it
Foundations sized for two floors, steels for any openings, and building control approval, all before anyone starts on site.
Build and sign-off
Vetted teams working under S&K build the extension, and we arrange the building control completion certificate at the end.

Questions about first-floor extensions in Thornaby-on-Tees

Do I need planning permission for a first-floor extension in Thornaby-on-Tees?

Thornaby-on-Tees is in England, so England planning rules apply. It may be permitted development if it goes no more than 3 m beyond the original rear wall (counting any single-storey part) and stays 7 m from the rear boundary; otherwise it needs planning permission. If in doubt, a lawful development certificate or a planning application gives you certainty before you build.

How much does a first-floor extension cost in Thornaby-on-Tees?

Our guide for North East England, which covers Thornaby-on-Tees, is £1,620–£2,340 per m² including VAT. A typical project with 4 m × 3.5 m of new first-floor space (14 m²) comes to about £22,500–£33,000. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.

Who handles building regulations for a project in Thornaby-on-Tees?

We handle it as part of the project. What's needed in England: building regulations approval (full plans recommended) from the council or a registered building control approver, with a completion certificate at the end. We prepare the drawings and the engineer's calculations, book the inspections and pass you the completion paperwork at the end.

Will the first-floor extension match my house?

It should, and in England matching materials and roof pitch is a condition of permitted development. We match brick, tile, mortar colour and window style where we can, and tell you at the design stage if an exact brick match is not available, so you can choose between the closest match and render or cladding.

What happens to the room below while the first floor is built?

The old roof is removed, so the room below is exposed for a short period. We fit temporary weather protection, plan the work so the roof comes off only when the new structure is ready, and agree with you which rooms will be out of use and for roughly how long.

Nearby

Close to Thornaby-on-Tees? We work across the area, including:

Other extensions in Thornaby-on-Tees

All two-storey extension services in Thornaby-on-Tees · First-floor extensions across County Durham

The quickest next step is a visit. We'll look at your house in Thornaby-on-Tees and give you an honest view of the options.