First-floor extensions in Darwen
For a first-floor extension in Darwen, the first questions are always the same: how big, which approvals, and what it will cost. This page answers all three for Lancashire.
Before you start in Darwen, here's what applies.
- Postcode districts
- BB3
- County / area
- Lancashire, England
- Price region
- North West England
- Guide price
- £24,000–£34,500 (£1,710–£2,470 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
- Blackburn (3.7 miles), Haslingden (5.8 miles), Accrington (5.8 miles)
- Distance to Preston
- about 10.8 miles
- First visit
- Free, no obligation
About a first-floor extension
A first-floor extension suits you if your house already has a single-storey extension and you need more space upstairs. You get a structural check of the existing walls and foundations, a design that ties the new floor into the house, the approvals your nation needs, building regulations approval and the build, managed by us and carried out by vetted teams working under S&K.
Around Darwen we also work in Blackburn, Haslingden and Accrington, and across the rest of Lancashire. Postcode district in and near Darwen: BB3.
What it costs in Darwen
In North West England, which includes Darwen, we'd expect a first-floor extension to cost around £24,000–£34,500 for 4 m × 3.5 m of new first-floor space (14 m²), or £1,710–£2,470 per m² including VAT. Kitchens, bathroom fittings and flooring are priced separately so you can compare like with like.
| Spec | Per m² | 4 m × 3.5 m of new first-floor space (14 m²) |
|---|---|---|
| Essential | £1,570–£2,270 per m² | £22,000–£32,000 |
| Standard | £1,710–£2,470 per m² | £24,000–£34,500 |
| Premium | £2,050–£2,960 per m² | £28,500–£41,500 |
Planning and building regulations in Darwen
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 Darwen homes
Tying into the house
The new floor and walls must line up with the existing first floor, often through a new opening in the rear wall above the old one. Floor levels, the position of upstairs windows and the existing roof all affect where the opening can go, so we survey both floors carefully.
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.
How we'd run your project
Free site visit
We visit your home in Darwen, look at the structure, access and boundaries, and talk through what you want the new rooms to do.
Get it approved
Once you're happy with the design we submit it to the council for Darwen and deal with any questions from the planning officer.
Detail and price
Construction drawings, engineer's calculations and building control sign-off, then a written fixed price for the build.
Build it
The new structure goes up first, then we open into the house, finish inside and get the completion certificate.
Questions about first-floor extensions in Darwen
How much does a first-floor extension cost in Darwen?
Our guide for North West England, which covers Darwen, is £1,710–£2,470 per m² including VAT. A typical project with 4 m × 3.5 m of new first-floor space (14 m²) comes to about £24,000–£34,500. Design and approval fees are extra, and we fix the build price in writing once the design is agreed.
Which postcodes do you cover around Darwen?
We cover Darwen (BB3) and nearby places including Blackburn, Haslingden and Accrington, plus the rest of Lancashire. The first site visit is free.
How long does a first-floor extension take?
Allow 4 to 8 weeks for survey and design, then the approval period (councils should decide householder applications within 8 weeks of validation), then 4 to 8 weeks of technical design, which can overlap. The build itself usually takes 14 to 20 weeks, longer for large or complex designs.
Can I build on top of my existing single-storey extension?
Often yes, but only after the structure has been checked. Foundations, walls and lintels built for one storey may not carry two. We dig trial holes, look for the original building control approval and ask the structural engineer to confirm what can stay, what needs strengthening and what must be rebuilt.
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.
Nearby
Other towns near Darwen:
Other extensions in Darwen
- Rear
- Side
- Wraparound
- Part two-storey
- Over a garage
- Semi-detached
- Detached
- Kitchen and bedrooms
- Bedroom and en-suite
All two-storey extension services in Darwen · First-floor extensions across Lancashire
Ready to talk it through? Call us or send the form and we'll arrange a visit to your home in Darwen.