Blackburn, Lancashire
Semi-detached rear extension in Blackburn
Semi-detached rear extension in Blackburn: single or two-storey extensions for semi-detached houses, with side access and one party wall. We measure the house, show you what the rules allow, price it in writing and build it.
Blackburn sits in Lancashire, with postcode districts including BB1, BB2 and BB3. Semi-detached houses are common across Lancashire, and their side access makes rear extensions quicker to build.
Book a free site visit
Tell us about the house and we'll call to arrange a free, no-obligation visit.
How much does a semi-detached rear extension cost in Blackburn?
In Blackburn, our guide for a semi-detached rear extension is £1,790–£2,910 per square metre including VAT for a standard specification. North West prices sit below our West Midlands baseline; access on terraced streets is the main variable.
| Size | Guide price inc VAT |
|---|---|
| 3 m wide × 4 m deep (12 m²) | £21,500–£35,000 |
| 4 m wide × 5 m deep (20 m²) | £35,500–£58,500 |
| 4 m wide × 6 m deep (24 m²) | £43,000–£70,000 |
Your written price follows a measured survey and an agreed design, so it reflects your house, not an average.
Do you need planning permission for a semi-detached rear extension in Blackburn?
Here is how the rules in England apply to a semi-detached rear extension in Blackburn.
Under England's householder permitted development rules you can usually add 3 metres at the back of an attached house, or 4 metres on a detached house, without a planning application, as long as the extension stays under 4 metres high and its eaves stay under 3 metres where it is within 2 metres of a boundary.
A two-storey rear extension can be permitted development in England only if it projects no more than 3 metres, stays at least 7 metres from the rear boundary, matches the existing roof pitch and has obscure-glazed upper side windows. In practice most double-storey rear extensions go through a householder planning application.
If 3 or 4 metres is not enough, England's prior approval route allows 6 metres on a terraced or semi-detached house and 8 metres on a detached house for a single-storey rear extension. Neighbours are consulted, the council decides within 42 days, and the fee is £249, rising to £310 from 8 December 2026.
Conservation areas, National Parks, National Landscapes and the Broads restrict permitted development, and listed buildings always need listed building consent. At the first visit we measure what is there now and tell you which route fits: permitted development, prior approval or a planning application.
What are homes in Blackburn like to extend?
Stone and brick terraces are common around Blackburn. Where houses back onto a ginnel or alley, deliveries can come in from the rear, which makes the build tidier.
How a semi-detached rear extension in Blackburn comes together
Look round and measure
A free visit to see the back of the house, measure it, check the drains and boundaries and hear what you need.
Design and guide price
Two or three layout options with a guide price for each, so you choose the design knowing what it costs.
Planning and regulations
We take the design through the right planning route, then produce the construction drawings and engineer's calculations for building control.
Fixed price and build
A written price for the agreed design, a start date and a named contact through the build, ending with the building control completion certificate.
Semi-detached rear extensions near Blackburn
Neighbouring places we cover include Darwen (3.7 miles) and Great Harwood (3.9 miles).
- Semi-detached rear extension in Darwen
- Semi-detached rear extension in Great Harwood
- Semi-detached rear extension in Accrington
- Semi-detached rear extension in Haslingden
- Semi-detached rear extension in Padiham
Other work in Blackburn:
FAQ
Semi-detached rear extension questions in Blackburn
Is the site visit in Blackburn free?
Yes. We visit, measure up and talk through the options with no cost and no obligation.
What postcode areas do you cover around Blackburn?
Including BB1, BB2 and BB3, and the surrounding Lancashire area.
What if my neighbour has already extended?
That can make a matching depth easier to justify. We measure theirs at the survey.
How long does a rear extension take to build in Blackburn?
Most single-storey rear extensions take 10 to 16 weeks on site and two-storey extensions 16 to 26 weeks, after design and approvals.
Can I get a price without a visit?
Our online guide-price tool gives a range in a couple of minutes. A written price needs a survey.
Do you handle building control in Blackburn?
Yes. We apply to the council or a registered building control approver and hand over the completion certificate.