Building out into a terrace yard
Ask a Blackburn builder where most extensions go and the answer is the back yard. The mill terraces were laid out with a scullery behind the front room and a flagged yard running down to the back street, where the toilet used to be. Nearly half the borough's homes are terraced, according to the council's housing strategy, so that small walled space is where the extra room usually ends up.
On a terrace the government's permitted development allowance for a single storey addition is 3 metres beyond the original rear wall, 4 metres high at most, with eaves no taller than 3 metres if you're within 2 metres of the boundary. In a yard you are always within 2 metres of something. Most jobs here fit those limits without a planning application. Going longer, up to 6 metres, means asking the council for prior approval first, and your neighbours get a say.
Bear in mind the word original. If someone added a back kitchen decades ago, the 3 metres is measured from the house as first built, so the allowance may already be partly used. Measure it before you get excited.
How each type of extension tends to work here
Single storey rear. The everyday job behind a two up, two down. It usually takes over the yard, so think about where the bins and the back gate go before the footings are dug.
Kitchen. The old scullery is often a narrow, dim room. Pushing it out with a roof light or two over the worktop does more for a terrace kitchen than an extra metre of floor.
Open plan. Knocking the back room into a new extension means taking out a load-bearing wall in a house that may be well over a century old. Get an engineer to size the steel before anyone quotes a fixed price.
Two storey. Permitted development allows one only if it stays within 3 metres of the original rear wall and 7 metres from the back boundary. Few terraces with a yard onto a back street have that depth, so expect to apply.
Wrap-around. Mostly a semi or detached house job, on the interwar estates and newer streets where there is room at the side as well as the back. The side part makes it a planning application on most plots.
Side return. Only works where the back kitchen leaves a strip of yard beside it. Many Blackburn yards are walled on both sides, which can mean a party wall notice to each neighbour.
Ground floor bedroom. A steep terrace staircase is a real problem as people get older. A bedroom and level shower room in the yard can let someone stay in their own street, but check the door widths and the step down from the house early.
Home office. Few terraces here have the garden for a separate cabin, so an attached room off the back is often the only option.
Planning. The council judges applications against its Residential Design Guide. In a conservation area, side additions and anything over one storey at the back lose their permitted development rights altogether.
Cost. We don't publish a Blackburn figure. Steel, drainage and the state of the ground under an old yard vary too much from one house to the next. Get three priced visits and compare what each includes.
Old pits under the streets
People forget Blackburn mined coal. Cotton Town, the borough's history site, dates it to around 1569 on what became Coalpit Moor, with shafts at Coal Pits Farm about 50 feet deep and bell pits at Brandy House Brow and Old Bank Lane. It records two deep shafts in Mill Hill and an old shaft below the back streets of Norfolk Street and Suffolk Street. Further out there were collieries at Cherry Tree, Whitebirk, Knuzden and Little Harwood, and a lot of shafts along Sough Lane.
Footings come first, so check the address on the Mining Remediation Authority's free map before a spade goes in. Inside one of its development high risk areas, any work that could disturb coal seams or old workings needs the authority's permit, and a mine entry close to the house is something to design around, not over. Hit anything odd while digging and you stop and ring its hazard line. One old quarry at Coal Pits Farm was later filled with rubble from the demolished Market House, which is the sort of thing a trial hole finds.
What the council's design guide asks of a bigger extension
Anything beyond permitted development is judged against the borough's Residential Design Guide, revised in September 2012 and still quoted in planning reports. It sets no fixed rear depth that we could find. Instead it applies the 45 degree rule to the nearest windows next door and wants rear additions subordinate to the house, with matching materials and a roof form that suits it.
Side extensions get firmer rules. Two storeys at the side should step back 1.5 metres at first floor, which the council says keeps them subordinate, helps where new brick or stone won't bond neatly to old, and stops semis joining into a terrace. Flat roofs on two storey side additions are ruled out. Behind, it wants 21 metres between facing habitable windows, or 13.5 metres to a blank gable.
Close to the River Darwen at Ewood or the Blakewater between Cob Wall and the town centre, check the flood map too. An extension in Flood Zone 2 or 3 needs a flood risk assessment with the application, and the Environment Agency's advice is to set new floors well above the expected flood level.
Where these facts come from
- GOV.UK: permitted development rights for householders, technical guidance
- Blackburn with Darwen Council: committee report on side extension policy and coal mining
- Cotton Town: coal mining in the area
- Mining Remediation Authority: home extensions in former coal mining areas
- Environment Agency: flood risk assessment standing advice
- Blackburn with Darwen Housing Strategy 2026-2036
- Cotton Town: terraced and modern housing
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Questions people in Blackburn ask
My terrace is near Mill Hill. Should I worry about mine shafts?
Check rather than worry. Cotton Town records deep shafts in Mill Hill, so look the house up on the Mining Remediation Authority's map before the footings are designed. If it falls in a development high risk area you may need a permit first. Your builder should stop and report anything unexpected found while digging.
Can I build over the whole back yard?
Permitted development caps a single storey rear addition at 3 metres on a terrace, and outbuildings plus extensions can't cover more than half the land around the house. A full yard infill usually goes past one or both, so it needs an application.
Will the council let me put a two storey extension on the side of my semi?
It can, but expect it to ask for the upstairs to step back 1.5 metres from the front, a pitched roof that stays secondary to the main one, and nothing that crowds the street. The idea is that pairs of semis don't end up reading as a terrace.
I'm in the Corporation Park conservation area. What changes?
Side extensions and anything of more than one storey at the rear need planning permission there, and cladding is out. A single storey rear addition can still fall within permitted development, but check the council has no other restriction on your house before you build.