Willow Brook and the lanes of the Old Village
If you live near the brook, start there. The Environment Agency runs a flood warning area called Willow Brook in Corby, and it names the properties it covers. Chapel Lane, The Jamb, Tunwell Lane, Boon Walk, Meeting Lane and Waterside Lane are on the list, along with Stanion Lane, The Grove and Geddington Road. Weldon has its own, taking in Church Walk, Maltings Lane, Bridge Street and Hillside Crescent.
Being on a warning list isn't quite the same as sitting in Flood Zone 3, so look up your own address on the government's flood map before anyone draws a plan. If the house falls in Zone 2 or 3 and the work needs an application, the Environment Agency's standing advice for minor extensions (anything up to 250 square metres) goes in with it.
Tunwell Lane also runs through the Old Village conservation area, among the listed stone cottages. So a back kitchen there can need a flood assessment, planning permission and, on a listed house, listed building consent as well. Three separate hurdles for one room. Find out before you book a builder.
Ten ways to add room, in the order Corby houses suit them
Single-storey rear. Most Corby semis and new town terraces can go 3 metres out from the original back wall (4 metres for a detached house) without an application. Outside the conservation areas, the prior approval route lets you go to 6 or 8 metres after the neighbours have been notified.
Kitchen. For most households here this is the reason to extend at all. Pushing a kitchen out into the garden is the natural move on a semi. Plan the rooflights early, as a deep room goes dark in the middle.
Open plan. Taking out the old back wall means a steel beam and new padstones carrying it. On filled ironstone ground or glacial clay, the engineer has to think about what sits under those posts as hard as the beam itself.
Wrap-around. Needs a corner or a wide plot with space beside the house. Outside a conservation area the side part must stay single storey, under 4 metres high and no wider than half the original house. Inside one, it needs planning permission.
Two storey. An application nearly every time. Permitted development stops a two-storey rear addition at 3 metres deep and 7 metres off the back boundary, and in any of the nine conservation areas it isn't permitted at all.
Side return. Corby has few of the Victorian terraces with a narrow yard beside the back wing. Where a semi has a passage to the garden instead, infilling it can work, though you lose the route for bins.
Downstairs bedroom. Useful where the stairs have become the problem in a two-storey semi. A bedroom and shower room on the ground floor, with level thresholds planned from day one, keeps everyone at home longer.
Home office. Attached rooms share the house's heating and wiring. A detached cabin on a shallow base can move on clay.
Planning. Check four things before you pay for drawings. The conservation area boundary, the flood map, whether the house is listed, and whether the permission for your estate took away householder rights. The council's planning register answers the last one.
Cost. We won't publish a Corby figure. Deep or piled foundations on backfill, a flood assessment by the brook and the size of the steel are what move a quote here, so get three builders to price one set of drawings.
No Corby rulebook, so your neighbour's light decides
Some councils publish a householder design guide with a 45 degree test and a maximum depth. Corby doesn't have one. North Northamptonshire inherited extension guides from East Northamptonshire (2020) and Wellingborough (2002), but those belong to their own districts. The Corby planning documents on the council's list are three village conservation appraisals and a planning obligations paper.
What an officer weighs instead is Policy 8 of the Joint Core Strategy, adopted in 2016. Council reports on Corby householder schemes sum it up as no unacceptable harm to neighbours through loss of light or overlooking, alongside the national push for design that respects local character. No fixed number, then. Judgement.
On the Lloyds estate the 2008 appraisal gives a flavour of that judgement. It calls a car-port tacked onto the side of one house an undesirable suburban intrusion and singles out a porch on white columns. Keep additions behind the house, in matching brick and render, and you're working with the appraisal rather than against it.
One more Lloyds point. Permitted development measures from the original house, and for anything built before 1948 that means the house as it stood on 1 July 1948. A lean-to added in the 1960s already uses up part of your allowance.
Ironstone backfill, clay caps and radon
Much of Corby stands on ground that was dug up. The British Geological Survey maps extensive areas of former ironstone workings around the town as infilled ground, and many of the pits were only partly restored. Priors Hall Park went up on backfill that the NHBC says had to be re-engineered before raft foundations went on. If your house has a raft, an extension tied to it needs an engineer who has seen the original design.
Higher ground is capped with glacial till, which the survey notes can shrink and swell as it dries and wets. Between Cottingham and Gretton the Welland valley sides have a history of landslips in the Whitby Mudstone. And the Northampton Sand that fed the steelworks has the highest radon potential in the district. BRE radon guidance covers extensions as well as new houses, so ask building control at the Corby office whether your new floor needs a membrane.
Where these facts come from
- Environment Agency: Willow Brook in Corby flood warning area
- Environment Agency: Willow Brook in Weldon and Kings Cliffe
- GOV.UK: Flood risk assessment standing advice
- GOV.UK: Permitted development rights for householders, technical guidance
- Supplementary planning documents (North Northamptonshire Council)
- Conservation areas (North Northamptonshire Council)
- Lloyds Conservation Area Appraisal, March 2008
- BGS: Geology of the country around Kettering (Sheet 171)
- NHBC: Priors Hall Park case study
- Building control, Corby office (North Northamptonshire Council)
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Questions people in Corby ask
My house is on Tunwell Lane. Can I still extend at the back?
Often, yes, but check three things first. Whether the house is listed, the Old Village conservation area rules, and the flood map, since the street is in the Environment Agency's Willow Brook in Corby warning area. Any one of them can mean an application.
Can I build a two-storey side extension on my Lloyds estate semi?
Not without planning permission. The estate is a conservation area, and there no side extension or rear extension of more than one storey counts as permitted development. Expect the council to look hard at how it sits against the matched pair.
Does Corby use the 45 degree rule for extensions?
There is no Corby householder guide setting one. Officers judge loss of light and overlooking under Joint Core Strategy Policy 8, so a scheme that keeps clear of a neighbour's nearest window stands a better chance.
We're at Priors Hall Park. Is the old quarry a problem for an extension?
Not a bar, but it shapes the foundations. The houses sit on rafts over re-engineered quarry fill, so ask the developer or your conveyancing pack for the foundation drawings and pass them to whoever designs the join.