How close can foundations be built to an existing drain?

Question

We’re planning an extension and there’s a drain running near where the foundations would go, so I’m trying to work out what’s allowed before we get into this

Grant · Answer

You’ll need at least half a metre around the drain, building control will give you the options once you have drawings etc

Barry · Answer

Your first port of call should be the architect if you have one. Then speak to your local water authority, they will have guidelines on how close and how the build should be done so as not to damage the drainage. Most likely there will be a fee but if your serious about getting the work done weighing up the cost against space and living in the future shouldn’t really come in to it.

Ken · Answer

Hi, Ive just had the same issue on a small extension, we had to make the extension smaller as it would have meant either applying for a build over agreement or altering a public sewer and that would have been too expensive, so the customer had to compromise.  Ideally the foundations should normally be 1.0m away at least.  If the drains are deeper than 1.0m ideally they should be 1.5-3.0m away.  Where this is not possible like our situation where the foundations ran along side a public sewer at 1.20m deep, we had to take the foundations down 300mm below the invert level, any pipes had to be sleeved and a double slip membrane with a compressible material applied to the inspection chamber so that the foundations did not put any pressure on the drain.  This was all designed by the structural engineer.

Don · Answer

It would either be a simple shift of the drain with matching sized rubber collars to free it from obstruction and install a rodding eye at ground level for easy clearing of any future blockages

Or laying founds and block either side and laying two 150 mm lintels above and carrying on the blockwork atop the lintels

How close can foundations be built to an existing drain?

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