Building Close To or Over Public Sewers

Whether you are planning a new extension, conservatory or a new build property you should consider if you are going to be building close to or over a public sewer, if you are it could well affect the cost of the project and in some cases if the project can go ahead at all.

The water companies are employed to maintain and protect our sewer systems and obviously the construction of the new property or extension in close proximity could quite easily cause damage through impact, ground heave or undue loading.

If you are building close to a sewer and in most cases `close` is deemed to be 3mtrs from the center of the sewer you have to apply for a building over/close approval.

In most cases if the sewer is 300mm diameter or less the water companies will give approval as long as they can specify the foundation construction, this can vary depending on the depth of the sewer, the ground type and obviously what you intend to build but it can greatly affect the cost of the overall project.

We have been involved in projects where concrete piles were specified to help bridge the load of the extension and where one guy was asked to excavate down to a depth beyond the sewer (in excess of 3 metres) and mass fill with concrete up to just below ground level, so you can easily put 20% or 30% of the cost on a small extension build before you see any brickwork above ground.

How Do You Know Where The Sewers Are?

If you apply to the water companies they have drawings showing where they think their apparatus is, unfortunately this is usually the main sewers running beneath the roads and not the small diameter shared pipe work to the rear of properties. If you do get a sewer record showing apparatus near your property there will be a disclaimer saying it is only an estimated position so you would usually be required to have some investigations carried out at you own cost before they will consider your application.

These investigations will at the very least involve a sonar trace to confirm the line of the sewer and its proximity to your property and proposed construction footprint, if it is close and a full structural report is required a drain camera inspection will have to be undertaken (sometimes before and after the construction) so again you could be looking at £200 to £400 on top of your application fee (anywhere from £150 to £500)

Drainage Transfer 2011

Just to complicate matters in October 2011 the responsibility for all shared drainage and any private drainage beyond the boundary line of the property it serves comes under the jurisdiction of the water companies, so the shallow 100mm or 150mm drain running along the back of a row of terraced properties is now classed as a sewer and you have to go through the same process as building over a main sewer.

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