Anyone had to build over a sewer line?

[MsShopper]

I've planned our upcoming extension to the last detail, except it seems the need to check whether it will be built over a sewer line. For some reason, I thought that only applied if there was a manhole in your back garden (which there isn't). But our builder recommended I contact the water company to check.

So, I've bought a site plan and it shows where the drains are (see pic - our house and the new extension marked in pink). They go across the back of the house, exactly where our new extension will be built. So am I right in thinking I now need to get a formal build-over agreement from the water company? I'm sure I do, but thought I'd seek out the wisdom of Mumsnet before I spend several hundred pounds that I'd rather not spend. Any insights very welcome!

Does the sewer actually go where it’s shown on the map? I’ve had experience of the sewer taking a different route to the one on the map

Might be worth speaking to your neighbours if any of them have had their extensions in recent years, since they seemed to have been built over the drains/sewer.

I think you probably do unfortunately, or at least you’ll have to have an inspection to ascertain if you building over it will be problematic from a maintenance perspective. We had a similar situation ( although we do have an inspection chamber actually within the house, not even in the garden). Ours turned out to be a private sewer in the end so not needed.

Yes you will need to seek a build over agreement. They may also need a manhole putting int your garden for maintenance if the sewer will be affected. .

Yes you will need a build-over agreement. You can either a agree to replace the pipe or have a camera survey done and ask your water company to approve the build over without replacement, if it’s all in good condition.

Fwiw, the sewer maps are often inaccurate, so a camera survey may be useful anyway to identify the exact location of the pipes. Our map shows the pipes running along our side alley, when they actually run along next door’s and the associated manhole cover is on their side too.

Another consideration you may beed to think about is where the rainwater run off from your new roof is going to go. That held up our build initially, as the building inspector refused to accept it could go into the existing combined waste water system. Again we needed written confirmation from our water company before building control would allow it.

Yes you do.

The foundations for your extension will need to be done differently, to ensure the gravitational forces from the weight of your extension don't crush the sewer line. This will make your extension more expensive.

I had similar plans and abandoned them once the full weight of the additional costs and time/effort became apparent. It is possible but it's usually cheaper and easier to move to a bigger house.

As PP have stated, yes you do need a build over agreement. The water company or buildings regs may require service access/a manhole to be put in place although sewers can be worked on remotely now.

When you get it the agreement be sure to keep all documentation relating to it (planning permission/buildings regs etc) safe because you will need them if/when you sell.

A belated thanks for all the replies!

I just wanted to update in case anyone else is finds themselves in our shoes. In the end, we didn't need a formal build-over agreement. We had a pre-build CCTV drains survey done (which we'd planned to do anyway), which confirmed that the drains were in good condition, there is no manhole / access on our property, and the pipes are 150mm and 100mm in diameter.

Our water company (Severn Trent) allows you to do a self-certification in these circumstances (the criteria are listed here, under 'Application forms'). We got the self-certification validated within a week. So all go on the build - phew!

Just to add that the CCTV drains guys explained how the builders would need to protect the sewer pipe. Effectively, they create a bridge that protects the pipe underneath so that it doesn't have to bear any weight from the new foundations. Fairly straightforward, he said. I've shared that with our builder, who seemed similarly sanguine! No mention of additional costs either, but I'll update if that changes.