Be careful of what lies beneath..
…because the surface never tells the full story.
If you’re building an extension, or a new building, always make sure you know what’s below the bit of ground you want to build on.
It’s very important you survey any existing drains on site. This needs to be completed early on, before design work starts. Need to map their position fairly accurately, how big they are [large pipes or small pipes], how deep they are and what direction they run in / the fall.
Existing drains often make things interesting, especially if the drains are shared with your neighbour as they are then effectively a public sewer and they fall under the jurisdiction of your local water authority.
Building over an existing drain or sewer can be risky. The weight of a new structure can crush the pipe, leading to serious damage and costly repairs from the local water authority. This means the drain will be blocked and the walls will subside.
This sort of work is controlled under the Building Regulations and building over an existing sewer is only allowed if the building work 'is constructed or carried out in a manner which will not overload or otherwise cause damage to the drain, sewer or disposal main either during or after construction.'
If the drains are shallow and not shared then it’s a fairly straightforward solution to drop a traditional strip foundation below the drain so there is no load above the drain run. The wall is built around the pipe with a lintel over the top.
If the drains are deep, over 1m deep, then it’s not usually cost effective to excavate and lay strip foundations that deep down. There are also health and safety issues with trench collapse to consider as well.
If this is the case then there are other ways of building the foundations. Read on to find out about one of these ways.
So, be warned and be careful, always check out where your drains go before planning any building work, or things could get messy, literally!
How we fixed it for Rose Villas and were shortlisted for an award....
On a recent project, called Rose Villas, we had this exact problem, the extension needed to be built over a deep sewer. The sewer was over 2 metres deep and excavations would have needed to be about 3 metres deep. Separate approval was also required from Yorkshire Water.
After considering several options, I decided on mini-piled foundations. This method uses deep steel and concrete "stilts" to transfer the building's weight well below the sewer pipe, ensuring no pressure is placed on it. The piles or stilts are then topped with a reinforced concrete ground beam. The walls are then built off the ground beam. This effectively creates a bridged structure over the top of the drainage.
This not only dealt with the risk to the sewer but also proved to be a safer, faster, and more cost-effective solution than the alternative deep trench foundations.
The Rose Villas project was also a finalist for The Architectural Technology Awards 2025 [AT Awards] | Small Project of the Year. Partially in recognition of the number of technical problems that we overcame. It also looks amazing, but then I probably am slightly biased.
Want to create something amazing and reduce risks? Please do get in touch and we can kick things off with a chat.
New Places...
I love buildings and nature. I also love travelling to new places and cultures. Whenever I go on holiday, I try to take a few photos of buildings and places I find fascinating and interesting. I’m by no means a professional photographer but I’d like to share some of these places with you, hopefully they capture something of what I experienced.
Every newsletter I’ll share a photo with you but I’m not going to tell you where the photo was taken. If you think you know where the photo was taken or like the photo and want to find out more, hit reply and let me know.
Whatever next?
Next time we will dive into how to choose sustainable materials, and what I think is important. Keep an eye out to find out more.
That’s it for now, look forward to sharing more with you soon.
