Advice Sought for Foundation System/Drains/Sewer

[Narthecius]

I am hoping to build a small (5.5mx4m) single-storey rear extension on the back of our 1930's end-of-terrace property. Prior to getting planning/building regs drawings approved, I want to confirm the feasibility of the project and approximate cost of the foundation system before I proceed.

 

Soil surveys I was given when I bought the property indicate the ground conditions are poor (made-up ground) and it's possible a pile or raft system is needed.

 

There is an adopted sewer (invert level 1m and all pipes <160mm) that converges in the alley next to my property. As far as I understand, building over pipes is relatively simple when using traditional footings, but more complicated with pile/raft systems. Having read through Thames Water's build over guide, piled foundations must be 1.5m from the sewer.

 

I have attached a (guessed & not to scale) drawing of the sewer system. Does anyone have a recommendation on how to proceed from here? CCTV drain survey? Engage a structural engineer? Ask Thames Water? Many thanks!

 

[Mr Punter]

Perhaps start by looking at what other houses nearby have rear extensions.  You could talk to Building Control and find out what foundation systems are typically used and whether there are any foundation issues locally.  You will probably want a structural engineer for the project so if you can find one that has dealt with the ground conditions and sewer proximity before they should be able to propose a sensible design.

[Narthecius]

Thanks @Mr Punter. The bottom 3 properties on my drawing have extended. Apparently 1 has a raft, 1 has traditional footings and 1 has piles, which isn’t very helpful 😂. Good idea re. Building Control - I will get in touch.