Blackpool

Hello I am wanting to extend my kitchen (flat roof) by 2m but there is a soil pipe on the wall which needs to be demolished. The soil pipe is from the upstairs bathroom (house has a pitched roof upstairs like a loft conversion). Soil pipe runs up the wall and then goes up into the eaves and then 1m across before it connects to upstairs WC. I would prefer not to see the soil pipe on the new extension wall so two questions are : Soil Pipe and Sewer Questions (1) If the soil pipe is boxed into the kitchen (I am keeping a nearby wall that it could be run down) what issues will I have trying to get the soil pipe over or under the new foundations? (2) If boxing in is not an option, can I use two elbows to create a step down (if needed) to allow the soil pipe (with 1:40 fall) to run inside the flat roof? In relation to (2) I want to keep the extension as low as possible, so eaves height or ideally less (so flat roof starts at or under existing gutter). 2.65m high walls in house so eaves is reasonably high. Hence elbow step down question. Foundation Question The extension will only be two new walls. The length (7.5m overall) and the (2m) return. The other (2m) return is already on the house. The question is, the length will join to a wall on the garage. So 5m will be new and 2.5m will be existing end of the garage wall, will the garage wall (single brick) need to knocked down to put deeper foundations in? Or would the inner leaf of the new garage wall element just require the deeper foundations? Many thanks