Neighbours soil pipe in my garden can I have it inside my extention?

Firstly check with Building control. Who then will in turn check with the water Authority who controls the drainage in your area. Depending on the size of the drain you should be able to build over it. As long as the foundations go below the invert of the pipe. And a simple Lintol over the pipe where it enters and exits the building will be fine. May help depending on location of the existing manholes. To put another one in, to enable you to rod the drains in the event of a blockage

Yes. When you start your extension u need the architect first to inspect that. Your extension is costing you bit extra.

In most cases an existing soil pipe can be re-aligned.

If the relocation of pipe is unavoidable, then you have to do the following: Protect the pipe by distributing any subjected load away from the pipe using concrete or steel panels resting on strong strata to transfer the vertical load toward the soil NOT the pipe. Avoid burying elbows or Tees under your extension, as potential of blockage is high. Usually it is preferable to provide manholes at intersection points instead.