Peter Taylor wrote:
"Pete" wrote in message
...

The bottom of the foundation must be at least 150mm below the bottom
of the
drain so it will be bearing on solid soil and won't put any load on
the drain.
That way you can go as close as you like, although put some formwork
in between
to stop the concrete surrounding the drain. And take care not to
move or damage
the drain, especially if it's an old stoneware one.

There are detailed Building Regs at:

http://tinyurl.com/6krzx

Look at section H 2.25 onwards

Hope this helps

Charlie