We are hoping to build a summer room or a conservatory and would
prefer it to be off the kitchen. Ideal scenario for access to this
would be to take out the kitchen window and chop out the wall below.

Moving the kitchen base units and even the sink is no big deal
but...as usual the rising main "rises" under the sink. I only need to
move the pipe say 18 inches to one side of the window opening.

Problem is, all ground floor rooms in our house have suspended
concrete floors, so the question is...

Is it safe to drill a hole in the floor large enough to get an arm
through so as to grab the rising main so that I can re-route the
offending pipe?

Has anyone tried this before? I'm not sure how deep the cavity is
below the floor so I've already realised I would need to tie some
string round the end of the pipe before pulling it below the floor (in
case I drop it).

cheers
Mitch