Which trade for soil pipe/drainage works? Plumber, groundworks or general builder? Thinking ahead...

ChasingtheWelshdream

Following our issues finding a willing builder for a definite job, I'm thinking ahead now to some future work.  I'm unsure which trade(s) is the most appropriate to approach to see what is feasible in order to start planning and budgeting..

We want to install an en-suite and possibly an additional bathroom above, offset due to a window (funds allowing). The actual plumbing work etc. we can do ourselves. But the external drainage will need looking at and possibly an additional soil pipe installed first. 

We have two cast iron soil pipes, one of which is not currently in use (it seems the WC was relocated at some point). This pipe is the closest point to the proposed new WC and
we've had general vague conversations with a couple of plumbers in the past. One said it's impossible to branch into cast iron, the other said it was fine. 

Now this pipe, even if could be cut and branched, is just on the cusp of building regulations acceptance (5.5m) away from the proposed new WC on the 2nd storey. It may or may not be possible, but it would look pretty ugly. Then again, as the whole side of the house is a display for a variety of pipes already, it's not the end of the world and we haven't discounted it as it would be the cheapest way. (If we don't touch the pipe, it will be left in situ as I may well reinstate the existing WC to it's original position at some point. And it would weigh a ton to remove!)

The other option is to install a new soil pipe to serve the proposed bathrooms. The underground drainage runs parallel to the house (we think), but is unfortunately under a lovely thick concrete path. So not guaranteed without investigation. It is (we believe) a combined sewer and Building Regs will be involved.  The gas pipe is also underneath this path, so reluctant to disturb the concrete without justification.

So, we need to determine (a) the most appropriate method of serving the new WCs, and (b) which trade is best placed to help. We essentially require the external connections work, so we can carry on the internal work.

Who would you approach first? Plumber as - well - plumbing? Groundworks as potentially underground works and bashing up concrete? Or a general builder? Or, are Building Regs able to advise the correct way to go?

As I say, just musing at the moment and not quite sure where to start.

ChasingtheWelshdream

This is not to scale, but show roughly what I mean. The top floor bathroom is a 'would be nice', but not set in stone. Not least as I struggle to end up with a non-awkward bedroom 5. But it would make sense to install at the same time if possible - and before the kids become teenagers.

If anyone can see a way to improve the existing bathroom, feel free to comment ;-)