I've got a victorian house. Currently the bath/sink waste goes into the SP. I have a drain which is next to the soil pipe and after digging, can see they both join around 10ft down the garden (hell of a lot of digging) The RW pipe leads into a Waste drain (as used by the kitchen).. Is it legal/feasible to run the RW pipe straight to the soil pipe since they all join further down the line under the garden? Basically, all rear of the house shares a single underground pipe. Yet 2 different inputs at the top of the ground.
it's already an extension (T shaped house extension) pipes are on the right as you look towards the back of the house.
I can't imagine there should be a problem if you're simply rerouting existing pipes to the exact same end point. But I do know planning have certain requirements with respect to (a) use of existing waste for new installs (b) routing of surface water into the waste pipes.
Might have been a little drainage skullduggery when the extension built? You are not generally allowed to add additional surface water to a combined drain so maybe head the pipe off to a soakaway then forget to build it? I'm a big advocate of "if it ain't broke....." so I'd want to be sure my improvements weren't going to cause problems.