Replacing Old Clay Sewage Pipe.

[SysMax]

Hi,
I'm building a kitchen extension and started the work on the underground drains. My property is an end of terrace semi.
I need to replace a sewage pipe that runs under our house (our rear kitchen) and connects to our 1st floor toilet. An outside toilet was once also connected to this pipe via smaller clay pipes but has been cemented over.
This larger clay pipe connects to our neighbours in series all along our street. After escavation I can see that I am the first house, there is no neighbouring raw sewage coming into our pipe work. All work is on my property.
I measured the clay pipe and it is aprox 7 1/2 inch (18-19cm) wide, outer width. So my question is with what size upvc pipe should I replace the old pipe, 160mm or 200mm? I was going to go with 160mm but am now unsure. Also if possible, what adapter should I use?
Btw I intend to one day build a Granny Annex at the end of our garden and connect to this pipe.
Thanks in advance.

 

[blup]

I would buy both and see what fits on the day you break the pipe, then measure the internal diameter, which may be greater on the plastic pipe than the clay. If so make sure they line up at the bottom so there isn't a ridge which catches any soil - i.e. waste - etc.

This youtube video is really helpful at 23.06 on.

Blup

 

[SysMax]

Thanks for your reply and advice Blub. The video was helpful. Yes, it's the inner diameter that's more important!
I'm going to go for the 160mm pipe and appropriate adapter. The inner diameter of my clay pipe is approximately 150mm wide.