Underground Pipe diameter

5 Jan 2004
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United Kingdom
Here's a question that's been puzzling me for a few days.

I am plumbing in a new sink in a new extension on the house. There is a external gully a couple of feet away from the proposed sink location. I was planning to run a standard 32mm sink waste pipe to a hopper outside the extension wall. From the hopper I was going to lay 100mm diameter (I assume this is the standard diameter) underground pipe directly into the gully . As this pipe will only be used for sink waste, is there a smaller diameter pipe that can be used (legally) underground to run to the gully?

Thanks in advance.

Ade B
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