Today the drains were installed. As well as installing a new drain that crosses the span of the extension to go into the existing sewer, the builders also updated the old drain from the WC that had cracked – apparently it had been leaking for years! Never smelt anything though, so I guess most of the waste was flushed away!
I think the second part of the drain is the soil stack, and just closed off for now while the building work is being done. Will ask tomorrow. I did not see the soakaway go in, but it is below the soil just over 5m from the rear of the extension. All works approved by building control today.
They also put in a temporary drain for the existing kitchen – I assume this will be removed when the new kitchen sink is installed – hopefully not before!