How close to existing foundations can a trench safely be? 
(water supply duct, assume 1m deep trench and 1m deep existing foundations)
Problem for the foundations?

Water only needs to be 750mm and if it’s a meter away and dug with a 12” bucket it won’t cause any harm but watch out for services to the other house when digging 

If you're not undermining the foundations of then the risk is low.

 

However, in some circumstances you need to consider the footings can become a small retaining wall if you excavate the soil along one side so safest is to do it in stages, excavate a length, backfill, then the next section. It's a lot of 'it depends' - footing size, soil type etc but whenever you deal with ground conditions, excessive caution is never punished.