Assuming you refer to a domestic drain as opposed to a public sewer, the general rules are that any foundation should be excavated to a level slightly below the drain invert (the bottom of the drain) where it is closer than about 1m. Drains can be run below extensions providing they are in good condition, the foundations are deepened locally as above, appropriate lintels are used in the walls that run overhead, and a suspended beam-and-block floor is installed. Good practice is to have an inspection chamber each side of any build above a drain, although one side only is acceptable. As a rule, only shallow drains need to be encased in concrete to protect them from, for instance, damage from vehicles.