Adding a room, a bathroom or a separate dwelling extends more than the floor area. It puts additional demand on systems that were sized for the original building.
Drainage is the first constraint. New fixtures need to reach existing drainage at an adequate fall, and where the new structure sits relative to that line determines what is practical. It also determines whether the existing line has capacity for the additional fixtures.
Water supply is the second. The existing supply pipe was sized for the original demand. Adding fixtures, particularly ones that run simultaneously, can reveal that the supply is now marginal, which shows up as pressure dropping when several outlets are used.
Hot water is the third, and it is the one most often overlooked. A separate dwelling may need its own unit rather than sharing, depending on distance, demand and how metering is arranged.
Then there is the question of what can be built where. Drainage lines and easements restrict what can be constructed over them, and that is established from the sewer service diagram and the property title rather than assumed.
All of this is cheaper to resolve at design stage than during construction.
