If the use of a room is changed and could result in the load (weight) on the floor structure changing significantly work to strengthen the floor may be necessary.
For example, if a new bathroom suite is installed in a room where the floor structure is constructed of timber joists and boards, there is a significant risk of the floor being overloaded from the bath once it is full of water and in use. Therefore, work may be needed to strengthen such a floor.
A structural engineer or surveyor can assess the floor and determine this for you. If the floor does need to be strengthened they can produce the paperwork that a building control body will require before you commence the work.