In this guide

4. Suitability of existing floor structures

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 building control will require before you commence the work.