An apartment owner does not strictly own "their" apartment, but has a share in the whole building with the exclusive right of use of certain parts (article 5:106(4) BW). In the Model Regulations 2017 those parts are called the private section: the part of the building and/or land that, according to the deed, is intended to be used by an owner as a separate unit (article 1.1(x)).
You may make changes in your private section without others' permission, provided this does not harm another part. You must notify the VvE of a change without delay (article 5:119 BW, from which the regulations may deviate). You may use the private section yourself or let someone else use it (article 5:120 BW).
Which elements are private and which are shared differs per building. Check the division drawing and the regulations; pipes or window frames in your apartment can be common property.
See also: Apartment right (appartementsrecht), Common parts (gemeenschappelijke gedeelten), Deed of division (splitsingsakte), Occupant (gebruiker)
Read more: What is a VvE?
Legal basis: Boek 5 BW, art. 5:106, 5:119, 5:120, Modelreglement 2017, art. 1.1