Exclusive use of common property

We have a small block of units. The ‘penthouse’ unit sadly, has the only access to the roof space that contains our water heaters and some electrical wiring etc. The floors are not boarded and there is no lighting. The owner of the penthouse wishes to use a ‘corner’ of the roof space for her exclusive personal use to store items. Most owners are not keen for her to do this and have cited insurance issues, fire hazard etc as reasons for not wanting this to happen. She does not have access unless given the security master key.  She has been told at an AGM that she will need clearance from our insurers, a by-law giving her exclusive use of commone property and perhaps some structural reports.  What sort of numbers are required in an EGM to block her application?  What other issues have we  not thought of?  All information is very much appreciated.