With all due respect you may have misread my comment about the noise issue of the adjoining apartment wall to your apartment being separate from the structural issue. I was specifically referring to your particular issue and not noise in general.
The example that you cited of an upstairs bathroom being re situated in a Lot above the bedroom of a lower Lot has extended the issue further than the original issue.
The original issue merely concerned removing a wall between a kitchen and a lounge room whereas your example went further than that. Your extended example may cause confusion to the readers of this site.
Re-locating a bathroom would trigger more than structural change by-laws issues. It would trigger other issues such as flooring types, which usually has its own by-law provision. It would also trigger common property issues with plumbing, and possibly common property slab and wall intrusions.
It is very unlikely that a downstairs Lot Owner would not be aware of a bathroom being installed above your bedroom before this renovation actually occurred.
Even if a committee was to approve such a change then this would be noted in the Minutes. The Minutes should be supplied to all Lot Owners in a timely manner – usually 7 to 10 days after the Meeting. Within that time frame no action is to be taken so as to allow any affected Lot Owner to mount a challenge to the Committee decision.
It is the Lot Owner’s responsibility to read the Minutes to ensure that the Committee is acting reasonably in decisions that impact the Lot Owner.
If it is the opinion of any Lot Owner that they will be negatively impacted by any of the Committee’s decisions made at a Committee Meeting, or that the Committee has not acted reasonably in making that decision, then the Lot Owner can challenge the Committee decision.
It is the responsibility of the Lot Owner concerned to provide evidence to the Committee that the proposed work would cause a nuisance, hazard or interfere unreasonably with the use or enjoyment of their Lot.
I live in QLD where a Committee and Body Corporate’s responsibility to “act reasonably” is covered in at two sections of the BCCMA.
Incidentally, in QLD it would possibly be unreasonable for a Committee to deny an upstairs Lot Owner an approval to lay a tiled floor merely based on noise from the tiled floor if the Lot Owner acknowledged that they were prepared to undertake and comply with all acoustic impact systems and to provide the maximum underlay and floor preparation to minimize any noise transference emanating from their Lot.