Not knowing the precise layout of the penthouse, the only other issues that I can think of are:
1) how the Owners Corporation ensures unrestricted and perpetual access to the roof space in order to maintain or to permit others to maintain the hotwater heaters and anything else there; and
2) how the O/C could periodically check upon what / how much / what weight of items were stored in the area; and
3) who is to be responsible for maintenance / repairs of the area should the worst happen (e.g. structural problems) – remembering that if that’s to be the Lot Owner then they need to agree to that provision in writing; and
4) how well subsequent Owners of the Lot would comply with any requirements of the O/C (because the benefits of a Special By-Law pass to all future Owners); and
5) who pays for the preparation and Registering of the necessary Special By-Law (typically the benefiting Owner); and
6) what if any increase in the market value of the Lot would arise from that Owner’s use of the Common Property in the way proposed and how that could be accounted for.
With regard to that last point (6), IF needed that’s best managed by way of a once-off or annual payment to the O/C that’s provided for in the terms of the Special By-Law (SBL).
So far as consideration of the Motion at the EGM is concerned, if it’s exclusive use that the penthouse Owner is requesting then that requires a “special resolution” where a minimum 25% of those Owners in attendance at the Meeting both personally and by proxy would need to vote against the Motion in order for it to be defeated, and with that percentage being determined from the sum unit entitlement of those against and the aggregate unit entitlement of the Plan (i.e. a “poll vote”).
In considering all of this, remember that once granted a SBL of the type proposed can only be rescinded by a special resolution at a General Meeting, and only then with the prior written consent of the then current Owner of the benefiting Lot – so be cautious!