fuco – in NSW it’s not up to the Building Manager, but rather your request needs to be considered by way of a Special Resolution placed as an Item on the Agenda of a General Meeting of the Owners Corporation (O/C) together with plans / diagrams / photos etc, where you’ll need a minimum 75% of those present by unit entitlement, both personally and by proxy, to vote in favour of your proposal in order for it to succeed.
If Consent is granted by the O/C, then it would customarily include Conditions such as (but not limited to) making you entirely responsible for maintaining that part of the Common Property (roof-space), you granting the O/C and its Agents (e.g. contractors) unrestricted access to other parts of the roof-space via your Lot where that’s necessary, and requiring you to make regular payments to the O/C to account for the increased value of your Lot consequent to it having exclusive use of a portion of the Common Property, and with all of the above being incorporated in a Registered Special By-Law attached to the Strata Title of your Plan at your cost.
Part 5 / Division 4 of the NSW Strata Schemes Management Act (1996) applies.