Unless the owner is a serial renovator, they may be quite shocked to learn that changes to the roof may result in them taking over ongoing responsibility for the roof (I say “may”) because nothing is decided yet. So this early process is informative for all owners.
We are at this same stage in my block. Serial renovators have moved in and are keen to make modifications. We are in discussion about what the OC is and is not likely to approve. No point in the owner paying for professional reports for anything that is going to be an outright “no”. Taking over the roof cavity (common property) for their hot water tank is likely to be a no. Putting aircon on the roof in a non-visible area is likely to be a yes with them taking on responsibility for that section of the roof. So it’s not worth them spending any more money exploring their hot water plan, but they can confidently spend money exploring the aircon plan in more detail. In fact, their aircon plan is likely to prompt the Committee to develop a broader aircon by-law that covers all lots.
If you don’t feel confident discussing drawings, ask for a physical walk-through. If they won’t allow that, then you can hold off having any discussions about their Reno. We had to do that with our serial renovators who didn’t want to provide access for the purpose of discussing their plans. Eventually they worked out that No access = no discussion = no approval, so we’ve moved on from that particular stand-off.