I really have no idea where your BC manager (or is it strata manager?) gets this idea.
You are right in as much as you will need a by-law to be passed which should include clauses that state that you and subsequent owners of the property will maintain the deck and should include a payment to the Owners Corp for the use of the airspace (based on the increase in value of your unit minus the cost of the deck's construction). You should also be responsible for the costs involved in drawing up the by-law and holding the general meeting to pass the by-law.
You may also require planning approval from your council – that will be assisted by the support of your executive committee (a simple majority will suffice).
As I said, I have no idea where the BC manager gets this idea that you require 100 percent approval and you should ask him where this is stated if he insists it's true – maybe you have a by-law that says any changes to common property have to have unanimous approval. If so, propose a change to the by-law … again that would only require a 75 percent vote in favour.
But honestly, I think the manager is wrong and you should worry about how well equipped he is to do his job if he doesn't have a basic grasp of strata law.