The owner of the unit needs permission before he can change common property (roof, loft space, ceiling) and the Owners Corporation needs a guarantee that he will maintain the skylight at his own expense. That requires a special resolution by-law which lays all this stuff out. For a special resolution by-law, he will need 75 percent of the owners to agree to this at a general meeting (it’s actually no more than 25 percent voting against and Unit Entitlements may come into play, but basically he needs six of the eight owners to agree).
Why does he have to pay for the drafting of the by-law (and calling the meeting)? Because he is the only person benefitting from this so why would anyone else vote in favour if it’s going to cost them money?
Why would the OC go to all this trouble? Because if you don’t the law quite clearly states that the skylight would become your responsibility if you don’t make it clear in a special resolution that it’s his.
And the other thing you need to know is that you should insist that he uses (and pays for) a specialist strata lawyer to draft the by-law. He can either use a lawyer of your choosing or pay for your lawyer to review his lawyer’s by-law but that would be a condition I would place on even considering the idea.
Why do you need a lawyer? Because they will tell you the important stuff like the correct section of the Act under which to create the bylaw (as Simone from Teys does below) which is different from the one that governs changes to common property.
By the way, if the owner just goes ahead and installs it, you can take him to the CTTT (and possibly to court) to force him to reinstate the roof to its original state.