Whales solution is totally correct but very bureaucratic and strictly by the book.
I’d be more inclined to grant them permission by a simple majority so long as the garden is appropriate to the complex and maintained by the lot owner at no cost to the Strata. Also be very clear about the area involved so that it doesn’t grow over time and a desc of what they plan to do.
Also the EC reserves the right to ask the owner to return it to it’s original state if the result doesn’t comply with what was described by the owner at the time of the request.
On virtually no circumstances would I consider granting them an Exclusive Use Bylaw because once passed they are virtually impossible to revoke and the Strata looses the area for good to the lot owner.