Hi
I think the first step is understand if contradicts the by-laws as the chairperson can rule it out of order if it conflicts with that. Otherwise it will be voted on at the meeting and you’d need to communicate the negative impacts to the enjoyment of your lot.
I assume your’ve spoken with the person that is wanting to undertake the garden to discuss the issues to work through them. If not that would be worth a try as sometimes people are unaware that they inconveniencing someone else.
Failing that as it’s something permanent I think you’re right not to risk this and get advice. I haven’t used Bannerman’s but I’ve heard David Bannerman on the Flat Chat podcast and he’s very knowledgeable and I know our strata manager uses this company as well. So perhaps that’s an avenue to follow up.