You will be financially better off to seek an exclusive use of this land.
If you buy it you will pay for a surveyor, avaluer, a lawyer,and the unit entitlements will need to be changed. A lot of expense
if you get an elusive use by law,you may just need a lawyer and a surveyor.
The result is the same – your title now extends to the garden area, and it’s yours to do what you want with it.