I think the EC could minute that an action for one of its members is to have a quiet word with the new member making the followng points:
-The EC appreciates the maintenance and improvements the unit owner is making to the area, does not have a problem with that, and is very appreciative of the efforts made.
-Nonetheless, explain to the owner that the EC (acting for the OC as a whole) has responsibility for maintenance of the common property. Consequently the EC is obliged to ensure that anything the owner does is safe and generally attractive and in keeping with the style of the place. So, please discuss any ideas you might have for the area.
-A fundamental principle in strata title is that all owners have a right to access and use common property. We recognise that in practice this particular bit only seems to be used by the owner of this one unit. Nonetheless, any improvements made should not restrict access to other owners or give the impression that it attaches exclusively to this unit.
As others have said, make it clear that this is common property while allowing the present arrangements to continue. Perhaps the owner does not realise this is not his bit. If that is the case, better to have been clear about it now.
We have similar arrangements in our set of townhouses:
-There are many little bits of common property that are really one used by one or perhaps two units and is generally maintained by those units. As a matter of long-standing policy, the style of our development is that unit owners area encouraged to blur the boundaries of their unit areas with the common property in preference to fences (although they are permitted). This creates a more open, parklike environment.
-One owner plants and waters in a larger area visible from that unit. She is pissed off with the EC for various reasons and does not communicate except when she thinks we have done something wrong, and then prefers to act via third parties. However, what she does on this bit of the common property is in keeping, harmless and generally useful. She never asks for reimbursement for the more expensive, more advance plants she buys. We seem to have an understanding that we leave this alone.
-In another area it is particularly understandable that particular strips were assumed by owners to be part of their unit areas although the unit plan shows they clearly are not. Two such owners put up fences to enclose the strips by their units. One fence was amicably removed when the unit sold and the new owner was made aware that the fence was unapproved. Another unit’s fence was not removed till we had explained it nicely, then again as a formal breach notice, then went to the ACT tribunal and still it was only removed after the unit owners were reminded in writing that failing to act on the orders of the tribunal was an offence.