Generally, any area that is open to other tenants is considered common area. In condos or other multi unit properties, the inside of the unit, the landing at the front door, and any rear yard or deck that is enclosed is typically considered part of the unit. The rest, unless it is spelled out in writing, is considered common area.
Is the area in front of your unit fenced off so only you have access? If so, it would probably be considered part of your unit. If not, then it is probably common area.
Do you maintain it, water it, keep it clean and repaired, pay for water & utilities for that area? If so, then it might be considered part of your unit. If not, then probably common area.
Since you are a tenant, the simplest way to find an answer would be to look at your lease. Is it included in your lease? If not, then it is not part of your unit and you cannot deny access to anyone. Continuous usage combined with the payment of rent for that part of the property might give you an argument that it is part of your unit. But if you did not pay additional rent for that, then you probably do not have the right to claim exclusive use.