If the land is privately owned (not part of the public highway) then the HA can do as it wishes in terms of who it allows access to the land.
A more usual arrangment is that the HA puts yellow lines down to stop OTHERS (non-residents) parking there, blocking off the spaces the residents should be in.
What does your tenancy arrangement say about your entitlement to parking space? - can't belive the HA would just turn up and do this with no notice to the residents.
What else there we haven't been told about this?