Yosef Karo

“ It is just like someone who has an opening to an alley that he doesn't use; he does not render forbidden. This assumes that the valley or enclosed area is bigger than two seahs, but if it is only two seahs, it is small and he does not like it as much. If a Jew has an opening to an alley and an opening behind his house to an enclosed area and he forgets to make an eruv, if the enclosed area is bigger than two seahs he renders the others forbidden because he cannot use it, so he is not removed from the alley. ”