Both are acceptable. Past participles with BE may be parsed in two ways: as components of the passive construction or as predicate adjectivals.In the first example, built is understood as a component of the passive construction: the sentence asserts a past event.In the second example, built acts as an adjectival: the sentence asserts a current state.Note that although a simple present will usually designate a state and a simple past will usually designate an event, it is the entire discourse context, not the tense of the auxiliary, which determines which interpretation is to be applied. Passive built may be employed in the present tense: