"Someone was building a new building."
In the active voice sentence, make the subject of the passive voice sentence the doer of the action. Make the verb in the sentence in the corresponding form of the verb, usually in base form. And, if the object of the passive voice sentence is essential, include it.
For example, in the sentence "A new building was being built," the subject is "building," the verb is "was being built," and there's no object. To convert this sentence to active voice, we can say, "Someone was building a new building."