Now on a purely architectural level it is certainly true that the thing underneath a roof and between four outside walls is a house, that the things behind the inside doors are rooms, and that the thing to which the inside doors open is a hallway. But is it legitimate to then say that, on an ecclesiastical level, the existence of multiple church groups and denominations equals one church?
It depends. If the "church" being described by Lewis's house is invisible, then sure, it is possible to say that the full number of elect souls who eventually wind up in heaven will have worshiped in varying local churches throughout history. No real controversy there.
But if Lewis's house is one of those houses that you can actually see from the street (i.e., the visible kind), then that changes things a bit. Sure, most Christians claim to accept other Christians as fellow housemates, but according to Paul, true unity is not only verbal, it is sacramental. If it is true that there is "one God," "one Lord," and "one Spirit," it is also true that there is "one body," "one faith," and "one baptism" (Eph. 4:4-5). This means that the claim on the part of an evangelical that he is "one" with a newly-converted cradle-Presbyterian is an empty claim if he insists that the Presbyterian be re-baptized upon his conversion. To give another example, if a Lutheran minister refuses to offer the bread and cup to a Reformed minister, or if a Reformed minister refuses to commune a Baptist, then any claim of unity rings rather hollow.
It seems to me that the number of visible churches directly corresponds to the number of binding-and-loosing authorities over those churches. So the pastor-led independent Bible congregation is one church, Calvary Chapel as a whole is another, and so is the PCA. NAPARC is not a visible church, but an affiliation of visible churches (for it has no ecclesiastical authority). The same can be said for evangelicalism in general. And the Catholic Church, of course, is a visible church because of its authority rooted in the Petrine See.
The question, then, is whether Jesus intended to build a single church with a visible head whose keys allow him to bind and loose, or whether many visible-yet-divided churches are permissible. And if the answer is the former, then what is it that unifies this one church's many congregations?