It's a little bit of an apples-to-oranges comparison:
建てられた = 建てる + られる + た (passive + past)
建てました = 建てる + ます + た ( polite + past)
One verb has the polite auxiliary 〜ます, the other has the passive auxiliary 〜られる. Both verbs have the past auxiliary 〜た. But these are all separate variables, and we can use any combination of them:
建てる = 建てる + ( )
建てた = 建てる + + た ( past)
建てます = 建てる + ます ( polite )
建てました = 建てる + ます + た ( polite + past)
建てられる = 建てる + られる (passive )
建てられた = 建てる + られる + た (passive + past)
建てられます = 建てる + られる + ます (passive + polite )
建てられました = 建てる + られる + ます + た (passive + polite + past)
But I think we can set aside the polite and past auxiliaries and just compare passive to active.
Forming the passive
Let's start with a basic active sentence:
新しい大学のビルを建てた。
Like many Japanese sentences, this one doesn't have an explicit subject. That's fine, of course, but it'll be easier to talk about the passive here if we put one in. We won't worry here about whether it's natural to do so or whether we should use は. I'll just add 私が to the sentence:
私が新しい大学のビルを建てた。
I built a new university building.
We can make this into a direct passive with three steps:
Add 〜られる.
私が 新しい大学のビルを 建てられた。
Replace が with に, and replace を with が.
私に 新しい大学のビルが 建てられた。
Move the が-phrase before the に-phrase.
新しい大学のビルが 私に 建てられた。
(Japanese word order is flexible and the が-phrase doesn't have to come before the に-phrase, but I include this step because I think it's the most basic word order for a direct passive.)
Now we've got a passive sentence:
新しい大学のビルが私に建てられた。
A new university building was built by me.
In this sentence, just like in its English translation, the building is the subject. (In Japanese the subject is marked with が, while in English it's marked by its position at the beginning of the sentence.) The person who built it isn't the subject anymore—they now appear as 私に "by me", a phrase that appears later in the sentence.
Of course, you might think "by me" sounds a little silly in English. That's true, and most often we'd leave it out in English. We can remove 私に in Japanese, too, but unlike in English we can also remove 私が from the active version.
Let's remove 私 from both versions:
新しい大学のビルが建てられた。
A new university building was built. (passive)
新しい大学のビルを建てた。
[I] built a new university building. (active)
And now we have sentences like yours. Hopefully you can see the difference :-)