そう is based on visual information by the speaker, whereas みたい does not have that restriction :)

Can this be put into negative, like 'it doesn't look ___'?

'mitakunai' for instance?

Aah, okay. thank you for the clarification! That was my second thought on how to make it negative.

Went ahead and expanded the usage parts, and will adjust the meaning to be a bit more inclusive.

First of, a small note: it should be な if it's preceding a noun (and に if preceding a verb), generally speaking. I would read the first as 'it's a school that looks like it was (once) a prison. I'm not sure what country you are from, but at least in the states, some restaurants have very distinct architecture, so that even if the restaurant goes out of business and some other company moves in, you can still look at the building and tell what it used to be. I would expect the だった form to be used in a case like that.

Ok, so did a bit of research on this one, @jamielin .

The second one doesn't really occur, except for a very specific usage (which I'll get to at the bottom). 

If you were to try and translate them literally, the first it "it looks to not be a dog", while the second is "it doesn't look like a dog."

However, you are not negating the feeling/idea you have (represented by みたい), but you are saying that your observation suggests that the thing you are looking at is not a dog.

Now, there is a way to use it, but it doesn't really have anything to do with みたい. You might know that the negative conjugation can be used to elicit a response from the other party*

A: かっこういいじゃない？ (Cool, isn't it?)

In this sense, you could attach じゃない to the end of the phrase to say that you think it is  dog, and you're looking for confirmation from the other party.

Hope this helps!