How should we know what we will be allowed to build

[Neda Navidnia]

I am a Realtor in Houston TX and I always deal with single families, buy, sell and buy and hold. I have a friend trying to buy a property, waiting for a while and then demolish and build something new. He is not sure what he is going to build, is it going to be a duplex, 2 townhouses or a single family.

Now we have found a duplex which is listed at lot value. 

How could we get the information about permits. In that area there are new SF s and old duplexes. But since this is his first time, he wants to know about all of his options.

City didn't give him the information because he doesn't own the property.

Please help us, how could we gather the information about permits and those kind of information?