Here is my idea of what they mean by environments. There are certain sections on the map where a new hood itself would not fit but if they expanded an already existent hood then it would be A Ok. The picture below for example there is a blank space where I have circled red. The neighborhood I highlighted would be an example of one that they would expand since it is small in public space. I would not think they would expand the neighborhood with the library, gym, etc. due to the already sprawling size of its public space but it is possible I guess. The second picture if what I would assume they mean by changing the environment. As you can see the houses are all clumped together kind of funky. If they added to the hood by expanding it lets say for a moment they expanded towards that area. They would essentially erase what appears to be there and create a more polished area since it is now a playable area.