Procedural Building Generator HELP PLZ!

Hello all! I am trying to use the pbg to create buildings using prefabs and I have been trying to follow several tutorials but in all of them they use the “alt” + mouse drag when in the “top” perspective to extend the building to the desired width, length, etc… MY problem is that when I hold the “alt” and drag the prefab it just makes a copy and I cant extend it from the corners either.

In the pic attached I tried to extend but all it does it make copies
Anyone can help? Plz?