I made a city by following Andrew Price’s tutorial but when I added the building group to the particle plane mesh, I get buildings sticking from below (facing down) and top(facing up) of the plane. How can I fix this? Urgent!!.
select all mesh and press E and give it a little tiny you know… what i mean 
or
maybe check normals?
Sounds like when you created the buildings you didn’t put the objects origin at the base. Check and see if all the buildings origin is at its base. If not, you need to move the mesh, in edit mode, up in Z until the origin is at the base.