GML depth issue

so i am making a game with isometric art can place buildings in it. now my problem is that when i have place a building and then a building above it the new building overlapse the old one what looks stupid. so how can i make it that it doesnt do that?

i am working in gamemaker 8.0 pro



I dont know much about GM8, but assuming its depth system is anything like GMS1.4 then you could use something like:
depth = -(y + x);