how to show light on particular area

i am working on a night scene, where i want to have light on my building, not the grass. I am using plane for grass. I have used even layers too. But it too didn’t work. I have attached pics for verification. any suggestions plz

I found the answer for that. Add a new layer. Assign the layer to the Light which we are using. In that light you would find culling mask. Select the newly created layer in the culling mask drop down menu. That’s it.