I have looked through the terrain API and haven't found anything that helps get you adjacent terrain cells. I'm also not certain if terrain has its own coordinate system or what the distance between two adjacent pieces of terrain is. Is there a recommended way of getting adjacent cells?
Terrain has his own coordinate system. This will set the cell where you click and the adjacent sells to asphalt.
mouse = game.Players.LocalPlayer:GetMouse() mouse.Button1Down:connect(function() local cellpos = Workspace.Terrain:WorldToCell(mouse.Hit.p) --Get the cell position in terrain coordiante if cellpos then for x = cellpos.x - 1, cellpos.x + 1 do for z = cellpos.z - 1, cellpos.z + 1 do Workspace.Terrain:SetCell(x, cellpos.y-1, z, 5, 0, 1) --Set cells end end end end)