Given players most times rush when they are buying land, it would be better for the warning to be after the purchase. If it's just before it players will not bother reading it because it would slow them down when buying the plot.
But yes the timing of the information of demolition should be coupled along with the purchase of land. Not when a player is a lvl 15 sprout and barely knows how the game works. Every other tutorial orientated message is displayed upon when a person can interact with the system it's for. Why not demolition too? It makes sense and it's more consistent.