What would be best solution for adding different sized objects to world?

Hello,

I am working on a game where I use different sized objects - such as houses, trees etc. My question is, which way should I determine is the place I am going to place my object free or not? World is generated randomly every time the game is launched.

(I'm only using x and z coordinates) For example, I have tree in a item pool with a size of 10x10, in the location 10, 0, 10 and I am going to add a stone which is size 5x5 for example. So how do I tell the stone "hey you can't place yourself to coordinates from 5 to 15 on x axis and 5 to 15 on z axis, place somewhere else".

Yes a simple way would be to just write down all the coordinates which are taken (5,6,7 ... 14,15), but what if I have 1000 trees? Is there any better and faster way to locate free spot for the item other than looping through a list of coordinates which have also looped to be written in taking a slot?

Thanks in advance, K