How do I make a part "placeable" only in a players area?

I am creating a tycoon and in some places you can build on a tycoon. I have created a part moving tool, but I do not know how to make the players be able to place these bricks only on their own tycoon.

How should I go about this?

When I have it check what the brick is hitting it can be placed off the platform because it may still be touching the platform.

Should I add walls? - Issues: He can walk through his tycoon entrance and keep the brick on the wall, but it goes through the wall.

I know this may seem like jimmer-jam, but I am running out of hope on trying to make this work. Maybe can I get some example code from a Sandbox Game maker?