If you are developing a "Mall Tycoon styled" game, the GUI is usually custom-built rather than using a standard tycoon kit. Expert developers recommend:
Modify the code that handles building placement to trigger a Pop-up UI instead. Mall Tycoon GUI
For a look at how these Set Pieces and their corresponding GUIs function during end-game play, check out this completion walkthrough: If you are developing a "Mall Tycoon styled"