Hello, fellow CEOs!
I saw people asking about this and wanting to do this, so I created a video tutorial on how to do it. But first, I would like to note, that this is not officially supported by the developers and is at your own risk. Neither I nor the developers are responsible for any bugs or lag you might encounter. I didn’t notice any performance difference with the area itself being bigger, but you can, of course, get more stuff in a bigger area which can lead to performance issues.
So, here is the video: [Airport CEO] How to expand the playing area - YouTube
If you have any problems or questions leave a comment or reply to this topic. I would also greatly appreciate any feedback.
Have a great day!
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Nice find! Thanks for letting everyone know about it!
Actually not my find, have to give credit to nice people at the Airport CEO Discord server. 
I’m glad if you found it useful anyway.
ah. Isn’t messing with save files really interesting?
I do like messing with the save files 
They are quite simple to understand. I already fixed a stuck vehicle on my airport this way 
Did You face any lag issues or any bugs when building on unlocked area
Didn’t notice any issues. I think the lag was more related to the airport being big as I noticed similar lag with a big airport only in the default tile. I noticed one bug where an aircraft taxied through the terminal because it had some trouble finding a path, but that occurred only once in like 5 saves where I did it and it likely has nothing to do with the unlocked tiles. As I said, can’t guarantee anything, but you should be fine.