There isn't anything like that but when you start up FreeSO there is a 'Sandbox' button in your upper left corner.
Click that, choose the empty lot from the list and build there. You can save what you are building with Ctrl+S and it'll be on the list for you the next time you go to Sandbox mode.
Hit P to pause so your Sims wont be a distraction (speeds 1,2,3 are also available in sandbox mode if you need to test any routing or anything.