Hello Ian,
thank you for posting on Architect 3D forum webpage.
If Undo Preferences are enabled (See Main Menu > Edit > Undo Preferences. Also Main Menu > Edit => Preferences > Undo Preferences) you may locate the backup file in the following folder:
Go to Windows’ File Explorer then local disc C: then enter folder Users => XXX(your user name) => AppData => Roaming => Architect 3D => XXX (the name of your Architect 3D (R) program) => Undo or Recovery => Note that the files will be deleted after you restart the program normally.
Keep in mind that, if you deactivate the UNDO option (Edit => Undo preferences) => this option deactivates also the backup => in this case, you won’t find the backup files of your work.
Kind regards,
Architect 3D Support team