there are 2 things to consider
1. you need to have the same folder structure that the orignal theme . So property.php should be in libs
2. on property.php – i assume you want to add new fields , in this case you need just to use the add_metaboxes function and you can do that without doing a new property.php file. You can put the code in functions.php – child theme.
Also , you offer way to little information about what are you trying to do . Because of that my answers cannot be accurate and in most of the cases will not help you. Try to explain exactly(too many details is better than no details) what are you want to do and my answers will be much more helpful. 🙂