Schema for a lease platform
If I were you I would make sure to separate all of the data. Is this going to be completely controlled in the database, or is there going to be an application? If it is an application I would have a users table and permissions tables for the people using the application. I would do a separate a customers table for your customers. I agree with the availability table, but if you are using a price based system I would store that separately. For billing I would set up a few tables. I would have a payment type table and a transaction table with due dates. I like the idea of a check in and out table as well.