I am considering creating an application that is able to work out a unique password for a site by hashing the url while using a secure master password as a salt (or technically pepper since it will always be the same?). I am incredibly inexperienced in the world of cryptology so if this is a stupid question then sorry for wasting your time (:
The aim of using salt is having a countermeasure against the Rainbow tables. In your case, we can think of as you are creating a password for each site by using a master password and the URL of the site as salt.
Normally, you will be secure against the rainbow tables. One, however, must consider the case that the master password is compromised. Then all of the sites are easily compromised since your URL is known and the hash algorithm is also known.
A better solution, first, use an off-line system to generate the password for each site using a KDF function ( as Bcrypt, Argon2, PBKDF2) using your master password. For the randomness in KDFs you can use the URL but add other random values, too. Now, save your master password as offline, or delete if you finished.
Then you can use these passwords directly. And, remember you will still need the salt and pepper in your websites against rainbow tables.