Should your title have a "to" on the end -- you're buying a house with a "lean to" roof, not a lean on the title, right?
If it didn't need planning permission, then why would there be problems with the "authorities"? What your local government requires and can do is very specific to your locality. You should post what city and state this is in, but even then you'll be lucky to find someone who knows the specific issues in your area. If the house was built at a time when approval would have been required, then you could have a problem. If it was built long ago, and no one knows when the extention was built, it is harder to prove a problem.
Another "authority" is a homeowners association. Usually, they get on people right away for things that don't conform, but they can have a great amount of power.