In my recent experience, I found it especially challenging getting a loan for such properties especially if the appraiser does their job well and dismisses illegal add-ons or even notes them. Lender's don't like words such as non-conforming when underwriting time comes.

There are two reasons why you see this a lot here
1. It is illegal to advertise a certain number of square feet if they do not actually exist.
2. We have a lot ot "Ohana" living situations, where lanais are enclosed, decks are built and rooms are added on.
These are not always done by getting permits, especially 20+ years ago. This a good reason for a home inspector to look over everything carefully. Even in homes where the square footage is the same, you may see bathrooms modified, walls constructed or removed and some suspect electrical connections.

When in doubt, verify everything including square footage. Ask lots of questions.

Aloha,
Frank