てもいい

「てもいい」expresses permission to do something.
If you turn this structure into a question, you get a sentence used to request permission from someone to do something, or to confirm whether a general rule allows that action. Accordingly, the responses will also differ. For example:
+ 君の隣に座ってもいいですか。 (Asking for permission)
➔ To grant permission: はい。/ はい、いいです。/ はい、どうぞ。
➔ To decline: いいえ。/ いいえ、ちょっと...
+ このマンションでは、犬を飼ってもいいんですか。 (Confirming permission according to a rule) ➔ Allowed: はい、飼ってもいいんですよ。 (In this case, people often do not omit the verb in the response.)
➔ Not allowed: いいえ、いけません。 etc.