You can finish the Drudanae quest before talking to Lanilor to get permission, but if you do I believe Otho comes around when you first enter the elf's garden and rightly chews you out for being a thief. I don't know how important it is to keep Otho happy, but if you don't want to tick the dwarf off make sure to help Lanilor free from his icy prison first. Once you get George's first quest (getting the Drudanae), Lanilor will have a speech option to ask for permission.
