* 화장실에 가도 돼요? (hwajangshire gado dwaeyo?) This is a polite and common way to ask. It literally translates to "Is it okay if I go to the restroom?"
* 화장실 좀 다녀올게요. (hwajangsil jom danyeoolgeyo.) This is also polite, but implies you're going to the restroom and will be back soon. It translates roughly to "I'll be right back after going to the restroom." This is a good option if you're already in the middle of something.
* 화장실 가도 되겠습니까? (hwajangsil gado doegesseumnikka?) This is a very formal and polite way to ask, suitable for elders or people of high authority.
* 화장실 좀 갈게요. (hwajangsil jom galgeyo.) This is less formal than the first two options, suitable for friends or close acquaintances.
In most situations, 화장실에 가도 돼요? (hwajangshire gado dwaeyo?) is a safe and appropriate choice.