285. Leaving During a Meal
42:18 When visiting at someone’s home, one should not ask for food until offered by his hosts. One may not eat from a meal that is not sufficient for his hosts. If someone does this, even if invited by the host, he is compared to a thief. This is actually considered a serious transgression.
42:19 It is forbidden to leave the place where one ate until he has said birkat hamazon (grace after meals). (One may, however, depart to attend to a timely mitzvah, such as if it’s time to daven – Mishnah Brurah 170:29.) One should not even go mid-meal to finish eating in another room with the intention to return for bentching. One must even be careful about stepping outside with the intention to return to finish the meal. (Mishnah Brurah 178:34 cites sources that stepping outside and immediately returning is not problematic.) If one did leave (even though one is not supposed to), whether he finishes eating in his new location or returns to finish the meal, he does not repeat the blessing of hamotzi since it is still considered the same meal. However, he must make sure to eat at least an olive-sized piece of bread in the place where he will bentch.