I do not have enough outdoor space to build a sukkah and will have to rely on people’s sukkah in the neighborhood. My question is whether I need to ask permission to enter their sukkah. Obviously if I plan on eating a meal with my family in their sukkah I will speak to them first. However, what about just to eat a snack and have a drink?
Yes, you may enter someone else’s sukkah without permission so long as there is no one currently inside. If there is someone inside then then you should ask their permission.
Explanation: Initially, one may not enter into another person’s Sukkah without permission even if he does not intend to steal it from him, if the person is currently in his Sukkah as perhaps the owner is particular in this as he does not want his friend to see him eating or doing his other needs [in his Sukkah] without his permission. However, it is even initially permitted to enter his Sukkah while the owner [or family members] is not there as certainly the owner will not mind, as a person desires that a Mitzvah be done with his property if it does not incur him any monetary loss.
Sources: See Admur 637:9; Rama 637:3; Taz 637:4; M”B 637:9; Kaf Hachaim 637:15; See Piskeiy Teshuvos 638:2