Q. Can one enter the unlocked suka of a friendly neighbor built on his front yard, without his knowledge (he is not home), to eat one’s own food and leaving everything as clean as it was?
A. Remah (O.H. 627: 3) rules that on the onset, one should not enter the sukka of a friend without his permission. Mishna Berura (ibid. 9) maintains that one may enter if he knows that the owner is not present and will not be arriving while he uses the sukka, and be disturbed by his presence. This permission is based on the principle that a person agrees that a mitzva should be performed with his property, as long as there is no loss (see question 1559).
Kaf Chachaim (ibid. 15) asserts that if the suka is well decorated and contains chairs with pillows and fresh tablecloths, one should not enter without asking permission.
Horav Shlomo Miller’s Shlit’a opinion is similar in case of need.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a