"Where did you build your house?"
Translation:Di mana Anda membangun rumah Anda?
Indeed I have the same issue, and do not agree. Kalian is used for speaking to more than one person. But in English this is both you. So we cannot know about the question of Duolingo means one or more persons so both kamu and kalian should be accepted as being correct. I recently asked this to my Indonesian teacher in Yogyakarta.
Anda (only written with the capital A at the start of a sentence) is the Indonesian polite word for you. In Dutch it would be "u". Kamu is the less polite word for you. Anda can also be used plural, like Terima kasih anda untuk penerbang dengan Garuda. (Thank you for flying with Garuda). Anda is used in officila letter and when you don't know the name you are speaking with. Kamu is used when you know the person you are speaking with very well like a family member or a friend, not with an older family member. At Java it is (important) appreciated to be more polite. It is considered un-polite to not use someones name, but anda. Like; Ya terima kasih bu Ugi. O tidak pak Sinto. Pak Sinto sudah makan? Is polite. To say to mister Sinto: Anda sudah makan? Is considered un-polite (if you know his name). Kalian is plural for kamu to address more then 1 person. Like saying to your children to go to sleep: Ayo kalian harus tidur.
The "nya" is indication of possession. In this case you can imagine you speek with your neighbour. Di mana kamu bangun rumahny?. Would be meaning: where did you build the house. Di mana Tono bangun rumahnya? where did Tono build his house. If I ask you "Bagaimana mobilnya? I ask: how about your car? I am not sure if you understand it now a bit better. I know exactly how and when I can use "nya" but I am not sure if I can explain it clearly to someone else. Partly because my native language is Dutch not English. If you are still confused, please reply and I try again.