It is confirmed in the chapter itself that the grandmother lived in the room beside the kitchen.
page 28: "Yeah... Especially since there was a pool of blood from the grandma's room to the kitchen."
Another thing that is odd: There are only five members of family, why do they need five rooms?
Usually the parents shared a room. Children might shared a room, but maybe they wanted separated rooms from the get go, so that's three rooms. The immobile grandmother should live in 1st floor. So there are two extra rooms that they don't really need.
Looking at the plan, if the children used the northern rooms, I don't think it would be any of a problem. That's basically a giant shared room. However, if they use all the southern rooms, [child room] [child room] [parents room], now it would lead to problems. Like, if in the middle of the night the grandma need something, then the mother have to pass all the children rooms.
Nice touch with all the police wearing masks on the year of 2020. Oh. If it happened during Covid Outbreak, I can see how the mother - grandmother's relationship get worse rapidly.
Also Tsama, did you really need to misaligned the text on the floor plan, because of the crease from the folding? /stand ovation