Chapter 10 house 1,820 pop for 15 squares:
A Chapter 10 house needs 724 culture and 1,5 roads (roads give culture - 51 per square, so thats 77 culture off) which results in 647 culture
647 Culture results in (with weeping willow) in 4,62 squares. So a Chapter 10 house needs 15 + 1,5 + 4,5 = 21 squares. Thats only 86 pop/square.
Btw.: can you fullfill your culture requirements with 10 weeping willows? I can't - at the moment I need 75000 culture (for Culture bonus 100%) and I will require even more in the next weeks. So for me culture is an important point I won't neglect in my calculations.