@Milanin no, the spirit is the house itself... her human body is just manifestation, an avatar...
since her real body, the Tree become foundation of that House...
thats why when they do the deed, it affected the House...
not each floor, entire building is their 'child'... well technically it isn't real child, but extension of the house, but since it created from their 'deed' it can be said that its their 'child'
and usual house in japan (the bigger one) has 'storage house' on its backyard... easy example is like "Nobita House" they have small storage house in the backyard...