What you can do is to get this tool
When clicking somewhere with this tool, it will give you the position (the lua vector) of where you clicked.
Now get your lua code with the localisation of the house :
houses.list[houseid].boxes = {{ Vector( 4509.2094726563, 13543.0390625, 0 ), Vector( 5829.8266601563, 15176.849609375, 948.96875 )}}
The the vector with the lowest Z ( like Vector(x, y, z)) is this one :
Vector( 5829.8266601563, 15176.849609375, 948.96875 ) So in theory it should be the dirt.
Now take the tool i told you to download, hit the cave and let's say you have something like
Vector( 5845, 14226, 400 ) Just take the 400 value, sub 1 or 2 to it just to be sure.
So you get 398.
Turn your
Vector( 5829.8266601563, 15176.849609375, 948.96875 ) into
Vector( 5829.8266601563, 15176.849609375, 398 )