This is going to be a bit new but what you need to do to get a multi-tile building to rotate as a single unit is to add “_north” to the end of all of the entries in overmap_specials.json, but no where else. This tells the game that the existing orientation of the map is north… I haven’t tried it but something like the below may work… but I’ve never used anything but _north to designate orientation.
{
"type" : "overmap_special",
"id" : "mansion_new",
"overmaps" : [
{ "point":[0,0,0], "overmap": "mansion_new_corner_north"},
{ "point":[1,0,0], "overmap": "mansion_new_side_north"},
{ "point":[2,0,0], "overmap": "mansion_new_corner_east"},
{ "point":[0,1,0], "overmap": "mansion_new_side_west"},
{ "point":[1,1,0], "overmap": "mansion_new_center_north"},
{ "point":[2,1,0], "overmap": "mansion_new_side_east"},
{ "point":[0,2,0], "overmap": "mansion_new_corner_south"},
{ "point":[1,2,0], "overmap": "mansion_new_entrance_north", "connect":"road"},
{ "point":[2,2,0], "overmap": "mansion_new__corner_west"}
],
"locations" : [ "wilderness" ],
"city_distance" : [3, 5],
"city_sizes" : [1, 12],
"occurrences" : [75, 100],
"rotate" : true
}