The key is located at the Fraternal Order of the Raven's main chapel:
On the altar table, next to a vial of infusion and a book, there is a chest key, that unlocks the Lansdowne Residence chest
Pick up the key to chest
You now need to find the lock that fits the key
Go back out and arrive at this street:
Arrive at the end of the street at plaza where "Dollar Bill" figure is standing:
Turn RIGHT and enter the door at the base of the stairs:
Go through the corridor and turn left at some room with knitting machines and go through this door that has the portraits next to it:
Now turn RIGHT and go through the door into the balcony:
Jump on to this building:
and into this one:
Go down the stairs to the middle floor and find the bedroom that has the big blue chest:
Use the chest key to unlock chest.
