The door next to the staircase thats on the right of the bedroom. Its a little confusing atm cuz its a Temporary door until the backyard entrance is finished.
There was a text box letting people know about the temporary door, before the hangout quest triggers, but ill have to double check to see if its still there. I might have accidentally gotten rid of it.
Eventaully there will be a proper pop-up with pictures showing you where it is.