I'm not sure how I got it. But make sure you have atleast 500K$+ because the cost to make the bathroom is 100K$ but if you bargain the price could goes to 200K$ or something.
To trigger the scene you must wait till your payment day or when land lord come and ask you is you want bathroom etc.
Once you paid you have to wait a few days. Maybe try go out more to trigger the scene.
When you go out you have a chance of meeting him at your doorway which he will install your toilet. Around 11-20 is the time?
And he will kick you and jun out for a while for the installation.