It should be overhearing the girls complaining about the bathroom at work. You then go to the business policies and pay the fee for the unisex bathroom remodel. (I have had some playthroughs that overhearing the girls seemed to take longer than others.)I am on day 86 and the option to upgrade the room (which leads to building the dungeon) did not trigger. Cannot find a reason for this occurring. What exactly is the trigger.
Then you can do your room. After your room, you can then do mom's, sister's, bathroom, dungeon. For the big harem remodel, you need at least one girlfriend that agreed to the polyamory relationship.