It's true that the house isn't easy to live in with 9 people. Mike designed the house but that was before he was married to Carol. During that time the house was designed for 6 or maybe 7 people. Mike and his late wife had one bedroom and their own bathroom, Alice had her own room and her own bathroom, Greg had his own room and Peter and Bobby shared a room but the boys all shared the same bathroom. Back then it was fine because it was all boys. Mike probably designed the house and had it built before Bobby was born if there are only 3 upstairs bedrooms. Peter and Greg probably both had their own room and when Bobby was born, Peter ended up sharing his room with him. Remember in the pilot, Peter and Bobby had bunk beds and Greg had his own room.
But when Mike married Carol, Greg's room became the girls' room and Greg, Peter and Bobby all had to share the same room and the boys and girls all had to share the same bathroom which is not easy when it's boys AND girls. They really should have a house where the boys have their own bathroom and the girls have their own bathroom.
I do like the beginning though, where Greg knocks on the door but Marcia doesn't hear him because she's playing her record too loud so she doesn't answer, Greg knocks louder and Marcia doesn't answer, Greg pounds on the door and Marcia still doesn't answer! Greg, now fed up, goes "Aw, screw it!" and just walks in on Marcia in her underwear! Very funny!