This is exactly the same thing I've been doing with my Thyme family dynasty! Their house was two stories up and two stories down and filled to the brim with all kinds of stuff, not to mention an enormous garden. It has taken me three days of my time so far and it's still not done.
What I originally had in the lower basement was 7 large, ensuite rooms with a large central communal area. I figured the easiest thing for me to do was actually set up three of the suites to house the family. They had just gotten back from an extended vacation, (15 Sim days in France), and were exhausted, so I felt I could put them to bed and then work on the rest of the house. I didn't want to spend a lot of cash since they had depleted most of their resources in France working on their mansion there. (Remodel of the Abandoned Nectary found in the swap shop.) No cheats are allowed so testingcheatsenabled/build buy on this lot was out, but I could still use the regular build/buy.
It took me a full day of my time to get their basement home built, complete with a kitchen etc. etc. and completed the museum as well. Then, Quick Time/Itunes decided to update and BAM! Black screen of death.
>:(Of course, stupid me I hadn't saved a darn thing. When I calmed down enough to go back into the game, (day two), we were back in France. I finished the day there and sent them home again and started over. While they waited at the gate, game paused, I started piling stuff into the family inventory and moving it downstairs.
Rebuilt the downstairs, (differently but ok), and decided to save a LOT, (I sometimes do learn my lessons!), and faced day three with a much better attitude. Yesterday, (day 3), I stayed up till 6 am and have the entire ground level and second story demolished, saving construction materials where I could, (doors and windows), most of the museum floor done, and the lower level livable and resonably attractive. The lower level should actually be large enough to house all 8 immortals but I didn't want them all being moles. My intention with that level was, as the next gen was born the previous gen would move down stairs for privacy and to keep them out of the way while I played with the current gen and heir. Now I'm not so sure I want to build a gigantic mansion above ground. It was a whole lot more work than I had anticipated and it was sad to remove some of my favorite features but the game was so laggy with all that stuff I had to do something. Still figuring it all out.