Quick question

The original household would have to sell the secondary properties first, back to the town, so the ones splitting off can purchase them anew.

If you also had NRaas StoryProgression and the SP Money module, then there would be the option of the original household retaining ownership of the secondary lots and becoming the landlords of the ones splitting off and moving into them, who would then become tenants and pay rent plus only a fraction of the property taxes with the usual twice weekly bills. But that option does not exist without the mod.

Thank you @igazor! You're the best!

Would you know if there was a way, through the NRAAS mods, since family size is not limited, to "assign" certain sims in your family to call one place "home" while others use another lot that they own?

No, playing with multiple lots unless they are right next door to each other or something like that can be really awkward. The entire household lives on the primary lot. You can use creative door locking techniques (with or without our GoHere mod) and bed assignments to keep the segments of the active household somewhat separated, but Service Sims, party and other invited guests, resets will frequently put your sims on the lot you didn't expect them to be on. The refrigerators will still share contents, mail will only be delivered to the primary lot, etc. Owning multiple lots can be helpful in some instances, for example if you have a large number of animals you wish to keep together, but it's not a very well thought out or developed part of the game.

So, in regards to beds and door locks, if I specify only Sims A B and C can use the doors on lot 1, and assign them beds on lot one, and then specify sims D E and F to be able to use the doors on lot 2, and assign them beds on lot 2, with D, E, and F go to lot 2 after work? I'm not terribly worried about mail or resets or parties or anything like that, I'd just like to keep Grandma and Grandpa under loose control but living separately if I can without too much hassle.

I think that any Go Home type of command issued by the game will send them to the front door of the primary lot even if they cannot use it. But it's worth a try to see how it works out.

nards! Yeah I'll try it out and let you know! :)

Yeah I don't even seem to be able to assign the beds or lock the doors for specific sims on the secondary lot. Ah well, c'est la vie!

The door locking can be achieved with NRaas GoHere, it's flexible enough to work on any kind of lot. But perhaps I was mistaken about the beds. Regardless, playing across two (or more) owned residential lots is always going to be some degree of awkward.

I have often broken up my households between lots. It has variable results. And seems a bit buggy.

But, I have found that if I'm persistent and keep sending the designated sims to the secondary lot they eventually get the idea and tend to stay there.

I had no problems assigning beds or locking doors for particular sims in the second house. I don't have GoHere in my game as the thing I like best about Plant Sims is their flower trail and GoHere was stopping that to happen. But I do have lots of other NRAAS mods.

What I did have problems with is persistent specific weird routing errors. For example, I could direct a toddler to read a book I'd placed on the floor but the toddler couldn't read the book. Displayed routing error thought bubble. And, my sim tried carrying her toddler to the birthday cake to blow out the candles and got routing error. Eventually I gave up on all of this and moved everyone back to the original house. Which I had to tell them to 'move to' as I'd made the secondary house the main house. Once they were back in their original home all issues (of which I am aware) were resolved.