Well, I was considering making a separate topic about it: How to build a extra large castle or something. But I can start here and see if I feel the need to split this topic later on.
Thing is: I can't tell you how to do it, just how I did it and what works for me.
In short; I build from macro to micro. Meaning that I first only focus on the bigger picture, placing all the walls, devising spaces and test out connections between rooms.
It is important that you have at least in your mind how many sims and animals will live in the building. In my case I wanted everyone in it so that means sims, occult or not, cats, dogs, small reptiles fishes and mammals and most importantly: horses.
If you want to include horses you better know that from the start of as horses need a lot of space. If you take a look at the mansion I've build you can see that over 50% of the ground floor is horses area.
I needed horses to be able to run, train, live but without interfering patting issues with sims while both freely could leave the area but without letting in the paparazi.
So if you want horses you should be aware of all these things.
Eventually I came up with a large wall surrounding the whole area which made it possible for horses to run around the house, litterly, yet in a closed an secured area.
When that's done I could focus on the house itself, the dividing of areas, choosing what areas are for what while making sure all sims have easy access to rooms that fulfill their basic needs.
Also there are plenty of beds on the fourth floor for foreign visitors to sleep as you can invite them if you have World Adventures installed. You can invite 8 guests from another country so I needed 8 bedrooms all with their own bathroom, to avoid them to use yours. Because they will.
In my case I made a big decision in dividing rooms not only based on function but most of all based on skill.
What you can learn in the kitchen and in the private gym should be obvious but in fact the whole multi-leveled Party Area is actually an almost complete room to train all of your hidden skills, with exaptions of the trampling, hotscotch and other outdoor activities.
Later on I decided to place the trampling indoors still though.
And when the space definition of where to put your animals and where to put your sims was decided and connecting skill and need based room at the right way together I could go into detail and did some internal architecture.
This was based on rules of course: all rooms must be properly lighted, all the right activities must in first and all rooms must be upgraded to give a sim the Nicely Decorated mootlet. If a room with light and activities didn't provide that on it's own than art must be added.
So I mostly worked on one big room at a day, sometimes two and eventually I came up with what you all see here.
I'm not sure if you'll see this or not, but I've swapped houses with Superstar Sim so living in the Millionaire's Mansion with my horse. When I first moved in, I had to use the move objects cheat to get my horse in the backyard - I placed it in the pen. He was able to get out, but now he can't get back in. I noticed when I go into build mode, if I click on the pen inside the structure, it turns red - the octagon seems not to be large enough to allow for a path for the horse. I thought maybe something had already been posted about it, but I have tried searching and found nothing. So I am sorry if this has been asked/answered! I tried moving the walls, but I can't. Looks like I'd need to rebuild it, but I'm newer to the game so I hope that's not the case!
Any advice would be greatly appreciated!
Thanks,
Cari