I usually build middle-sized houses, with all the necessary basics, but without too many extras. The must-haves are: a living room, usually with a TV, a stereo, a sofa+two armchairs set, a bookcase (or two), some end tables and a coffee table, sometimes a dresser if there is a space for it, and in most cases, with a fireplace; an eat-in kitchen for smaller houses, and a separated kitchen+dining room for larger ones; bedrooms for all the family members, and usually a guest bedroom (as for kids, if there are any: in smaller houses, they share one room, in larger homes, they get separate ones, or at least i don't make teens live with toddlers). There can be either a separate study, or a desk with a computer in each bedrrom. Skill objects can be placed all over the house without any special room for them: for ex. instruments in the living room, a gemcutter in the garage/basement, an easel on the terrace or balcony, etc. Or, in large houses, there can be special rooms for skill objects. Also, large mansions can have gaming rooms, music rooms (if the owners like to play instruments), saunas, winter gardens, kinda two living rooms (one is smaler, with a TV, and another is lager,with a stereo,a bar,and lots of space, this is for parties). There can be even an indoor pool beside the outdoor one in a large house, but this is suitable only for worlds with colder climate. A spa room with a hot tub and a massage table is also a good idea (this idea actually came to my mind now, I've never buit such a room before, gotta try that
) Anyway, building ridiculously large mansions on huge lots is usually not a good idea, as it slows the game (on not very powerful computers), and makes sims waste too much time getting from one part of the house to another.