I mainly build living buildings above ground and, thus, use real or, at least, fairly realistic medieval buildings for reference, but, to get the point across:
Early settlers and the lowest workers get 3x5 (walls not included) one-story huts with no internal walls, one bedroom set (bed, cabinet, chest) and two dining sets (table, chair). Somewhere along the corners, there are two one-tile stockpiles - food and booze. The remaining space is walkable, so no crampled furniture. If it's a family with children, they get an attic with additional beds. This I consider the bare minimum, just a place to eat and sleep.
Later on these huts almost always get an expansion: a cellar, an additional room or a couple of
overhanging floors. There are either more bedrooms and dining rooms for family members, a personal kitchen or a personal "guest room" with some statues. This is a good place for living already, the owner only has to go out for public activities and work.
Later, some may get additional attachments, such as a workshop building (5x5 or less), a small garden, an enclosure/barn for livestock, a small individual temple or a "kennel" with a restraint for a pet (edit: also, if you have a lot of waterworks, you can make personal bathhouses, optionally, with mist). The resultant farmstead receives a fence around: the largest ones can get up to 20x20 (usually in a different proportion, but you get the idea), including the fence itself. Once all of the above are present, this is pretty much a personal mini-fort, the owner only needs to go out for public activities.
The
reasonably sized houses for nobles rarely get any larger than a 17x17 manor complex, and that includes richly furnished personal rooms for all of the noble's family members, which mostly means more containers, statues etc. The design itself is usually based off the many google pics for "medieval manor", with uneven height and such fancies. Similarly to the farmstead above, it can also get the same additional buildings, only larger and richer.
Then, indeed, the land holder noble can get their own castle. Those I build based on terrain, but generally they are pretty similar to the manors described above. The keep where the noble lives is some variation of a 11x11 five-story building. The lower floors are used for stockpiles and barracks, the top one is battlements and the remaining are, again, similar to manors. The outside can get several nicely sized buildings, such as a lord's own tavern, temple and "art studio" (to fullfill the crafting/creative needs). Be it a siege or a loyalty cascade outside, the lord should be able to get whatever they could possibly want without ever leaving the castle.
The main premise is to think both in terms of in-game and real-life practicality. As mikekchar have mentioned, there really is no reason for a dwarf to have a house that takes hours to navigate through, from both of these standpoints.
Now, indeed, the designs I've described above are not very well fit for underground buildings, but the idea stays the same. Comparatively, I would personally recommend to have less floors, but larger rooms, as this allows for some better geometric designs and keeps most of the stuff in the player's sight. For inspiration on what
exactly these designs should look like, I'd recommend googling something like "apartment plan" and pick the whichever ones you find most aesthetically pleasing - naturally, they are all designed to be practical already; then, apply the in-game and in-universe practicalities towards things that are out of place (such as bathrooms and tv-rooms) and switch them for something that is of use and makes sense (such as the additions I've described above).
Not sure if my rambling was on point, but hope it helps somewhat, at least.
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