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Creating a (fictional) building can be very tricky. It doesn't matter if it's a house for your main character, a massive castle for a king and queen or a large space station with all sorts of high-tech gizmos. However, they can all be created in relatively the same way, which is by starting small and continuing from there.
In this guide I'll go over the steps you may wish to take when creating your own fictional building. The steps aren't necessarily in a strict order, although it's usually best to do them in order anyway, if only to make sure you don't run into issues later.
It's a common theme in my guides, but it's common for a reason. Do your research. This includes what parts are usually added to whichever building you wish to create, what building styles are often used and things like size and shape.
The research phase is also meant to help with inspiration, as while you research you will come across a wide range of different variations of the building you wish to create, all of which might spark new ideas in you.
If you're really stuck on inspiration try searching for images, especially concept art images, as there's a wealth of resources available. If nothing else they might at least help you with the later phases of creating and decorating your own building.
Now it's time to start actually designing your building. I personally find it easiest to start with 1 single part of the building, usually it's either the most important part or something like a hallway. The reason is that this way I can more easily add other parts to my building without having to fear I might mess up a crucial part.
So when starting with this first part all you really need to know at this stage is what is it used for, how many people will it have to support and what kind of shape do you want it to be. Everything else will mostly be decided later or has already somewhat been decided in the research phase, like the kind of style you're likely going to use.
From here on you add new rooms one by one. Let's say we're building a castle and we started with the main entrance hall. Next you may wish to add the throne room, likely at the end of the entrance hall. To each side of the entrance hall, right at the entrance, you may wish to add towers, or since we're on the bottom floor the staircases leading up to the top of the towers.
Those two staircase rooms might lead to two more smaller hallways leading further outward to two more towers.
Then you'll need to add a dining room, perhaps a ball room, the bedroom of the king and queen, kitchens, servant quarters and so on. From here on it can become a bit like a puzzle you need to solve, almost like Tetris in that you have to try and fit pieces together, but by doing it one piece at a time it becomes much easier. It's also not as strict as a real puzzle, as you can change the pieces to your liking.
Do take into account that if you're working with multiple floors some of the space may already be taken up by rooms below or above it. The towers in the previous example will reach to the top, so I can't add rooms in those spaces. Other rooms might have incredibly high ceilings, like the ball room, so that space will be gone too.
At this point it's also important to keep in mind any special features you may wish to add and the space they take up. This is mostly aimed at secret passages, but it applies to all sorts of features.
If part of your building contains a hidden room or a secret escape passage you will need room for that. For example, if there's a secret escape passage between the hall and the dining room, the "walls" between these two rooms would be very thick to accommodate the space needed for that passage. You can often disguise these spaces though, like building the entrance underneath a staircase or in a corner of a room. Of course, how obvious or not the passages are will also depend on what you need from them in your story.
You hopefully now have a pretty decent outline of how you think the final result will look like. The best thing to do now is to visualize it in some way. The simplest way to do so is by simply drawing it out. Make a type of blueprint of your building, perhaps already including doors and windows.
Alternatively, if you feel like your drawing skills aren't good enough or if you feel like you need a more visual blueprint you could always use games like The Sims or even Minecraft to build your outline in game. I'm sure there are plenty of other games and other tools which could do this, so pick whichever you like.
In this stage it's also the right time to make any changes you feel like you need to make and thus create the finalized version of your building. At least, the finalized foundation version, next up is adding the details.
So it's time to add the details. There's not a whole more to say on this as you've done most of the detail work in the research phase. If you did it right you'll know what style the outside has, what materials were used and just overall how to describe your building in your story. At least, the outside anyway, the inside will likely need a little more work.
The way I work is similar to building the foundations. Start at the most important part and work through the rooms from there. Some rooms may not be important at all, the rooms which never really appear in great detail in your story, but others might need to be fully decorated for full detailed descriptions.
The best way to decorate a room, at least the best way that works for me, is to start with the windows and doors (if you skipped this step earlier) and then work on the main part first (there's a theme in this guide). If you're decorating a throne room, start with the throne area, if you're decorating a living room, start with the sofas, the tv and whatever else you absolutely need.
After that you can fill in the room with smaller details, but note that these details aren't as important. From a writing perspective you can cheat a little here and there, your readers won't know the exact measurements of the room so if there is or isn't a big cupboard in the corner won't be too much an issue.
The exception to this is when you're adding items which are crucial to the plot or when the room itself is crucial to the plot, like in a murder mystery where the room is thoroughly inspected. But even then the measurements of the room can be skewed a little to make room for items and furniture.
Other than that you're pretty much done. If you're stuck on the interior decorating part you may wish to do some more research and image searching, just to get some ideas for rooms. This is especially important if you care about authenticity in your design.