Just to add my agreement to Puddintane's suggestion... I too really like Sketchup for that sort of thing. It is quick to learn (there are excellent tutorials available over on YouTube), and very flexible in both creating, and modifying, various models.

I remember too well one of my favourite authors (David Weber), who made a simple and obvious mistake... that lots of people noticed... in his otherwise great series about "Honor Harrington". Sometimes referred to as "the Great Resizing", his goof involved a glaring discrepancy between the length of his starships, and their masses. [The mass of something depends on its density multiplied by its volume... and its volume is directly linked to how long and how big around something is.] In short, the first books in that series had his warships too long for their stated masses -- they would have the density of "cigar smoke". Not good. His later books in that series left the mass (weight) of the ships unchanged... but shortened them drastically, to bring their average density up to around "0.25". Still lightweight, as one might expect a starship to be... but much more reasonable, for a construction with internal walls and decks, armour, missiles, powerplants, life-support equipment, etc, in it...

Smile. My reading about that [he makes no secret of it -- it is mentioned in the free digital disks that accompany his hardcover books] made me determined to actually model out my science fiction creations, so that I would have a better understanding of them... and their physics... and hopefully avoid that sort of mistake. As with your own story, this lets me know exactly where in a building/spaceship my character is... what they should be able to see from there... the path they would have to follow, in order to reach somewhere else... etc. A larger model using the landscaping tools of Sketchup has the general terrain mapped out for the parts of the various planets that my characters visit as well. No major detail -- just terrain and vegetation map texture colouring, rather than actual rendered trees and things... but it again lets me visualize where things are.

[I do tend to go overboard a bit though. For a few of the scenes in one of the books I am working on, I decided to model them in extreme detail -- so that I could photorealistically render the scenes, for illustrations in the book. THAT is/was a HUGE amount of work... and detracted considerably from my free time for writing. Or in other words, it might be a mistake, unless you enjoy such modelling for its own sake. Smile. To give an example of how over-detailed I am being... I also have some free astronomical simulation software ["Stellarium" and "Celestia"] installed on my computer -- which I have used to render the background stars in the space scenes, accurately portraying the real view you would have in that year, at that point in space, looking out the window from a spaceship there. Silly, I know. How many readers will even notice those "random" points of light in the background?]

Anyway, I suppose I would just add that what I like about Sketchup -- as apposed to a couple "real" CAD programs I own -- is that it has a really easy to use interface, that lets you quickly "sketch" and modify your creations. If all you need is the "overview" of how a building is more or less laid out -- you can do that, in just a couple minutes. If you want to create a landscape, again it just takes a few minutes to create a plain, then "deform" it any way you want to make hills, mountains, etc. Copy and paste your "house" model in from the earlier project... and now you have a visualization of what the view out the window is. And if you want more details? As Puddingtane mentioned, there is a large "free" library of objects out there, that you can drop into your model. [The company that makes "Sketchup" was purchased by Google as the "best" of its type, so that Google would have a free program to offer people to encourage them to model real buildings on the real planet Earth, as part of the Google Earth project... so that library contains a lot of real world stuff already -- but you can create anything you can imagine in it, and many people have also uploaded various sci-fi prop's into the library as well].

One thing that Sketchup does *not* do well, is model people. It is a "surface" modeller, rather than a "solid" modeller. Plus humans have a simply enormous amount of detail in their "construction". Things like hair, eyeballs, fingers, clothes, etc. The Sketchup library has some reference "people" in it, that let you sort of position them in a model to see the scale, etc... but I would not recommend it for attempting to render a scene with a model of your character in it.

For that, I would use several programs. Sketchup, to build the scenery... the terrain, and the buildings, etc. Render that image. Then use another free program, "DAZ Studio" (which also has a lot of free stuff available for it, although there is also a lot of "pay" items as well -- be careful, or you can end up spending a lot there), to model the people, posing their bodies just the way you want them, with their clothing, equipment, etc, including setting their basic facial expression, then rendering the person from the angle you want, with the lighting set to match the Sketchup scene. [If you are doing a space story... the astronomical simulation software previously mentioned can also render the sky for your background... or just the sky as seen from Earth, at a particular place on its surface, and time].

Once you have the "raw elements", you can then open whatever image software you like [my list of "free" software includes "GIMP", (which is equivalent to Photoshop -- but overkill in its complexity for many simple tasks), "MS Paint" (okay for basic stuff... and it is there on your computer already, for most of us), and "Ultimate Paint" (the freeware version available on their downloads page, is a step up from MS Paint, with a few special effects that I like... but not as sophisticated as GIMP/Photoshop). "Paint.net" is another option, roughly equivalent to "Ultimate Paint" -- they both have pluses and minuses, so I might use either one depending on exactly what I was doing. Anyway, regardless of which tool you use... the next step is to composite your various rendered scenes together, overlaying the sky background into the appropriate area of the Sketchup scenery... then overlaying your DAZ Studio humans onto that background, positioning them where appropriate.

This is good enough for some purposes... but the figures will still look a bit "plastic". If you want even more detail, there is another freebie program called "Kerkythea" out there that can import Sketchup models, and render them a whole lot more realistically than the built in Sketchup render engine can. But that adds yet another program to master to the list of tools, which is already getting too long. [Be careful... you can easily get lost in all this, and spend years learning all these programs...].

Regardless of all the automated rendering tools that you use, I suppose I should add that there is a final stage to any story illustration -- the human touch. I have a graphics tablet (basic ones are not that expensive) which makes this easier to do... but I suppose you could also do it with just a mouse. But what should be done as a "finishing touch" is to pull up the image created above... and use that as the starting point for your own "digit painting". The facial expressions created by DAZ Studio, for example, need to be "polished" by hand, to make them look like an actual person, and not some sort of plastic "dummy". All of the above stuff helps you get the viewpoint angles right... the body proportions, and perspective right... but it still needs the "human touch" added, to make it come to life.

Smile. But that is getting much farther into "digital artwork" than you asked for. For now... know that I agree with Puddingtane that Google Sketchup is a good start -- and keep in mind, that you can play around with this stuff in your spare time (when your muse is being stubborn about the next step in your story, or whatever). Just be careful... I had an idea for an illustrated science fiction story over a year ago -- and have barely started writing it now. The reason for the delay? Fooling around with all these free image manipulation software packages, learning their quirks and how to achieve the illustrations I wanted. Grin. It's reached the point where I am debating forgetting about putting illustrations into my novel -- as otherwise, this story may never be told.

But for simply making a reference model, you should be able to achieve the needed skill levels with Sketchup in just a few hours -- which makes it a good solution to your problem.

Good luck...

[Updated to edit a couple silly typos...]