When I take over a settlement, the first thing I do is scrap everything that is scrap-able, which often leaves the place looking quite empty. Inevitably there will be some mound of debris or broken wall or roof that I will have to overcome (or ignore) in order to re-built the place, but this post isn't so much about that --- one would hope that when the tools become available we'll be able to safely clean such stuff from settlements.
No, this post is about the ~theory~ of settlement building. Settlements need beds, water, food, defenses, and happiness. There are many ways to achieve these goals including ~not~ achieving them on purpose (a "private hideout" settlement that is just for you and your gear).
Water
A 'necessary evil', providing water is rarely "sixy" but it's needed. For a 20-person settlement, you're looking at a row of seven or so water pumps or, if you are lucky, some water purifiers if there are sufficient bodies of water in the settlement area. I am using well over 100 mods and one of these lets me place a water ~well~ just about any place and it generates around 100 water or so --- more than enough. Although the thing IS rather large, it has animated water falling into rain buckets and if the property is big enough I can plant it off in a corner somewhere (clearly would not work in Hangman's Alley, for example).
Food
Again, necessary but not exactly thrilling to work with. Requires dirt (unless you are using a mod that lets you place planters anywhere ala Graygarden). For the longest time I used only Mutfruit since it provided 1 food per plant instead of the 0.5 food per plant that other food items offered. I've since gotten mods that give the same food value regardless of the crop, but I still find myself liking the bushy look of the mutfruit plants. You can use Tatos and Corn (I believe) to create adhesives and other crafting ingredients, which is a good reason to use those. If restricted to dirt, your placement of crops will be pretty limited and may dictate how you build, for example, fencing around the farm area.
Happiness
I'll confess I am not too versed in how to boost settler happiness, but I usually try to place radios around the settlement so that their sounds do not overlap (much) but so I can always hear the radio station anywhere in the settlement. But that's more for ME than for the settlers.
I also place couches or chairs around the place since settlers love to sit on their butts instead of working. This can start to edge into ~design~ of the settlement, placing couches just so. If I have the money and resources (and perks) I also place a Bar and a Clinic and sometimes a general trader just to give the area a more "complete" look. Considering the space you need around a Tier IV vendor stall, though, including space ~above~ it, the placement of the vendors can change the flow of your settlement.
In the early stages of building up a settlement, I will often just plop down a vendor stall, or a couch or a bed and move on. If I ever get satisfied with the basics, I go back and try to flesh out the area, adding end tables or containers or seats for settlers to sit ~at~ the bar / clinic table.
Decorations may influence happiness (seems I heard that) and I have a mod that adds some saucy pictures, so i will sometimes liberally scatter those around. I am usually a bit hesitant to do so until I am happy with the general structure of the settlement, though, since if I move a wall here or there, it may cause issues.
Lighting
This is really an aesthetic choice and one where I find my sense of good taste at war with my need to be able to ~see~. That is, I can place a table lamp here, or a wall sconce there, and it will cast a soft, warm glow in a small area around a lounge chair for example. But then I can't ~see~. So I slap a four foot long fluorescent light on the ceiling and everything gets washed out in a blinding white light.
I ~do~ like being able to see my settlement lit up from a distance, but I fear I may over-do it and need to prune my lighting back to what is ~necessary~. Perhaps using the Street Lights outside and table lamps, floor lamps, or even candles inside (which would also necessitate some end tables perhaps, further giving the place a "lived in" look).
Beds
This is what spurred my post in the first place. My Bunkhouse Dilemma.
Each settler needs a bed. You could give them a sleeping bag, but I read a theory that the ~nicer~ the bed, the higher the happiness. Whether it's true or not, I'd rather place a clean bed than one of the dirty, blood-stained mattresses. I've also seen some beds that provide two "beds" for one item, although I am not sure that two settlers will use the one, physical, bed. I dimly recall seeing two settlers on a queen size bed, but I don't think anyone ever uses the top bunk of a bunk bed, for example.
To place twenty or so single-user beds can take a lot of space. In Sanctuary, it was relatively easy to place queen size beds in the actual bedrooms of the existing ramshackle houses. Likewise, the "Free The Robots" hippie commune settlement has several existing buildings that can each hold several beds. Other places, though, have only one or two ratty sleeping bags tucked in the corner of a single shack.
So in many settlements I will place Foundation pieces to level out uneven terrain (and maybe to hide some weeds inside the concrete foundation)... then I build a ramp or staircase up to that flat area, wall it and put a roof over it. Then I slap twenty beds inside, making sure there is enough room on both sides of each bed to walk around (so they will ~use~ the beds). The problem is, what I have created inevitably looks like a barracks or bunkhouse.
Granted, there's a reason for that --- it's efficient. Both from the stand point of a military organization and from the standpoint of a player trying to maximize use of limited space and resources. If you only have to worry about four walls, a roof, a floor, and twenty beds, the simplest configuration is a box with rows of beds.
I tried a different tactic once, at the Starlight Drive-In. I placed water purifiers in the puddle of water and then built a square wall around that puddle using the Hallway End piece. The back wall (the 'end' of the hallway) was against the puddle / water purifiers and the open end of the "hall" was the entrance to a sort of monk-like 'cell' where I placed a bed. Wrapping around the centrally located water source, this created a row of 'cells'. I say 'cells' in reference to the cells of a monastery, but they are probably quite smaller than ~prison~ cells. BUT ---- they are distinct, and separate. Each citizen has his or her own little "cubby" where they can sleep. I think I even fit a chair and a footlocker in each one.
With four sides to my "prison", I ended up with three sides with six or seven cells and then one "side" I used to create a sort of common area where I placed some diner booth couches, a Bar vendor stall, disco lights, radios, wall-mounted animal heads and pictures, etc.
In terms of resources used, it was probably a lot more expensive than a simple bunkhouse but I liked it more. I can easily imagine a bunkhouse / barracks format being a likely starting point for a new settlement, but if these people are actually planning to ~live~ here long-term, raise families and so on, then it makes more sense that they would start to personalize "their" space. Easy to do in Sanctuary or hippie-commune-town where there are already separate buildings that are big enough for one family-of-four and little else. Easy to do in large areas like Spectacle Island or Somerfield Farm but less so in small or uneven areas like Hangman's Alley.
Another assumption that I might need to address is that I ~want~ twenty settlers. I usually turn on the recruitment beacon and leave it on. But why? I mean, you have to do that to "complete" the mission, usually, but once you turn it on, you can turn it right back off again. And then you can create settlements that only ~have~ six or seven settlers in them.
Simple, but to me profound realization, there. There's no ~need~ to max out your number of settlers. Only possible advantage I could see would be that you could task more settlers with farming more food and using scavenging stations to generate more salvage for you (excess water gets placed in the workshop regardless of number of settlers, I imagine). But I think that's a sacrifice I'd be willing to make, in order to create a more personal settlement.
So the # 1 way to avoid the impersonal barracks look is to limit the number of settlers. Obvious, I suppose, but it's against my nature to ~not~ do things "full bore" and get the most bang for the buck. But in this case, there's no real advantage to pushing the system, trying to max out number of settlers, etc. I think I may just turn off all existing recruitment beacons and send excess settlers to my new settlements as I find them. That should let me trim back on the 'barracks' as well as the number of crops I need, the number of water pumps, etc etc.
Today's Motto: Less Is More.