While settlements are admittedly not my favorite aspect of Fallout 4, I've nonetheless spent a lot of time figuring out how to make the most of their space. Settlements can hold a total of 20 settlers, but in some locations, that number seems impossible. Nowhere is this more true than Hangman's Alley, where I recently documented my efforts to design an arrangement both efficient and comfortable for my settlers. Using pre-fab units and wood bridges, I was able to lift some my housing dorms to allow extra room for crops and water pumps underneath, providing necessities for the settlement without sacrificing the market or crafting tables.
Since then I've learned another little trick, one that allows for even more space beneath your housing units. Use this to get your crops and water pumps out of the way so you can use that room for trading posts or craft tables. Mutfruit, Corn, and Tato can be combined at a cooking station with Purified Water to create a vegetable paste that will break down into 5 Adhesive. Adhesive is vital to weapon and armor mods, so any work-around that lets you grow more crops is imperative (the water you can obtain by pumping the water pumps, which will store Purified Water in your Workshop inventory).
In the Floors section of the Pre-Fab tree of the Workshop menu, you will find platforms that you can use to build elevated buildings. Skip those. While they are helpful, they are unnecessary from here on out. Instead select the wood bridge from the Miscellaneous section. Stack two next to each other to form one long bridge, then choose the second unit in the building prefab menu, the frame with a flat roof and floor but no walls.
Place them side by side on top of the bridges. Repeat until you've made a building that fits your size needs. Personally, I prefer a 3 X 4 arrangement, with back walls made of metal and railings lining the sides, to keep the unit "open" enough to house many settlers (I find that they do not like to sleep with more than 3 other people in an enclosed room). So long as each prefab structure snaps into place with the one next to it, the bridges will hold as many as you make.
Next, remove the bridges and scrap them. Instead of falling on the ground, your new structure will stay in place. Settlers will wander underneath, and tend to crops they are assigned.
Think of some of the elaborate buildings you can make with this tip. The full possibilities have yet to be explored. Remember to put pylons and generators and turrets on top of your buildings, instead of the ground level, so that the settlers have even more room to grow crops and move around. I like to put my housing units near the fields, not only so the settlers are adjacent to their work area, but also because it's pretty. Feel free to incorporate some of my designs into your own. I will continue to tweak my settlements and dormitories and update you all with more tips in the near future.