Floor Planner is a great website for making your own floor plans to use as guides for your builds. It’s free and easy to use. Just drag and drop various items to your ‘graph paper’ grid and draw walls where you need them.
It’s a user-friendly intuitive program that takes very little time to figure out. Try it to draw your own custom home, shop or other architectural floor plan. Once done you can save it as a .gml or, in the preview tab, as a .jpg or .png image. Apply the completed plan to a prim for inworld building or import it as a background image in Blender to create a mesh version of your custom building.