I draw all my plans in 3D Sketchup program, that gives you the ability to see what your building will look like, (free at Google) before you cut one piece of material. I use no plans, sometimes I may refer to a picture, or I may just come up with the entire building on my own.
You are wanting to work with plans in 1/2" scale, that is the easiest ratio, 1/2" equals a foot, A building 30 feet wide in real life is 15" wide in your model world. If you buy your windows and doors, either get them first or get their measurements, before you draw up your plans, I build all my own windows and doors, so I build to my plans, thats another story. Most single story structures walls are 9-10 feet tall, depending on your era, old buildings with 10 foot ceilings have usually a 1 foot flooring, a 1-2 foot foundation, then the roof, Seven foot doors makes modeling simple, 3-1/2" tall doors, four foot windows are 2'' tall.
You can use a 1/2 scale ruler to make life simple if drawing all your measurements out on your building products, wood or plastics, ect. A 1/2" scale ruler helps to prevent mistakes, check out therailscale.com . 1/2" is simple to figure, it is a common and simple scale to use. You know most of the buildings built will look fine if they are perportioned out close to the original building, you don't have to be exact, If your friends are critical, change friends.
Dennis