how do i draw a plan for a 15x30 addition
You can usually get a set of plans done for about $300. Money well spent since you can't even start without plans and permits. The plans are also used for material estimates.
Figure out a scale to use, like 1/4 inch equals a foot. So every inch on your line on the paper will equal four feet.
So your box will be 7.5 inches x 3 3/4 inches. Then you draw another box inside that one that is the thickness of your foundation. Poured walls are 8 inches thick or 2/3 of a foot so your line would be 2/3 of a quarter inch inside the first box. A wood foundation would probably be 2x6. So it would be 1/8" from outside line. (6 inches is 1/2 of 1 foot = 1/4inch)
Make those likes dark. Then your poured wall sits on a footing that should be 16 inches wide. Add a dotted line to each side of the wall lines. That is four inches wide or 1/3 of a quarter inch on your scale. (Your footing should be twice the width of your foundation wall so it might be 12" (1/4") for 2x6 wood foundation)
You should have four lines just for the foundation. Starting outside the box and moving to the center you should have; The outside most line should be dotted showing the outside edge of your footing. The next line will be dark and be the outside edge of your foundation wall. The next line will be dark and should the inside edge of foundation and the forth line should dotted showing the inside edge of the footing.
For the details of the frame walls start with a drawing of just the foundation and lay it out. If you want a 36 inch window then leave an area 3/4 inch open. Mark it window.
You don't have to get down to a gnat's butt for the drawings. Write up a separate window and door schedule. Your drawing will show a 36 inch window, but you need to leave the opening for the window alittle bigger like a 1/2" for the rough opening. Same with doors.
Code enforcer's like screwing with do-it-yourselfer's. They'd rather have their buddy the carpenter do the work, they know his work and don't have to inspect it as hard. They can also be your friend and save you from major headaches.
I have had friends sketch out additions and submit them to the building officials. as long as they know what your planning to do.