If you struggle with paper drawing, then the steep learning curve of doing a similar thing on a computer will be a waste of time.
I suggest starting with a dimensioned sketch - which you probably already have, but will need however you work - copied accurately onto graph paper. For your straightforward project, you might need enough time for a couple of cups of coffee.
Yes I'm thinking the old skool approach and pencil to paper, as I can easily work the scale the council requires too. It's not a complicated structure, it's a simple shape. The planning people just want to know it's basic dimensions as a floor plan and elevation.
If you use pen and paper, scan it, then use a free simple vectoring software (tidies up the drawing) then just save it in Word, PDF etc.
Another way is use MS Paint, you be surprised how quick you can knock something up with box'es, circles, arrows and text etc.
Or use Word itself, but I tend to use Paint for simple stuff then just paste it to word.