Marketing Plan - exactly how many sole traders/microbusinesses would bother to do this? Even if they did, things change so quickly that it may be a waste. Short, action orientated and ever evolving (but occasionally think about this stuff to see what's changed)! ;)
Good website design is based on a number of factors, many of which have to do with taking into account the human factor since you are, obviously, building a site to be viewed by humans. An infographic that explains website design.