In this day and age SEO is the chasis to the car, it's the foundations to the building and the soles on the shoe. It should come first and the website you are building or redesigning should draw it's structure from it.
It is still difficult for some businesses to understand why SEO is so important. You don't know the business your missing by not having a properly optimized site.
You can't build a new house until the foundations are laid. So with SEO you need to start with keyword research and analysis to understand how the site should be structured. It doesn't hurt to have a content plan but allow your content plan to adapt to the results of your SEO research.
Search Engine Optimization analysis will tell you what your potential clients are looking for which in turn allows you to target them accordingly with effective landing page and copy writing.