DESIGN is always last, if you mean graphic design or layout related.
Been doing this since 1995.
Functionality is first.
Then the content for the functionality.
Once you have functionality to meet your goals, You know for ecommerce, your goal is to get the credit card number/payment.
Then the content for the SEO, then the SEO.
The content is used to drive the interest and get the payment, that you can not have without the functionality. The content for the functionality, help guide through the functionality. You know instructions for paying.
The SEO for content is to give the SEs reason to list your site in the SERPs.
The content and the SEO are becoming more like one now, but you have to create good content FIRST, before you can have good content with good SEO.
Part of the functionality is also SEO. When you are talking about the theme design (WordPress). Meta tags, proper title and header use, etc.
Back in the DAY SEO planning came before buying the domain.
With Google recent changes the domain name has become even more important than it was a few months ago. MUCH MORE Important!