SEO should be part of the design and development phase. If your site isn't built to be search engine friendly the first thing your
SEO will want to do is rebuild your site. How you develop code, organize your content, etc impacts your
SEO.
Going beyond
SEO your marketing should really be included in the process from day 1. It's no longer enough to have a product and then figure out how to market it. You should build marketing into the product itself. You should do research to see if there's a market for your product before building it. You should think about your unique selling point so it can be built into your product. YOu really need to consider marketing as early as possible.
And you need to consider
SEO early in the design phase of your site. As someone who offers both design and
SEO services, trust me when I tell you it'll cost you a lot less money to build
SEO in from the start than it will be to try to tack it on later. It'll also be more effective if you build it in from the start.