If you had the chance to build a SEO strategy while creating a website or before that, you would appreciate the chance to make it in your way. If your client has a clear idea what he wants to rank for and give you an opportunity to make it as you intend, this will work for sure.
Starting with filling a list of keywords to target, you would continue whit collecting a content based on what people intend, general info about any kw topic, single or multiple keyword page focus. And voila!
You build a content. Of course you could use already existing website to optimize it with a new strategy if the website owner agree with you (sometimes the man factor is harder problem then a technical one).
I look forward to read your comments!