Being a Web Development firm at our core, we create a lot of big projects from the ground up. The most affordable, successful, complete, on time projects always start with a detailed specifications sheet. Literally a 10 page, outline format description of every view and page on the site that a visitor would see. There are multiple reasons for this...
The only thing that could be done to improve on this, for pre-production documentation is a full set of wire frames ensuring information architecture is ideal. This is a service we offer, regardless of if your planning on using us or putting out the spec for bid.