Creating a site under temporary url (replacing existing site)

OK, I have sent bluehost a question about this, but have yet to receive a reply. I am approaching the second instance where a new client has an existing site and wants me to create a new one. They want the existing site to stay up while I am creating the new one, but when I’m done they want to use the same domain and host it with me (bluehost)instead of (I think register.com).

What is the best option for me so the transition (with respect to indexing, SEO, and ease of transfer) will be relatively painless? This will certainly not be the last time I will need to do this, so I’d love for someone to give me some advice here.

It would be perfect if I could just create the site in the wordpress admin and then just launch it when we are ready…instead of creating it and having everything get indexed under a temporary (and incorrect) url…and having to back up databases and transfer files, etc.

Any help would be appreciated. Thanks!