Develop WP blog behind HTML site?

Hello WP forum!

I’m a former HTML programmer now working with WordPress. I have a static HTML site I’m in the process of converting to WordPress. My question is this:

How can I build/develop the site BEHIND the HTML site? I’d like to develop the WP site/theme without disturbing the HTML site…. then, when the WP site is ready, I’ll move it over. Is there a way to keep the directories in order and still “turn it on” later?

I don’t mind if it’s live/public while I’m developing, I’m more concerned with making sure the HTML site is not disrupted while I’m in the process of QAing the WP site.

Any help is appreciated. I’ve looked all over for this answer and can’t find the solution.

Ryan