How to develop WordPress theme without removing current website (not WordPress)?

Hello,

I am going to install WordPress and develop a custom theme for a company. Currently, the company has another, non-Wordpress website up and running. I don’t want visitors of the website to be able to view the unfinished WordPress site as I am developing it, so my question is: where should I install and develop the theme so that no one will see it in its development phase? Should I get a wordpress.com site? Or another domain name? Please let me know any tips you may have. Thank you!