Creating a sandbox "inside" a redirected URL

I’m currently redirecting a URL for a domain I plan to host in the future to its current site. I want to build the new site within my Dreamhost hosting, then remove the redirect to have the URL point to the “natural” place, where this site will reside.

Simply put: is there any way to do this? Is there a way I can directly access the files as though they’re live in my hosting folder while the URL is redirected, so I can sandbox/work on the site until it’s ready, then simply remove the redirect and have the site be ready to roll?

Failing that, what are my best options? Create a subdomain that doesn’t redirect, build everything in that subdomain, and then…?