Developing websites on alternate domain options

Firstly, I'm new to these parts so sorry if this is in the wrong section! I'm a web desginer/developer and have a Linux server that I use to build and host clients sites. This is generally my process for both new sites or clients who have access to their current site/DNS is as follows: - Set up cPanel account for new client/website - Build site hidden from search engines but accessible on a sub-domain (dev.clientname.co.uk) - On completion move site from sub-domain to main (removing & redirecting old site if there is one) and allow indexing on www.clientname.co.uk To me this is as simple as possible and means minimal fuss on migration as website files and associated databases all stay put and is done with just a few changes to DNS etc. I'm trying to find a work around for when a client has an existing site but no access to it or their DNS - we would get this access in the end as the domain would be transferred to us. Ideally I'd like something like: - Set up cPanel account for new client/website (www.clientname.co.uk) - Build site as usual using their cPanel account but make site accessible on a sub-domain of our website (clientname.mywebsite.co.uk) and also give me access to Wordpress etc through that subdomain. Could this be done using redirects/aliases etc? I get the following error when trying to use an alias "There was an error when the system attempted to create the alias. (XID sjd3sc) The domain "clientname.mywebsite.co.uk" already exists in the Apache configuration." So I'm guessing it doesn't like the fact that both domains are run from the same server? If it isn't possible and I should stick to physically moving files/db etc from a staging area to a live site on completion. Any thoughts much appreciated!