[support] How to lock the website while moving website?

[Afan Pasalic]

  Hi,
the website on my local machine is done and ready to be moved to online 
server. After reading other people's 
questions/answers/posts/comments/opinions - I think it shouldn't be a 
big deal. Though since I'm doing it for very first time I expect some 
"sudden" issues. Even the website doesn't have a lot of traffic, I would 
like to avoid messages visitor can get, like "Page not found" or 
something. And because of website owner too - he's not going to like the 
message for a couple of hours, or even more.

Right now old website is visible (live) and to replace it with new one I 
have to remove it.
Now, my question is: How do I make "The site is temporarily down for 
upgrades" message on screen WHILE upgrading the website?
Is it possible to create a file, something like 
"under_maintenance.html",  with the message in it, and redirect all 
visitors to that page using .htaccess? But it shouldn't compromise 
installing Drupal, moving stuff round etc.?