If you want to use a static HTML homepage you have a few options.
Move the site out of the server root directory
Move the entire site into a sub-directory like "blog", "dev", etc.
Moving the site will require updating the database and all the other things that go along with moving a site to a new domain.
Leave the site at the server root directory
Option 1
Use an htaccess file to redirect based on IP.
You can create an HTML homepage and use your htaccess file to redirect anybody that does not have your IP address. Do not name the HTML page index, home or default because these may override index.php. In this example I use homepage.html.
Upload the file to the root directory where Wordpress is. Add the following code to your .htaccess file at the very top. Replace the IP address with your own (what is my IP?), replace the domain name with your own, and change the page name if you didn't use homepage.html.
# BEGIN maintenance
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !^/homepage\.html$
RewriteRule ^(.*)$ http://www.example.com/homepage.html [R=307,L]
RewriteCond %{REMOTE_ADDR} !^00\.000\.000\.000
</IfModule>
# END maintenance
Option 2
Use a plugin to control maintenance mode.
I have used the one called Maintenance Mode without problems. If you are logged in you can see everything on the front-end and back-end of the site. If you are not logged in you get the maintenance page.
This is by far the easiest solution and will make launching the site a breeze when it's time to go public with it.