During development phase you can use this plugin instead of showing ”Under Construction” page. This plugin will redirect all visitors to a specified url and allow developers or admin to login via wp-admin panel.
Advantages of using this plugin from Under Construction Plugin
1> Your old site is still accessible when your new site is in construction state
2> Admin or developer can login using site-url/wp-login.php
3> Easy to configure
4> Can even redirect to external urls
