If you are in the process of building a new website there are many points you need to consider prior to the go live date. If you are going to be hosting the new website on the same domain name then you will need to build the site offline and then switch it over to the new site when it is ready. This will allow you to keep the one site live whilst the other one is in progress. You can of course take the old site offline but if there are delays with the new site you may find your company with no site at all for a while and this is not advisable.
If you are going to be having the new site on a new domain name then it is often easier as you can work on the new domain and get it ready to go live with a password on. On the day of switch over all you need to do is remove the password and turn off the old site. You may need to redirect old URL’s on to new ones if you do not want to lose any of the traffic from rankings you have already got.