Adding new website as multiwebsite under existing drupal website
Domain modules will be your best bet. It will allow you to use same codebase and database for all your sites. You will also have an option to push content to a specific site if need be.


  The Domain Access project is a suite of modules that provide tools for running a group of affiliated sites from one Drupal installation and a single shared database. The module allows you to share users, content, and configurations across a group of sites