A big questions customers ask when we are doing a new SharePoint implementation is when you choose to create a new site collection or a sub site.
Steve Goodyear wrote in this post (from 2009!!) how and when he decides to create a new subsite, site collection or web application:
Some of the key things I look at when determining whether to split content into multiple site collections rather than a single site with many sub-sites, beyond the content size and number of databases I would like to span, include:
- If the top-level site reaches the end in its lifecycle, can I delete all the sub-sites when I delete it? If not, maybe those sub-sites should be in a different or their own site collection so they can have a different lifecycle.
- Will the permissions for the sub-site be the same or a subset of the top-level root site? If not, this might indicate the permission hierarchy is getting overly complex with broken inheritance and would be more manageable in the future separated into different site collections.
- Will I want to build site reports differently between sites? If I want to report on several sites as a group, it is often easier using out-of-the-box reports if the sites belong to a single site collection.
- Will the navigation change frequently? Having multiple site collections point to each other is as easy as setting a global navigation for each site that links to each other. Nevertheless, if that global navigation changes frequently, it might be easier to have the sites as sub-sites. Or, keeping in mind that ease of updating the global navigation across site collections is achievable still by implementing a custom site map provider, site collections might work just as well.
He even has this handy diagram that clearly shows when to use what. Before you use this in your implementation / organisation you need to review this on how you want to work and create sites in your organisation.
This diagram can be a perfect add-on for your governance plan. It can be communicated to the people that create sites in the organisation.