Microsoft recommends maintaining a flat architecture when you plan for your SharePoint application. Having multiple child level subsites will make your site structure complex. In this article, I am about to explain to you why do I prefer to create site collections instead of maintaining sites as Subsites under an existing site collection.
Governance Advantage
The site collection is accessible only to the selected group of users like the owner, member, and visitor of the site collection. These group of users doesn’t cross over to view the contents of another site collection. This gives you an opportunity to have a protection from the illegal sneak peek into your business-critical data which you maintain within your site collection.
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Controlled provisioning of SharePoint features
You can enable/disable the SharePoint features in the site collection level. This will ensure that only the features needed for this site collection will be enabled. Features which are enabled in one site collection will not affect peer site collections.
Access Provisioning
You can publish a label to be viewable only within a site collection. For e.g., If the Finance team alone needs to access a list which is the root level then the link to the list can be added as “Quick link” only in the Finance team site collection alone. This will not be shown to any other site collections of the web application or tenant. The same way you can work on search rule to bring in results only from a site collection.
External Sharing
It is easy and secure to share a Site collection externally whereas in SharePoint you don’t have an option to Share at the subsite level.
Administrative Advantage
Admin will have an option to allocate the storage associated to a site collection. In SharePoint on-premise, the individual site collection can have its own secured database. Admin does have an option to identify the unused/sparingly used site collection. Through this insight, admin can take up a decision to delete/achieve these site collections which are independent of other peer site collections.
Apart from the above pointers having a flat architecture will ensure better crawling and indexing thus improvising your search capability. Flat site structure is simple to manage even when the Organization’s structure gets changed e.g. Initially, IoT was a part of development. We created it as a subsite under Development Team Site collection. Later IoT team and projects have grown exponentially to become a separate practice. Now when I wanted to promote the subsite to a site collection it is tedious and involves a lot of manual processes.