Sometimes it is required to customize users/groups permission for the whole site or sub sites also and adding custom permission level with the existing permission levels. This article will show how to add new permission level and also customize existing permission level.
Permission levels are the various user rights which you can assign to different people / groups in a SharePoint site. By default, each site comes with the following number of permission levels such as Full Control, Design, Contribute etc. which you can customize to suit your needs.
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Full Control - Has full control.
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Design - Can view, add, update, delete, approve, and customize.
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Manage Hierarchy - Can create sites and edit pages, list items, and documents.
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Approve - Can edit and approve pages, list items, and documents.
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Contribute - Can view, add, update, and delete.
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Read - Can view only.
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Restricted Read - Can view pages and documents, but cannot view historical versions or review user rights information.
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To create additional permission levels, go to the Site where you want to create a new permission level. Then click on Site Actions > Site Settings > People and Groups > Site Permissions > Settings > Permission Levels.

Here, you can create your new custom permission levels. You can create permission levels from scratch, or copy existing permission levels also. If you want to copy an existing permission, you have to go inside that permission level and click on Copy Permission Level. After that you can edit according to custom permission and create. Or you can edit the permission of existing permission level. Or you can start to create Permission Level form the scratch using Add a Permission Level.
In every case (edit or add) three types of permission. Those are List Permission, Site Permission, and Personal Permission. Each of them contains a list of permission item with description. You have to choose the permission and press “Submit” in case of edit or press “Create” in case of new permission.
In this example, I created a new permission level “STDesigner” by copying the existing design permission level and reducing some right access such as delete items checked-off.

Now you can add some users with “STDesigner” permission those users can create and edit item in the site as design permission but cannot delete any item from the site. For example you can add one user for your site using “STDesigner” permission level .
ST Group > Site Settings > Permissions > Add Users

Then login by that user and you will see that there is no option to delete any item for this user. Hopefully, it will helpful for all sharepoint user.
Most appreciate to give any feedback
MJ Ferdous