In SiteVision, you can build on existing templates. However, in the new template, you cannot change all the areas in the template, only the areas that are marked as a template area. A template area is a defined area where the editor/administrator may continue to develop the template.
If a template does not have a template area, you cannot base other templates on it.
To indicate that a particular area is a template area, convert an area to a template area. This area must always be in a layout (in responsive templates, they can also be in a grid row or column). Right-click on the layout and select "Convert to content area".