Another approach is to have two app layouts. One for regular users and another for admin. Any logic or markup shared between the layouts can be moved to the root layout or into function components. So the layouts are very similar, except for the sidebar.

You can send a Phoenix Component in the controller assigns instead of sending the sidebar content.

In your controller:

defmodule NewPhoenixWeb.PageController do
  use NewPhoenixWeb, :controller

  def home(conn, _params) do
    render(conn, :home, sidebar_content: apply(NewPhoenixWeb.Layouts, :default_sidebar, [[]]))
  end

  def test(conn, _params) do
    render(conn, :test, sidebar_content: apply(NewPhoenixWeb.Layouts, :admin_sidebar, [[]]))
  end
end

In your layouts.ex:

defmodule NewPhoenixWeb.Layouts do
  use NewPhoenixWeb, :html

  embed_templates "layouts/*"

  def default_sidebar(assigns) do
    ~H"""
    <p>default sidebar</p>
    """
  end

  def admin_sidebar(assigns) do
    ~H"""
    <p>admin sidebar</p>
    """
  end
end

(There’s probably a better place for the above code, but layouts.ex exists in all Phoenix 1.7 apps.)

Finally, in your root.html.heex (or any other layout file you want to use):

<aside class="sidebar">
  <%= @sidebar_content %>
</aside>

If your sidebar is a component, you can use slots instead.

defmodule NewPhoenixWeb.Layouts do
  slot :inner_block

  def sidebar(assigns) do
    ~H"""
    <aside>
      <%= render_slot(@inner_block) %>
    </aside>
    """
  end
end

<.sidebar>
  <%= @sidebar_content %>
</.sidebar>

Things can get complicated depending on the number of sidebar variations, but it can be a good start.