How can I specify layout when using redirect_to

You don't. Layouts are associated with the controller, or the
controller's action

render :layout => "admin"

If you want to change it, the better approach would be to do something like

before_filter :login_required
before_filter :choose_layout

def index
  render :layout => @layout
end

private

def choose_layout
  @layout = current_user.is_admin? ? "admin", "public"
end

Or you could have one layout that has helpers and partials that
changed based on the user.

Are you saying that after the redirect_to it shows the view from
views/builder/index.html.erb but uses the layout from views/layouts/admin.html.erb instead of views/layouts/builder.html.erb?