documentclass{book}
usepackage[rmargin=1cm]{geometry}
usepackage{xcolor}
usepackage{fancyhdr}
usepackage{tikzpagenodes}
usepackage{lipsum}
usetikzlibrary{calc}

pagestyle{fancy}
fancyhf{}
renewcommandheadrulewidth{0pt}
fancyhead[EL]{%
  begin{tikzpicture}[remember picture,overlay]
  path
    let
    p1=(current page.west),
    p2=(current page.east)
    in
    node[fill=gray!20,inner sep=0pt,text width=x2-x1,anchor=west,minimum height=1cm]
    at (current page header area.west)
    (box)
    {}
    node[fill=black,inner sep=0pt,minimum height=1cm,text width=10pt,anchor=west]
    at (box.west)
    (bbox)
    {}
    node[font=Huge,anchor=west]
    at (bbox.east)
    {thechapter};
  end{tikzpicture}%
  }
fancyhead[OR]{%
  begin{tikzpicture}[remember picture,overlay]
  path
    let
    p1=(current page.east),
    p2=(current page.west)
    in
    node[fill=gray!20,inner sep=0pt,text width=x1-x2,anchor=east,minimum height=1cm]
    at (current page header area.east)
    (box)
    {}
    node[fill=black,inner sep=0pt,minimum height=1cm,text width=10pt,anchor=east]
    at (box.east)
    (bbox)
    {}
    node[font=Huge,anchor=east]
    at (bbox.west)
    {thechapter};
  end{tikzpicture}%
  }

begin{document}

chapter{A test chapter}
lipsum[1-20]

end{document}