Here's a possible solution using titlesec, having in mind Incompatibilities between KOMA-Script and titlesec. The \widestnum length controls the separation between the numbers and the titles; I set it to be 3em, but you can change it to be the value that best suits your needs:
\documentclass[oneside,openright,BCOR=5mm,paper=a4,fontsize=11pt]{scrbook}\usepackage[T1]{fontenc}\usepackage[latin1]{inputenc}\usepackage[ngerman]{babel}\usepackage[automark]{scrpage2}\usepackage{xcolor}\usepackage{geometry}\geometry{a4paper, top=35mm, left=30mm, right=60mm, bottom=25mm, headsep=15mm, footskip=12mm}\usepackage[demo]{graphicx}\usepackage{textpos}\usepackage{blindtext}\usepackage{lipsum}\usepackage[explicit]{titlesec}\usepackage{mparhack}\setlength{\marginparwidth}{3.2cm}\setlength{\marginparsep}{0.7cm}\setheadwidth[0pt]{textwithmarginpar}\setfootwidth[0pt]{head}\newlength\widestnum\setlength\widestnum{3em}\titleformat{\chapter}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \colorbox{gray!20}{% \parbox{\textwidth}{% \parbox[t]{\widestnum}{\thechapter}% \parbox[t]{\dimexpr\textwidth-\widestnum\relax}{\strut#1\strut}% }% }% }% }\titleformat{\section}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \fbox{% \parbox{\textwidth}{% \parbox[t]{\widestnum}{\thesection}% \parbox[t]{\dimexpr\textwidth-\widestnum\relax}{\strut#1\strut}% }% }% }% }\titleformat{\subsection}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \fbox{% \parbox{\textwidth}{% \parbox[t]{\widestnum}{\thesubsection}% \parbox[t]{\dimexpr\textwidth-\widestnum\relax}{\strut#1\strut}% }% }% }% }\titleformat{name=\chapter,numberless}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \colorbox{gray!20}{% \parbox{\textwidth}{\strut#1\strut}% }% }% }\titleformat{name=\section,numberless}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \fbox{% \parbox{\textwidth}{\strut#1\strut}% }% }% }\titleformat{name=\subsection,numberless}[hang] {\normalfont\bfseries\sffamily} {} {0em} {% \makebox[\linewidth]{% \fbox{% \parbox{\textwidth}{\strut#1\strut}% }% }% }\titlespacing*{\chapter} {0pt}{20pt}{12pt}\begin{document}\chapter*{Test unnumbered chapter}Text\section*{Test unnumbered section}Text\subsection*{Test unnumbered subsection}Text\chapter{Test numbered chapter}Text\section{Test numbered section}Text\subsection{Test numbered subsection}Text\end{document}An image showing the settings for unnumbered sectional units:
An image showing the settings for both numbered and unnumbered sectional units:
