Align chapter and section headings to outer edge

Question

I am trying to achieve the following result (made with Photoshop):

I already managed to get the right page formatted correctly, but this does obsiously not work for double sided documents:

\documentclass[a4paper,11pt,twoside,chapterprefix=true]{scrbook}

\usepackage{lipsum}
\usepackage[includehead,
            top=25mm,
            left=40mm,
            right=35mm
]{geometry}

\makeatletter
\def\@seccntformat#1{\llap{\csname the#1\endcsname\quad}}
\makeatother

\begin{document}
\chapter{Sample}
\section{This Is a First Level Section} \lipsum[1]
\subsection{And Second Level} \lipsum[2] \pagebreak
\section{First Level Again} \lipsum[3]
\subsection{Second Level} \lipsum[4]    
\end{document}

How can i align the headings to the outer edge?

Answer 1

Somebody has asked Markus. There are two suggestions on the KOMA-Script website: Überschriften am äußeren Rand ausrichten mit Nummer im Rand (German). Both suggestions needs some runs to get the numbers right.

The first solution needs KOMA-Script version 3.19a or newer. I have changed this code a little so it works now with longer section titles too.

\documentclass[chapterprefix]{scrbook}
%\providecommand*\Ifthispageodd{\ifthispageodd}% needed up to and including KOMA-Script version 3.27, see https://komascript.de/faq_deprecatedif
\usepackage{blindtext}% dummy text
\renewcommand*{\raggedsection}{\Ifthispageodd{\raggedleft}{\raggedright}}
\renewcommand*{\sectionlinesformat}[4]{%
  \ifthispagewasodd
    \parbox[t]{\linewidth}{\leavevmode\raggedsection#4}\makebox[0pt][l]{\hskip\marginparsep #3}%
  \else
    \makebox[0pt][r]{#3\hskip\marginparsep}#4%
  \fi
}
\renewcommand*{\sectionformat}{\thesection\autodot}
\renewcommand*{\subsectionformat}{\thesubsection\autodot}
\renewcommand*{\subsubsectionformat}{\thesubsubsection\autodot}
\begin{document}
\chapter{A Chapter}
\section{A section with a long, long, long title that needs more than one line}
\Blindtext
\section{Another section}
\blindtext
\blinddocument
\end{document}

The second example uses package marginnote but works also with older KOMA-Script versions.

\documentclass[chapterprefix]{scrbook}
%\providecommand*\Ifthispageodd{\ifthispageodd}% needed up to and including KOMA-Script version 3.27, see https://komascript.de/faq_deprecatedif
\usepackage{marginnote}
\usepackage{mwe}
\renewcommand*{\raggedsection}{\Ifthispageodd{\raggedleft}{\raggedright}}
\renewcommand*{\sectionformat}{%
  \marginnote{%
    \usekomafont{disposition}{\usekomafont{section}{%
        \thesection\autodot}}}}
\renewcommand*{\subsectionformat}{%
  \marginnote{%
    \usekomafont{disposition}{\usekomafont{subsection}{%
        \thesubsection\autodot}}}}
\renewcommand*{\subsubsectionformat}{%
  \marginnote{%
    \usekomafont{disposition}{\usekomafont{subsubsection}{%
        \thesubsubsection\autodot}}}}
\begin{document}
\chapter{A chapter}
\section{A section with a long, long, long title that needs more than one line}
\Blindtext
\section{Another section}
\blindtext
\blinddocument
\end{document}