Second Update:
If the chapter and section titles etc. should be aligned to the indented text you can use:
\documentclass[
paper=a4,
%chapterprefix
]{scrbook}[2015/10/03]
\newcommand\secnumwidth{1.5cm}
\makeatletter
\renewcommand\chapterlinesformat[3]{%
\IfArgIsEmpty{#2}
{#3}
{\@hangfrom{\hspace{-\secnumwidth}\makebox[\secnumwidth][l]{#2}}{#3}}%
}
\renewcommand\sectionlinesformat[4]{%
\IfArgIsEmpty{#3}
{#4}
{\@hangfrom{\hspace{-\secnumwidth}\makebox[\secnumwidth][l]{#3}}{#4}}%
}
\makeatother
\usepackage[
layoutsize={\dimexpr\paperwidth-\secnumwidth\relax,\paperheight},
layoutoffset={\secnumwidth,0cm},
inner=1cm,
outer=2cm,
showframe,
]{geometry}
\usepackage{blindtext}% dummy text
\begin{document}
\tableofcontents
\chapter{Kapitel}
\blindtext
\section{Abschnitt}
\subsection{Unterabschnitt}
\subsubsection{Unterunterabschnitt}
\blindtext
\addsec{Abschnitt ohne Nummer}
\blindtext
\end{document}
Result:

Update: It is better to use only the geometry options like @egreg showed in his answer. But the following code works also if the KOMA-Script option chapterprefix is used.
\documentclass[
paper=a4,
chapterprefix
]{scrbook}[2015/10/03]
\RedeclareSectionCommands[indent=-10mm]{section,subsection,subsubsection}
\renewcommand*{\chapterformat}{%
\hspace{-10mm}\mbox{\chapappifchapterprefix{\nobreakspace}\thechapter
\autodot\IfUsePrefixLine{}{\enskip}}%
}
\renewcommand{\chapterlineswithprefixformat}[3]{%
#2\hspace{-10mm}\parbox[t]{\dimexpr\textwidth+10mm}{\raggedchapter#3}%
}
\usepackage[
layoutsize={20cm,27.9cm},% cut 1cm from the width
layoutoffset={1cm,0cm},% put it in the offset
inner=1cm,
outer=2cm,
showframe,
]{geometry}% from http://tex.stackexchange.com/a/280792/43317
\usepackage{blindtext}% dummy text
\begin{document}
\tableofcontents
\Blinddocument
\end{document}

Original answer:
Maybe something like that
\documentclass[
chapterprefix
]{scrbook}[2015/10/03]
\RedeclareSectionCommands[indent=-10mm]{section,subsection,subsubsection}
\renewcommand*{\chapterformat}{%
\hskip -10mm\mbox{\chapappifchapterprefix{\nobreakspace}\thechapter
\autodot\IfUsePrefixLine{}{\enskip}}%
}
\renewcommand{\chapterlineswithprefixformat}[3]{%
#2\hskip -10mm\parbox[t]{\dimexpr\textwidth+10mm}{\raggedchapter#3}%
}
\usepackage[inner=10mm,outer=20mm,nomarginpar
,showframe% shows the page layout
]{geometry}
\addtolength\hoffset{10mm}
\addtolength\textwidth{-10mm}
\usepackage{blindtext}% dummy text
\begin{document}
\tableofcontents
\chapter{Kapitel}
\parbox{\linewidth}{\blindtext}
\section{Abschnitt}
\begin{center}
\blindtext
\end{center}
\Blinddocument
\end{document}

twosidelayout? – Tobias Kienzler Nov 30 '15 at 13:05twosidelayout and using\usepackage[inner=10mm, outer=20mm]{geometry}how could I increase only the "left" margin no matter whether the page is odd or even? Wouldbcorwork here? – Tobias Kienzler Nov 30 '15 at 13:16