I'm using the documentclass ttthesis based on book from here. I really like it, but I don't want to have a new page for each chapter. Sadly every approach I tried failed, and now I noticed it's because there is no clearpage or cleardoublepage at the chapters (that's why redefining or ignoring these like described in all answers regarding this topic didn't work out).
Here is the part that redefines the chapters (not my code, just a snippet from the template):
\newlength{\chapnolen}
\newlength{\chapparlen}
\newsavebox{\chapno}
%% Kein ``Kapitel'' bei Kapitelüberschrift
% \renewcommand{\@chapapp}{} allein reicht nicht!
%% \chapter{...}
\renewcommand{\@makechapterhead}[1]{%
\vspace*{0.2\textheight}%
%\hrule
\vskip 15\p@
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\savebox{\chapno}{\chapterheadfont\huge\bfseries \thechapter.}
\settowidth{\chapnolen}{\usebox{\chapno}}
\parbox[t]{\chapnolen}{\usebox{\chapno}}\nobreak\leavevmode
% \par\nobreak
% \vskip 20\p@
\fi
\fi
\interlinepenalty\@MM
\setlength{\chapparlen}{\textwidth}
\addtolength{\chapparlen}{-1.0\chapnolen}
\addtolength{\chapparlen}{-2ex}
\leavevmode\nobreak
\parbox[t]{\chapparlen}{\raggedright\chapterheadfont\huge \bfseries #1\par\nobreak}
%\the\textwidth
%\the\chapparlen
%\the\chapnolen
%\vskip 20\p@
%\rule{\textwidth}{1pt}
\vskip 40\p@
}}
I just can't find out why there even is a pagebreak for each chapter. I also searched the other parts of the template, but didn't find anything.
Has anyone an idea where these pagebreaks come from or, more importantly, how I can prevent them?

\chapteris defined in thebookclass, where it starts with\clear(double)pageand after a few other things it calls\@makechapterhead, which you are redefining here. You may want to give a look here – campa Mar 22 '16 at 13:46etoolbox, I have to useinputinstead ofincluded, that solved it - would you create an answer so I can accept it? or do we want to close as duplicate? – TheFlow0360 Mar 22 '16 at 14:15tex/ttthesisand is loaded as such to avoid a warning. The class does quite a bit of other strange stuff as well. Look at all the language conditionals that shouldn't be in any modern document. – Johannes_B Mar 26 '16 at 10:10