# Change report from one-side to two-sided

I wrote my diploma thesis in a one-sided layout (report) and now I want to print it, but don't want to have the back page blank on every single page. So I want to change the paper format to a two-sided layout in order to have a different offset on odd and even pages. For that reason, I changed my preamble which looks like this now:

\documentclass[a4paper, 12pt, twoside, openright]{report}
\usepackage[showframe]{geometry}

\usepackage{lipsum}
\usepackage{ngerman}
\usepackage{fancyhdr}
\usepackage{a4}
\usepackage{setspace}
\usepackage{graphicx}
\usepackage{sectsty}

\makeatletter
\renewcommand*\l@section{\@dottedtocline{2}{3.8em}{4em}}
\renewcommand*\l@subsection{\@dottedtocline{2}{3.8em}{4em}}
\renewcommand*\l@subsubsection{\@dottedtocline{2}{3.8em}{4em}}
\renewcommand*\l@figure{\@dottedtocline{1}{2.8em}{3em}}
\renewcommand*\l@lstlisting{\@dottedtocline{1}{2.8em}{3em}}
\makeatother

\parindent=0pt
\parskip=12pt
\onehalfspacing

\begin{document}
\pagestyle{fancyplain}

\renewcommand{\sectionmark}[1]{\markright{\thesection\ #1}}
\renewcommand{\chaptermark}[1]{\markright{\thechapter\ #1}}
\cfoot{}

\lipsum[1-100]
\end{document}


When I compile it, the following PDF is generated:

Obviously, the offset is the wrong way around. Page 1 should have the offset of page 2 and inverse. Does somebody know a simple solution for this problem?

The reason why you are having this inconsistent margin is because you are manually adjusting the textwidth when you write \addtolength{\textwidth}{...}. A better way to achieve this is to set the width of the inner and outer margins using the geometry package (which should be loaded last I think). An exaggerated solution (where I also adjust the top height rather than doing \setlength{\headheight}{15pt}) would use:
\usepackage[showframe, left=50mm, right=20mm, top=20mm]{geometry}