# How to prevent offsetting a margin?

I have basic class file and I would like to use fancyhdr. Here is the markup of some-class.cls:

\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{some-class}[2013/07/29]

\renewcommand{\normalsize}{\fontsize{10pt}{12pt}\selectfont}
\DeclareOption{a4paper}{
\setlength\paperheight {297mm}%
\setlength\paperwidth  {210mm}
}
\DeclareOption{landscape} {
\setlength\@tempdima   {\paperheight}%
\setlength\paperheight {\paperwidth}%
\setlength\paperwidth  {\@tempdima}
}

%Default options for design
\ExecuteOptions{a4paper}
%Process user given options
\ProcessOptions

\RequirePackage[top=20mm, bottom=25mm, left=25mm, right=0mm]{geometry}
\RequirePackage{fancyhdr}
\fancypagestyle{normal}{
\fancyhf{} % this clears all six fields (left, center, right in header and footer)
}
\pagestyle{normal}


Content of my document.tex:

\documentclass[]{some-class}
\usepackage{lipsum}
\author{roncsak}
\title{some title}
\date{2013/07/28}
\begin{document}
\lipsum
\lipsum
\end{document}


When don't use fancyhdr package every margin I set are what I wanted. But when I'm using \headsep command the whole document (except header) gets an offset of 20 mm. It would be ok but in this case the bottom margin is 'disappearing'.

I would like to use some kind of code that could subtract from \textlength, or make margin "sticky".

Do you know something like this?

-
Why is that \headsep inside the pagestyle and not a general part of the page layout, it has nothing to do there. – daleif Jul 30 '13 at 14:26
I don't know of any \textlength, do you mean \textheight? And \addtolength{\textheight}{-20mm} is what you are looking for? – karlkoeller Jul 30 '13 at 14:46
I would take your suggestion, @daleif ! – roncsak Jul 30 '13 at 17:23
@karlkoeller, I had meant that, and your code is what I was looking for! Thank you, all! (How could I accept your answer?) – roncsak Jul 30 '13 at 17:24

I don't know exactly what you are going to have in your class file, but I think you want to modify the value of textheight.
\addtolength{\textheight}{-20mm}