how to extend header over the page margins?

Question

I have defined the page margins and header as appears in the codes bellow, but I want the header to cover the page margin width too, in the pages after page 2 (all pages except the first page). I searched over the net but did not find any solution to it.

\documentclass[standalone]{article}

\usepackage{marginnote}
\usepackage{fancyhdr}
\usepackage[pages=some]{background}
\usepackage{geometry}

\geometry{a4paper,
          total=          {210mm,297mm},
          left=            37.5mm,
          right=           12.5mm,
          top=             25mm,
          bottom=          25mm,
          bindingoffset=   0mm}

\pagestyle{fancy}

\lhead{\Huge \bf Title1}
\rhead{\Large \bf Title2}

\begin{document}

\noindent Some text here.Some text here.Some text here.
\reversemarginpar\marginpar{\raggedleft {\large \bf Subtitle} \\*[-.8pc]}
\\Some text here.Some text here.Some text here.

\end{document}

Answer 1

I'm not entirely sure what you would like the result to look like. I assume one of the two images but which?

Here is a start using the offset option of fancyhdr:

\documentclass[a4paper]{article}

\usepackage{marginnote,calc,kantlipsum}
\usepackage{fancyhdr}
\usepackage{geometry}

\geometry{%
  left=37.5mm,
  right=12.5mm,
  top=25mm,
  bottom=25mm,
  bindingoffset=0mm,
  headheight=30pt,% output from geometry tells you what this needs to be set to as a minimum
}

\newlength{\myoddoffset}
\setlength{\myoddoffset}{\marginparwidth + \marginparsep}


\pagestyle{fancy}
\fancyheadoffset[leh,roh]{\marginparsep}
\fancyheadoffset[loh,reh]{\myoddoffset}
\lhead{\Huge \bfseries Title1}% Don't use \bf it is a LaTeX 2.09 command, long deprecated
\rhead{\Large \bfseries Title2}

\begin{document}

  \noindent Some text here.Some text here.Some text here.
  \reversemarginpar\marginpar{\raggedleft {\large \bfseries Subtitle} \\*[-.8pc]}
  \\Some text here.Some text here.Some text here.

  \kant[1-20]
\end{document}

If you add the twoside option to geometry, you'll get something like this:

Update

This undoes the changes to the right offset. (You don't need to set it to zero - you just have to not change it as I did initially based on the images you posted.)

It also sets up a separate style for the first page which is applied at the beginning of the document using \thispagestyle{}. If you want no heading there, just use plain or empty rather than fancyfirst depending on whether you want a page number in the footer or not. In that case, you can dispense with the complexity introduced by defining two fancy styles.

\documentclass[a4paper]{article}

\usepackage{marginnote,calc,kantlipsum}
\usepackage{fancyhdr}
\usepackage{geometry}

\geometry{%
  left=37.5mm,
  right=12.5mm,
  top=25mm,
  bottom=25mm,
  bindingoffset=0mm,
  headheight=30pt,% output from geometry tells you what this needs to be set to as a minimum
}

\newlength{\myoddoffset}
\setlength{\myoddoffset}{\marginparwidth + \marginparsep}

\fancypagestyle{fancyfirst}{%
  \lhead{\Huge \bfseries Title1}% Don't use \bf it is a LaTeX 2.09 command, long deprecated
  \rhead{\Large \bfseries Title2}%
  \fancyheadoffset{0pt}}
\fancypagestyle{fancyoffset}{%
  \fancyheadoffset[loh,reh]{\myoddoffset}%
  \lhead{\Huge \bfseries Title1}% Don't use \bf it is a LaTeX 2.09 command, long deprecated
  \rhead{\Large \bfseries Title2}}
\pagestyle{fancyoffset}

\begin{document}

  \thispagestyle{fancyfirst}

  \noindent Some text here.Some text here.Some text here.
  \reversemarginpar\marginpar{\raggedleft {\large \bfseries Subtitle} \\*[-.8pc]}
  \\Some text here.Some text here.Some text here.

  \kant[1-20]
\end{document}