Shifted text when setting custom pagestyle with long header using fancyhdr

I'm trying to make a custom header using fancyhdr package. For some reason if I haven't specified the \headheight=3cm, the rest of the document gets wrong header height, even the pages with \pagestyle{plain}. The page with long header is exactly as it needs to be, though. I need to add the header to just one of the pages where a new chapter starts. I just need to use the blankspace without affecting the rest of the layout. I would be happy to use other solutions without fancyhdr package as well.

\documentclass[12pt]{report}
\usepackage[top=1in, bottom=1in]{geometry}
\pagestyle{plain}
\usepackage{fancyhdr}
\usepackage{lipsum}

\fancyhf{}%
\fancyfoot[C]{\thepage}%
}

\begin{document}
\chapter{}

\chapter{}
\lipsum[1-7]
\end{document}

• Welcome to TeX.SX! That's probably one problem of using many line breaks in a header... – TeXnician May 25 '17 at 14:09
• So how to add headers which include more than one line? – Kaur May 25 '17 at 14:28

You could use a top aligned \parbox and hide its depth. Note that the resulting header overlaps with the text area and you have to ensure that there is enough space.

\documentclass[12pt]{report}
\usepackage[top=1in, bottom=1in,
]{geometry}
\pagestyle{plain}
\usepackage{fancyhdr}
\usepackage{lipsum}
%\usepackage{showframe}% to show the page layout

\fancyhf{}
\raggedleft a\\b\\c\\d\\e\\
\rule[\ht\strutbox]{\linewidth}{.4pt}%
}}
\fancyfoot[C]{\thepage}