How to display big chapter number right aligned with paragraph, without titlesec

Question

I would like to display on the first page of every chapter, the chapter's name, followed by some spacing and then the chapter's number in big numerals. It should look something like the following, except in black or dark grey. My problem is, that I found a few solutions, but they are all with titlesec, but I'm using fancyhdr and it doesn't seem to be compatible. And I'm using scrreprt and can't switch to memoir. And it's also important that only the chapters are affected and not table of contents or list of figures.

I really like this example, but I don't understand how to make the number look different (more like the one in the picture) and how tho align the number with the paragraph.

Answer 1

Here is a suggestion based on the code linked in the question:

\documentclass
  %[parskip=half]
  {scrreprt}
\usepackage{lipsum}% only for dummy text
\usepackage{xcolor}
\usepackage{graphicx}
\usepackage{etoolbox}
\colorlet{feline}{darkgray}% change the color of chapter numbers here
\makeatletter
\newsavebox{\feline@chapter}
\newcommand\feline@chapter@marker[1][4cm]{%
  \sbox\feline@chapter{%
    \resizebox{!}{#1}{\setlength{\fboxsep}{0pt}%
        \colorbox{white}{\color{feline}\usefont{U}{eur}{m}{n}\thechapter}}}% change the font of chapter numbers here
  \raisebox{\depth}{\usebox{\feline@chapter}}%
}
\renewcommand*{\chapterformat}{%
  \sbox\feline@chapter{\feline@chapter@marker[1.61cm]}% adjust the height of chapter numbers here
  \makebox[0pt][l]{%
    \makebox[\textwidth][r]{%
      \usebox\feline@chapter}}%
}
\renewcommand\chapterlinesformat[3]{%
  \Ifstr{#1}{chapter}
    {#2\parbox[b]{\dimexpr\textwidth-\wd\feline@chapter-1em\relax}{\raggedchapter #3}}
    {@hangfrom{#2}{#3}}%
}
\makeatother
\preto\chapterheadendvskip{%
  \vspace*{-\parskip}%
  {\noindent\setlength\parfillskip{0pt plus 1fil}\rule{\textwidth}{.4pt}\par}%
}
\begin{document}
\tableofcontents
\chapter{Ten}
\lipsum
\chapter{Lorem ipsum dolor sit amet, consectetuer adipiscing elit}
\lipsum
\end{document}

It works even if you set an option like parskip=half or parskip=full.

---

Update regarding a comment below

If you want to color the line below the chapter title, define a color and use it for the rule:

...
\colorlet{felinerule}{red}
...
... \textcolor{felinerule}{\rule{\textwidth}{.4pt}} ...
...

And if only numbered chapters should use the changed layout:

\documentclass
  %[parskip=half]
  {scrreprt}
\usepackage{lipsum}% only for dummy text
\usepackage{xcolor}
\usepackage{graphicx}
\usepackage{etoolbox}
\colorlet{feline}{darkgray}% change the color of chapter numbers here
\colorlet{felinerule}{red}
\makeatletter
\newsavebox{\feline@chapter}
\newcommand\feline@chapter@marker[1][4cm]{%
  \sbox\feline@chapter{%
    \resizebox{!}{#1}{\setlength{\fboxsep}{0pt}%
        \colorbox{white}{\color{feline}\usefont{U}{eur}{m}{n}\thechapter}}}% change the font of chapter numbers here
  \raisebox{\depth}{\usebox{\feline@chapter}}%
}
\renewcommand*{\chapterformat}{%
  \sbox\feline@chapter{\feline@chapter@marker[1.61cm]}% adjust the height of chapter numbers here
  \makebox[0pt][l]{%
    \makebox[\textwidth][r]{%
      \usebox\feline@chapter}}%
}
\newif\ifchapterrule
\renewcommand\chapterlinesformat[3]{%
  \global\chapterrulefalse
  \Ifstr{#1}{chapter}
    {\IfArgIsEmpty{#2}
      {@hangfrom{#2}{#3}}% unnumbered chapters: original definition 
      {#2\parbox[b]{\dimexpr\textwidth-\wd\feline@chapter-1em\relax}{\raggedchapter #3}%
        \IfArgIsEmpty{#2}{}{\global\chapterruletrue}%
      }%
    }
    {@hangfrom{#2}{#3}}% other sectioning levels with style=chapter: original definition
}
\makeatother
\preto\chapterheadendvskip{%
  \ifchapterrule
  \vspace*{-\parskip}%
  {\noindent\setlength\parfillskip{0pt plus 1fil}\textcolor{felinerule}{\rule{\textwidth}{.4pt}}\par}%
  \fi
}
\begin{document}
\tableofcontents
\chapter{Ten}
\lipsum
\chapter{Lorem ipsum dolor sit amet, consectetuer adipiscing elit}
\lipsum
\end{document}

Answer 2

Improved answer

\documentclass[chapterprefix=off]{scrreprt}
\usepackage{lipsum}
\usepackage{anyfontsize}
\newcommand{\HUGE}{\fontsize{40}{44pt}\selectfont}
\renewcommand{\chapterlinesformat}[3]
  {\mbox{#3}\hfill\HUGE{\mbox{\thechapter}}\[10pt]
    \hrulefill
  }
\begin{document}
\chapter{First}
\lipsum
\end{document}

Below is the original answer that did not address the question. I just leave it in for reference.

You can setup the plain pagestyle to generate this header on the first page of each chapter:

\documentclass{scrreprt}
\usepackage{lipsum}
\usepackage{fancyhdr}
\pagestyle{fancy}
\fancypagestyle{plain}{%
  \fancyhead[L]{\LARGE\textbf{\leftmark}}
  \fancyhead[R]{\Huge\textbf{\arabic{chapter}}}
}
\setlength{\headheight}{30pt}
\renewcommand{\chaptermark}[1]{\markboth{#1}{}}
\begin{document}
\chapter{First}
\lipsum
\end{document}

Please note that the \headheight has to be increased to avoid nasty LaTeX warnings, and that this causes the header also to be bigger on all other pages.