Custom width for chapter

Question

In my current LaTeX project, I want to have a wide margin next to my text to add notes there. But the problem is that the chapter heading looks a bit "too short". Especially with long headlines it looks strange if a line break has to be made so early. Is it possible to extend the width so that the title is also above the margin notes?

I am using titlesec and marginnote.

MWE:

\documentclass[12pt,a4paper]{book}
\usepackage{titlesec}
\usepackage[left=3.5cm,right=6.5cm,top=4cm,bottom=4cm,headheight=15mm, footskip=15mm,marginparwidth=3.5cm,marginparsep=0.5cm,asymmetric]{geometry}

\usepackage{marginnote}

\titleformat{\chapter}%
  {\sffamily\bfseries\Huge}{\thechapter.}{10pt}{}

\usepackage{lipsum}

\begin{document}

\chapter{This is a long chapter title}

\marginnote{\lipsum}
\lipsum

\end{document}

Answer 1

The following allows the chapter titles to cover the entire width of your text block and margin paragraph by using a \parbox of that width and letting it silently overprinting the text block using \rlap.

I'd also suggest to allow ragged section titles as generally justification works poor for short paragraphs (which headings effectively are).

\documentclass[12pt,a4paper]{book}
\usepackage[explicit]{titlesec}
\usepackage[left=3.5cm,right=6.5cm,top=4cm,bottom=4cm,headheight=15mm, footskip=15mm,marginparwidth=3.5cm,marginparsep=0.5cm,asymmetric]{geometry}

\usepackage{marginnote}

\titleformat{\chapter}%
{\sffamily\bfseries\Huge}{\rlap{\parbox{\dimexpr\linewidth+\marginparwidth+\marginparsep\relax}{\raggedright\thechapter.\
#1}}}{10pt}{}

\usepackage{lipsum}

\begin{document}

\chapter{This is a long chapter title that grows longer and longer and longer}

\marginnote{\lipsum}
\lipsum

\end{document}

Answer 2

Although it is not the same as your output,have a look to the following approach:

\documentclass[12pt,a4paper]{book}
\usepackage{titlesec}
\usepackage[left=3.5cm,right=6.5cm,top=4cm,bottom=4cm,headheight=15mm, footskip=15mm,marginparwidth=3.5cm,marginparsep=0.5cm,asymmetric]{geometry}
\usepackage{marginnote}

\titleformat{\chapter}%
  {\sffamily\bfseries\Huge}{\makebox[0pt][r]{\thechapter.\hspace{10pt}}}{0pt}{\Huge\bfseries}

\usepackage{lipsum}

\begin{document}

\chapter{This is a long chapter title}

\marginnote{\lipsum}
\lipsum

\end{document}