How to obtain a swiss letter layout?

Here is an example using scrlttr2:

\documentclass[
  SN,% lco file for swiss letters (SN 010 130))
  refline=dateleft,
  firstfoot=false,
  enlargefirstpage,
  backaddress=false,
  foldmarks=false,
  parskip=half-
  ]{scrlttr2}
\usepackage[english]{babel}

\setkomavar{fromname}{My Name}
\setkomavar{fromaddress}{My Street \\ My city}
\setkomavar{date}{The place where I am, \today}
\setkomavar{subject}{The title of the job \quad --\quad 100\%}
\setkomafont{title}{\bfseries\normalsize\raggedright}

\makeatletter
  \@setplength{firstheadwidth}{\textwidth}
\makeatother

\renewcommand\raggedsignature{\raggedright}

\usepackage{blindtext}% dummy text
\begin{document}
  \begin{letter}{Compagny\\Contact person\\Street\\City}
    \opening{Dear Mr. Forexample,}
    \blindtext

    \blindtext
    \closing{Best regards}
    \encl{enclosures}
    \cc{other recipients}
  \end{letter}
\end{document}

---

To shift your address a little down add the wanted shift to the pseudolength firstheadvpos.

    \documentclass[
      SN,% lco file for swiss letters (SN 010 130))
      refline=dateleft,
      firstfoot=false,
      enlargefirstpage,
      backaddress=false,
      foldmarks=false,
      parskip=half-
      ]{scrlttr2}
    \usepackage[english]{babel}

    \setkomavar{fromname}{My Name}
    \setkomavar{fromaddress}{My Street \\ My city}
    \setkomavar{date}{The place where I am, \today}
    \setkomavar{subject}{The title of the job \quad --\quad 100\%}
    \setkomafont{title}{\bfseries\normalsize\raggedright}

    \makeatletter
      \@setplength{firstheadwidth}{\textwidth}
      \@addtoplength{firstheadvpos}{.5cm}% <- shift the head with your address .5cm down
    \makeatother

    \renewcommand\raggedsignature{\raggedright}

    \usepackage{blindtext}% dummy text
    \begin{document}
      \begin{letter}{Compagny\\Contact person\\Street\\City}
        \opening{Dear Mr. Forexample,}
        \blindtext

        \blindtext
        \closing{Best regards}
        \encl{enclosures}
        \cc{other recipients}
      \end{letter}
    \end{document}

---

If you load visualize.loc in your preamble you can show the position and size of some fields:

\LoadLetterOption{visualize}
\showfields{head,foot,address,location,refline}

Well, have you read the documentation to chletter (try texdoc chletter on your console/terminal)?

On page 25 you will find chapter "5.5.1 Letterhead layout" with the definitions you have to change.

Please run the following MWE (and check the additional code between \makeatletter and \makeatother (needed because the code uses @)):

\documentclass[%
  %leftwin % Adresse Empfänger links, sonst rechts
 %,leftsig
 %,foldmark 
 %,footfill
]{chletter}
\usepackage[english]{babel}


\makeatletter  % <=====================================================
\if@leftwin
  \def\titletopmatter%
  {\splitfield
   {}{\fromlocation\par\fromname\par\fromaddress\par\telephonenum}}
  \def\titlemidmatter%
  {\returnaddress\par\toname\par\toaddress}
  \def\titlebotmatter%
  {\splitfield
  {\@date}{}} % <===== position of date in letter head!
  \else
  \def\titletopmatter%
  {\splitfield
  {\fromlocation\par\fromname\par\fromaddress\par\telephonenum}{}}
  \def\titlemidmatter%
  {\splitfield
  {}{\returnaddress\par\toname\par\toaddress}}
  \def\titlebotmatter%
  {\splitfield
  {\@date}{}}\fi  % <===== position of date in letter head!
\makeatother  % <=====================================================


\name{My Name}
\address{My Street \\ My city}
\date{The place where I am, \today}
\title{\textbf{The title of the job \quad --\quad 100\%}}

\begin{document}
\begin{letter}{Compagny\\Contact person\\Street\\City}
\object
\opening{Dear Mr. Forexample,}
My letter body.
\closing{Best regards}
%\encl{enclosures}
%\cc{other recipients}
\end{letter}
\end{document}

and see the result:

I have never been in favour of letter-like classes. They tend to specify a large amount of settable content in the preamble, making small adjustments to layout very difficult (or cumbersome in the very least).

Letters are easily set inside article:

\documentclass{article}

\usepackage{lipsum,graphicx}
\usepackage[margin=1in]{geometry}

\setlength{\parindent}{0pt}
\setlength{\parskip}{\baselineskip}
\pagestyle{empty}% Remove any headers/footers
\begin{document}
\sffamily% Default font for this letter

\begin{tabular}{@{}l}
  Jean Dupont \\
  Chemin des Capucines 3 \\
  1200 Gen\`eve
\end{tabular}

\hfill
\begin{tabular}{@{}p{.5\linewidth}@{}}
  A l'attention de Madame Lara Meier \\
  Chef du personnel \\
  centrale \'electrique de Limmattal \\
  Hagenholzstrasse 74 \\
  8037 Limmattal
\end{tabular}

Gen\`eve, le 20 f\'evrier 2015

\bigskip

\textbf{Candidature pour le poste d'ing\'enieur au sein de votre d\'epartement WORLD}

\bigskip

Madame,

\lipsum[1-3]% Your letter content

Dans l'attente de vos nouvelles, je vous prie de recevoir, Madame, mes salutations les meilleures.

Jean Dupont

% http://tex.stackexchange.com/q/32911/5764
\includegraphics[height=3\baselineskip]{example-image}

Annexes: CV, dipl\^omes at certificats de travail

\end{document}