documentclass[
               pagesize,
               usegeometry = on,
               DIV = calc,% for the line spacing, KOMA-Script manual, page 40
               ngerman
              ]{scrartcl}

usepackage[
            left = 25mm,
            bottom = 20mm,
            right = 45mm,
            top = 20mm
           ]{geometry}
usepackage[onehalfspacing]{setspace}% for the line spacing, KOMA-Script manual, page 40
usepackage{
            babel,
            csquotes
           }
usepackage[
            backend = biber,
            sortlocale = auto,
            sorting = nyt,
            style = ext-authoryear-comp
           ]{biblatex}

% for the line spacing, KOMA-Script manual, page 40:
AfterTOCHead{singlespacing}
KOMAoptions{DIV = last}
% adjusting the bibliography
AtBeginBibliography{renewcommand*{mkbibnamegiven}{textsc}}% small caps for the author name
AtBeginBibliography{renewcommand*{mkbibnamefamily}{textsc}}% small caps for the author surname
DeclareFieldFormat{biblabeldate}{#1}% year without the surrounding brackets
DeclareDelimFormat[bib]{nameyeardelim}{addcommaspace}% comma before the year
% changes the type @online
DefineBibliographyStrings{german}{
                                   urlfrom = {verfügbar unter},
                                   urlseen = {zuletzt geprüft am}
                                  }
% changes the format of the fields url and urldate
DeclareFieldFormat{url}{bibstring{urlfrom}addcolonspaceurl{#1}}
DeclareFieldFormat{urldate}{mkbibbrackets{bibstring{urlseen}addcolonspace#1}}
renewbibmacro*{url+urldate}{% changes the default order of the fields url and urldate
                             usebibmacro{urldate}%
                             newunit
                             usebibmacro{url}
                            }

begin{filecontents}{jobname.bib}

  @ONLINE{Chemie,
          EDITOR   = {LUMITOS GmbH},
          LOCATION = {Berlin},
          URL      = {http://www.chemie.de/lexikon/Kerosin.html},
          TITLE    = {Kerosin},
          URLDATE  = {2018-06-22},
         }

end{filecontents}
addbibresource{jobname.bib}

begin{document}

  Text. Text. Text. Text. Text. Text. Text. Text. Text. Text. Text. Text. Text autocite{Chemie}.
  printbibliography

end{document}