The piece has an asymmetric design, with one armrest the same height as the backrest and the other sitting just proud of the seat so that the user can drape their legs over the side.
"My idea was to offer different ways of sitting: normal, sideways, straight or slouchy," Sempé told Dezeen. "As all edges are upholstered, there are no hard parts to avoid."
"An armchair is almost as expensive as a sofa so I believe that it should be as comfortable as the main piece of the living room," she continued. "Sometimes the armchair is more like the poor and less comfortable member of a range that includes a sofa."
The design is available with the higher armrest positioned on the left or the right, and is intended to be used with an existing ottoman in the range.
The frame comes in natural or varnished beech, blue-grey or red, while the upholstery can be made up in a choice of Ligne Roset fabrics including velour, wool, thick cloth, microfibres or leather.
Ligne Roset will showcase the new piece at Maison & Objet trade show in Paris from 24 to 28 January 2014, where Sempé has chosen to present it in red and taupe.
"I have chosen this colour to contrast with the red structure, and to be rather happy and enlightening as it has to be presented at this dark time of the year in Europe," she explained.
"I have to say that I was not behind the choice of the sofa's colours," she added. "It often happens that the company does not want to involve the designer on the colours, and so one discovers it at the fair. Sometimes one could cry; sometimes one can be lucky."