Anyone who's had to deal with a combo living/work/entertaining/eating space knows a trick or two about utilizing every inch. Take that empty space behind the sofa. One way to make the most of your square footage is to place a desk behind the sofa.
And bonus — you free up a wall in the process. Take a look at these artful organizers with various ways to dress up the sofa desk.
And for those interested in the 5th photo (which also triples as a bed) it's the Warhol Sofa by Simone Brewster.