The Story Behind This Sofa
Long sofas are a structural problem before they are a design one, and it is worth knowing what changes as length increases.
The seat rail is the critical member. It spans between legs and carries everyone sitting on it, and deflection increases sharply with span - double the unsupported length and the bend grows several times over. So beyond roughly seven feet we add intermediate legs front and back, increase the section of the seat rails, and in the longest builds add a cross member beneath the seat. None of it is visible in the finished sofa; all of it is why a long sofa still sits flat in year fifteen.
Weight rises too, and not proportionally - a longer frame needs heavier timber throughout, so a four-seater is considerably more than a third heavier than a three. That has consequences for how it is carried and who carries it.
And access becomes the binding constraint. A long sofa cannot be rotated in a stairwell or turned on a small landing, and unlike width, length cannot be reduced by removing cushions.