Peter Gray – If the furniture is able to be moved then it is not permanent.
The hypothetical scenario of “fill(ing) an area of common property with whatever they like” would possibly trigger an ‘obstruction’ By-law (if your scheme has such a By-law). Although, ‘obstruction’ By-laws generally pertain to walk ways, and paths etc. From what you have described, the downstairs occupant is currently not causing an ‘obstruction’.
Is the furniture located on the lawn and does the furniture need to be removed when the lawns are mowed? If the moving of the furniture is creating an additional cost to the Scheme for the grounds maintenance contractor then the Committee can either ask the occupant to remove it when the mowing occurs.