In this case I would suppose that the 'average' refers to the number defined previously, while 'open plan' refers to the following:

Flexible method of space layout where divisions between separate areas are implied by easily movable elements (such as screens and skeletal construction components, and modular furniture) instead of being defined by permanent walls and fixed shape furniture. Based on the ideas of the US architect Frank Lloyd Wright and French architect Le Corbusier (who called it 'Le plan libre').