Who pays for painting extension?

Good Morning all, please help, NSW self managed town houses, an extension was done a few years ago to a unit, and by law signed off that they are responsible for repairs and maintenance of the extension. We are now in the process of painting the entire complex and the unit that has the extension insists that the painting of their extension should be included in the scope to paint the entire property and this is the trade off seeing if they had no extension we would have to paint their external walls which is now their internal walls. Who should pay for the painting of their extension, and is there such a thing as a “trade off”. thank you..