Claim over common space

Respected experts kindly answer my query.we have a common space of 600 sqft shared by four seperate land owners.its all been partiotioned from one single parcel of land and registered on individual names.one of the owner had built approximately over 60Sqft of the common space and its been almost 15 years.now if we want to sell our property then does it mean that we can divide the whole 600 sqft by four and then sell our share in the common space or we need to minus the space occupied ie 600 -40= 560 and then divide 560 by four and then sell.kindly reply