Buying common space

Can't seem to find a thread on this: What's the policy on buying common space from a Co-op? If you wanted to combine two apartments and buy some common hall space to make thecombination work better. Assuming it's all legal, and up to code, and the building ok's it, how do you go about determining a price? Does the building board get an appraisal? Does the buyer? Is the price based on the square footage price for the sale of one of the apartments? Which apartment? And if so, is there a precedent to argue that unrenovated common space should not be comparable in square footage price to something from a nicely renovated apartment? Who sets the starting price, buyer or co-op board?