If you mean the area running from the bowling green towards the north-east, it's the former Council cleansing depot, now demolished [aka Booth Street car park].
It was used for a number of years (till 2017?) for parking on match days, but is now destined for housing, courtesy of Stockport Homes.
Apparently, County/Sale Sharks were actually offered the land by the Council in the early 2000s but they failed to provide what the Council wanted money-wise.