You could go to town hall and ask to see the building permits issued for the last month. On those permits, there will be a cost of the renovations. Sometimes those are close to actual.
Lots more info needed.
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Typically a basic room addition can cost anywhere between $115 - $250 per square foot and up.
Single room addition, crawl space foundation, minimal electrical, minimal structural work, no plumbing, average finishes, average number of windows, easy tie-in to existing home, tie into existing heating system. (Generally â€“ the larger and more open the spaces are the less per square foot.).
Let me know if you are in need of a contractor. A have a few names of great contractors in your area.