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If your home requires improvements, it is best to do them before it goes on the market. Nothing will turn off a buyer more than showing up at your house while there is aplumbing or electrical van in the driveway, or roofers peeling off shingles. You never want to show a defect to a buyer and explain that you will fix it. Fix it first and then it's not an issue.
Don't go overboard and decide that now is the time you want to add another bedroom dormer. Keep rationality first and foremost and you will see that a little elbow grease and a mild investment may just be all your house needs to show its best and maximize the value of the offers presented.