This all depends on your local Building and Zoning regulations. Check with you local jurisdiction to find out how much you can add to your individual property.
Costs vary from $80 to $250 per square foot. Additions with plumbing involved cost more than those without, such as bathrooms, kitchens, etc.
On average, $6-10k if not adding any plumbing fixtures. If adding plumbing (such as a full bathroom), cost can go up to $30k-$40k.
$25,000 to $30,000, on average for a standard, two car garage.
On average, you will be looking at about $180 per square foot, especially if there is going to be a bathroom involved. For a standard 500-700 SF addition, you will be looking at somewhere in the range of $130k to $170k.