Add a 12 x 12 (3) seasons room onto our townhome.  We already have a concrete slab but would likely need to add another 18" or so. 

Looking for a REASONABLE estimate to add a 12 x 12 (3) seasons room on to our townhome.  There is an existing concrete slab but will likely need about 18 inches added to it.  Best guess would be $18-$20K