Trust me, with the high resolution aerial photography and/or highly accurate laser tools that assessor's regularly rely on (and love!), it won't be too long before they get the additional square footage added to your records - all without ever setting a foot on your property. 

Your only 'protection' is to own acreage, with areas of it not visible from public roads, and you build under a thick canopy of trees.  Then the assessors would need your permission to inspect your property.