Wordnet 3.0
ADJECTIVE (1)
1. 
 not subject to locally assessed property taxes; 
- Example: "unratable properties"
The Collaborative International Dictionary of English v.0.48:
nontaxable \nontaxable\ adj.
   Not subject to taxation; -- of goods imported into a country
   or sold at retail outlets; as, most laws imposing sales taxes
   make food nontaxable. Opposite of taxable. [Narrower terms:
   duty-free; tax-exempt, tax-free; unratable; untaxed ]
   Also See: {exempt.
   [WordNet 1.5]
WordNet (r) 3.0 (2006):
unratable
    adj 1: not subject to locally assessed property taxes;
           "unratable properties"