Extensive knowledge of land-registration and data-privacy issues and terminology.
I believe that in the vast majority of jobs what is needed is a translator with a firm grasp of good English. Sector-specific terminology can usually be found, whether by research, connections with fellow translators (such as Proz members) or direct queries to the client. Of course, expertise in the target subject helps, as it speeds the translation process and does lend quality to the finished product. The expertise I offer, however, is not necessarily in subject matter, but in constructing proper English text.