Standard techniques used in multilingual terminology management fail to describe legal terminologies as they are bound to different legal systems and terms do not share a common meaning. In the LexALP project, we use a technique defined for general lexical databases to achieve cross language interoperability between languages of the Alpine Convention. In this paper we present the methodology and tools developed for the collection, description and harmonisation of the legal terminology of spatial planning and sustainable development in the four languages of the countries of the Alpine Space.