Key questions in physics can be answered only by constructing a giant underground observatory to search for rare events and study terrestrial and astrophysical neutrinos. The Astroparticle Roadmap of ApPEC/ASPERA strongly supports this, recommending that: “a new large European infrastructure of 100'000-500'000 ton for proton decay and low-energy neutrinos be evaluated as a common design study together with the underground infrastructure and eventual detection of accelerator neutrino beams”. The latest CERN roadmap also states: “a range of very important non-accelerator experiments takes place at the overlap of particle and astroparticle physics exploring otherwise inaccessible phenomena; Council will seek with ApPEC a coordinated strategy in these areas of mutual interest.”Reacting to this, uniting scientists across Europe with industrial support to produce a very strong collaboration, the LAGUNA FP7 design study has had a very positive effect. It enabled, via study of seven pre-selected locations (Finland, France, Italy, Poland, Romania, Spain and UK), a detailed geo-technical assessment of the giant underground cavern needed, concluding finally that no geo-technical show-stoppers to cavern construction exist. Building on this, the present design study will address two challenges vital to making a final detector and site choice: (i) to determine the full cost of construction underground, commissioning and long-term operation of the infrastructure, and (ii) to determine the full impact of including long baseline neutrino physics with beams from CERN.