Surface-enhanced　Raman　spectroscopy　(SERS)　has　attracted　increasing　interest　for　chemical　and　biochemical　sensing.　Several　studies　have　shown　that　SERS　intensities　are　significantly　increased　when　an　optical　interference　substrate　composed　of　a　dielectric　spacer　and　a　reflector　is　used　as　a　supporting　substrate.　However,　the　origin　of　this　additional　enhancement　has　not　been　systematically　studied.　In　this　paper,　high　sensitivity　SERS　substrates　composed　of　self-assembled　core-satellite　nanostructures　and　silica-coated　silicon　interference　layers　have　been　developed.　Their　SERS　enhancement　is　shown　to　be　a　function　of　the　thickness　of　silica　spacer　on　a　more　reflective　silicon　substrate.　Finite　difference　time　domain　modeling　is　presented　to　show　that　the　SERS　enhancement　is　due　to　a　spacer　contribution　via　a　sign　change　of　the　reflection　coefficients　at　the　interfaces.　The　magnitude　of　the　local-field　enhancement　is　defined　by　the　interference　of　light　reflected　from　the　silica-air　and　silica-silicon　interfaces,　which　constructively　added　at　the　hot　spots　providing　a　possibility　to　maximize　intensity　in　the　nanogaps　between　the　self-assembled　nanoparticles　by　changing　the　thickness　of　silica　layer.　The　core-satellite　assemblies　on　a　135　nm　silica-coated　silicon　substrate　exhibit　a　SERS　activity　of　approximately　13　times　higher　than　the　glass　substrate.