This　investigation　examines　the　structure-soil-structure　interaction　(SSSI)　problem　between　a　tunnel　and　nearby　aboveground　building,　considering　the　impact　of　the　stiffness　of　tunnel　and　building＇s　foundation.　A　twodimensional　(2D)　model　is　utilized,　which　includes　a　flexible　circular　tunnel　and　a　three-story　building　anchored　by　a　flexible　semi-circular　foundation　embedded　in　a　homogeneous　half-space.　The　response　of　the　system　to　incident　plane　SH-wave　excitation　is　solved　analytically　using　the　wave　function　expansion　method.　The　analysis　focuses　on　how　the　system　response　is　affected　by　the　stiffness　of　the　tunnel　and　the　building＇s　foundation.　Tunnel　stiffness　plays　a　leading　role　in　tunnel　response　analysis.　The　model　with　a　rigid　tunnel　and　rigid　foundation　can　offer　a　reasonable　system　response　overall,　but　it　fails　to　accurately　represent　tunnel　deformation　and　stress　distribution.　Depending　on　the　stiffness　of　the　building＇s　foundation,　the　maximum　value　of　the　inter-story　displacement　of　the　first　floor　can　be　overestimated　by　around　70　%,　with　even　greater　overestimations　for　higher　floors.　The　results　help　understand　observed　seismic　behaviors　and　provide　insight　into　the　effects　of　foundation　and　tunnel　stiffness　on　the　tunnel-building　interaction　system.