I had a building that was not only built over a property line but it crossed over the two adjacent city's boundaries. The permit was issued by the adjacent city and I only became aware of teh problem when the elctrical contactor tried to take out a permit from my city and there was no building permit. i went to teh site and met the other City inspector who also had no clue that the building involved two separte cities. Fortunately, both cities were under contrcat with LA County so the inspections had been made. The County plan checker screwed up but so did the planning department. A deed restriction was required to join both properties. TI"s were issued depending on which part was in what city.
Each city then had to assess that portion of teh building within each city for tax purposes. What a mess!!!
To get to your question, the Code does not allow a building to cross over property lines. But, there is a solution if owned by the same person/entity. Require that the lots be joined or require a deed restriction to be held in perpetuity stating that the properties are to be treated as one.
think about: buildings in urban settings with basements that extend under public sidewalks and streets... similar urban projects that have bridges spanning between buildings and sometimes across streets... what about multi-building projects with tunnels beneath, campuses, hospital complexes, which frequently involve multiple legal parcels under common ownership (and sometimes not common ownership, but based on attached 'conditions' as chuck notes)... etc.
similarly, a single building can span over one or more property lines... as chuck points out, there are solutions to such problems and they involve zoning/planning and other requirements, perpetual 'conditional' approvals, deed restrictions, etc... this should not be confused with building code requirements relating to 'property line' protections, which would then be evaluated on the configuration resulting from these 'approved conditions or modifications' and not on the undelying legal parcel lot lines.