2. According to the "Urban City Appearance and Environmental Sanitation Management Regulations", it is illegal to post promotional materials on city buildings without approval.
3. Posting promotional materials in violation of regulations may result in fines and other penalties.
4. Therefore, it is illegal to install billboards on the second-floor exterior walls of first-floor shops without approval from relevant departments.
5. However, it may be feasible if the billboard only occupies part of the second floor exterior wall and does not affect the building structure.
6. Building codes and safety standards must be followed when installing billboards to ensure quality and safety.