A novel, two-layer hierarchical architecture based on surface-initiated atom transfer radical polymerization was investigated. It combines a thin and highly dense first layer, for nonfouling properties, with a loose second layer for high immobilization levels of active biomolecules. Sodium azide treatment, to reduce the concentration of macroinitiators on the first layer for reinitiation, and by controlling the polydispersity allowed one to achieve three polymer architectures with low, moderate, or high azide substitution. Moderate substitution enabled the highest immobilization levels with a nonfouling background. Integration with dual-functional zwitterionic poly(carboxybetaine) made this platform suitable for applications in undiluted complex media such as blood. It was demonstrated via a surface plasmon resonance biosensor that antigen accessibility and antibody loading were greatly improved. These results indicate the two-layer strategy as a generic concept suitable for applications from diagnostics to medical coatings in order to maximize and minimize specific and nonspecific responses, respectively.