In this paper, a new strategy to design semi-decentralized output feedback H-infinity controllers for continuous-time systems is presented. The feasibility problems that appear in solving the corresponding linear matrix inequalities are overcomed by adding convenient small perturbations to the state matrix. A decentralized design approach, based on the overlapping decomposition and the inclusion principle, is used to reduce the computational cost and complexity of controller design in the case of large buildings. The proposed methodology is illustrated through numerical simulations of the vibrational response of a five-story building under a seismic excitation.