The paper illustrates the design, analyses, materials, works for the full seismic retrofitting of a heritage-protected building, located in Sulmona (Central Italy). The redevelopment project, considering the safety requirements related to the use as a school building, provides for not reducing the nominal life and for carrying out the safety assessments as for a newly built building. The greatest criticality of the seismic resistance of the building is determined by the insufficient mechanical characteristics of the original materials. The lack of diaphragm behaviour of the floors and the shortcomings of connections between masonry walls and between decks and walls also contribute to the seismic inadequacy. These conditions compromise the optimal box-like behaviour and also determine torsional effects in the dynamic response, with a further reduction in the seismic-resistant capacity. The design strategy provided for multiple synergistic interventions aimed at increasing the mechanical characteristics of the masonry and eliminating the critical issues: closing niches and openings by indenting; consolidation of walls with mortar injections, repointing and transverse connection of the facings; insertion of new masonry walls; insertion of top steel curbs; in-plane stiffening of floors and connection with walls; connecting angles and hammers of walls; strengthening openings with perimeter steel profiles; thickening thin walls; strengthening r/c beams with metal profiles; insertion of tie rods; stabilising partitions with anti-tilting arrangement. The adopted interventions, compatible with the preservation of the heritage value of the construction, allowed to upgrade its seismic safety level to that of a newly built school building.