Three different case studies will be presented involving unforeseen conditions while performing construction administration services during structural repair and alteration projects on early-to-mid 20th century buildings in New York City. Rational engineering judgement was employed during investigation and design phases for these projects; however, the exposed conditions resulted in entirely different courses of action to be taken than what was originally anticipated. For each case study we will look at the unforeseen condition, state the problem, and explore feasible solutions to mitigate the problem. The first case study involves a late 1800’s mansion, with a retaining wall running along two of the building faces, which surrounds a light well and courtyard at the building cellar level. Displaced sections of the retaining wall required immediate shoring and structural repairs to prevent collapse of the soils and to avoid disruption and safety concerns for the sidewalk and pedestrians above. Prior to commencement of repair operations, it was found that the displacement condition was non-structural in nature. The second case study involves a high-end condominium building constructed in the early 1900s. While performing interior renovations at one of the units, the contractor exposed wire-reinforced cinder concrete slabs which were irregularly trenched through their full thickness, as well as severely disrupted concrete encased steel beams, all of which was meant to accommodate drain pipes for the unit above. The third case study involves a replacement canopy structure being built outside of a 1960’s reinforced concrete high-rise building. The new canopy, spanning approximately 25 ft from the building face to two posts on the street side, was initially intended to be supported on top of the existing footings which had been in service and supporting the old canopy for many years. However, following demolition of the old canopy, lack of sound footings and significant site constraints presented themselves. Structurally feasible, economical, and easily constructible solutions were designed and implemented to address these unforeseen conditions. Structural design considerations and key details for each case study will be presented.