Non-destructive tests (NDT) and partially destructive testing (PDT) are two major techniques available to evaluate the in-situ concrete strength of an existing reinforced concrete (RC) building. The various requirements such as seismic requalification, addition-alteration, and re-evaluation at the end of the design life of many structures have made the structural audit exercise an expanding research topic and execution challenge as well. Guidelines of international codes suggest various schemes to arrive at the single representative characteristics strength of concrete for the RC structure, using the results of the NDT-PDT exercise on that structure. In this context, the methods of developing a correlation expression between NDT and PDT have been discussed extensively in many works. However, in certain conditions such as apparently deteriorated health or functional constraints in critical facilities, carrying out PDT might not be feasible and the in-situ strength must be estimated based only on the NDT results of the building. Presently no technique or approach is available in the literature for such application. To bridge this knowledge gap, this study proposes a novel approach to arrive at the in-situ concrete strength of an RC building using NDT results from that building and NDT-PDT results from adjacent similar buildings. This work will point out the necessity of looking beyond a single regression equation to correlate NDT-PDT. This will be followed by enunciating the idea of characteristic strengths for different confidence levels. The proposed approach has been presented with a case study and the enumerated methodology would be very useful for practitioners across the world.