In low-rise buildings in Indonesia, beam-column joints frequently violate ACI 318 requirements through undersized columns (<300 × 300 mm) and the absence of joint hoops. This study numerically evaluates these deficiencies and the effectiveness of concrete haunch strengthening in improving joint behaviour using VecTor2’s nonlinear finite element analysis based on Modified Compression Field Theory (MCFT) and Disturbed Stress Field Model (DSFM) which is known as 2D Plane Stress Model for Reinforced Concrete. Results demonstrate undersized columns without hoops did not significantly alter failure modes; as long as the fulfilment of joint shear capacity, rebar anchorage length, and strong-column weak- beam (SCWB) are complied. In addition, concrete haunch retrofitting was found effective in improving the seismic performance of brittle joints by relocating the plastic hinge away from the joint core, therefore enhancing strength degradation, stiffness, energy dissipation, and structural ductility. As a result, initially brittle joints exhibited more ductile failure behavior. On the other hand, retrofitting already ductile joints provides no significant benefit, and may introduce overstrength conditions by promoting strong-beam weak-column mechanisms. These findings offer valuable insights for seismic retrofitting of substandard beam-column joints in earthquake-prone regions.