Power distribution network (PDN) of a multilayered printed circuit board is designed to supply low noise and stable power to ICs. Reduced voltage levels and increasing current-supply requirements accentuates the PDN design complexity. It therefore becomes necessary to have multiple design iterations to achieve an optimal impedance profile for the PDN. 3D full-wave electromagnetic solvers, like the Partial Element Equivalent Circuit (PEEC) method, are accurate but suffer from high compute time requirements, which prohibit its use in the early design phase iterations. On the other hand, pure 2.5D methods, like the non-orthogonal 2.5D PEEC approach, have lower time and memory requirements but fail to capture coplanar coupling due to the underlying TEM assumptions. This affects the accuracy of PDN modeling for coplanar power-ground or signal-power configurations. In this work, the non-orthogonal 2.5D PEEC formulation is extended to include coplanar mutual coupling. Numerical results using quadrilateral meshes demonstrate good accuracy reasonably close to 3D full-wave formulation for planar geometries.