This paper presents a top-down design methodology for the design of EBG structures meeting design targets identified in terms of S/Z parameters. The design of EBG structures are based on multiple design considerations of central cell structures as well as cell to cell interconnect structures. Possible iterations of cell structures and cell to cell interconnects are captured in libraries for design trade-offs. The RLGC first order based approach followed by precise s-parameter models extraction (2.5D/3D) is proposed in this paper. RLC libraries created are generic and reusable for top-down design methodology. This methodology is generic and has high value add in terms of systematic EBG design, cost consideration and design trade-offs. Further library augmentation, flow automation can be applied and this flow can be offered as a design tool. Methodology validation is carried out by one real design case wherein fabricated structure results (S11, Z11) are correlated with the mentioned design flow. The real design case EBG structure is having stop-band from 750 MHz to 5.10 GHz with isolation better than -30 dB and target impedance of around 0.5 ohm up- To 200 MHz. A close correlation is achieved with s-parameter using the top-down design methodology and measured results.