Two-dimensional lead halide perovskite nanoplatelets (2D LHP NPLs) have been emerging as one of the most promising semiconductor nanomaterials due to their narrow absorption and emission line widths, tunable bandgaps, high exciton binding energies, high defect tolerance as well as highly localized energy states. Colloidal synthesis of 2D LHP NPLs is generally performed using hot-injection or ligand assisted precipitation techniques (LARP). In the LARP method, perovskites are synthesized in polar solvents, which decrease the stability of the 2D LHP NPLs due to their weakly bonded nature. In fact, the presence of residual polar solvent in the LHP NPL colloid can cause deterioration of thickness uniformity, degradation of NPLs to parent precursors, and undesired phase transformations. Herein, for the first time, we report facile seed-mediated synthesis route of monolayer, 2-monolayers, and thicker lead halide perovskite nanoplatelets without using A site cation halide salt (AX; A = Cesium, methylammonium, formamidinium and, X = Cl, Br, I) and long chain alkylammonium halide salts (LX; L = oleylammonium, octylammonium, butylammonium and, X = Cl, Br, I). The seed solution has been synthesized by reacting lead (II) halide salt and coordinating ligands (oleylamine or octylamine and oleic acid) in nonpolar high boiling solvent (1-octadecene). The seed mediated synthesis has been carried out in hexane by reacting seed solution with A-site cation precursors (Cs-oleate, FA-oleate, or diluted MA solution in hexane) under ambient conditions. More importantly, the seed mediated growth of NPLs has been tracked for the first time by performing in-situ optical measurements. Furthermore, the optical properties and morphologies of the seeds have been extensively studied. We find that our facile synthesis route provides highly stable, monodisperse NPLs with narrow absorption, and photoluminescence line widths (68-201 meV), and high PLQY (37.6-1.66% for 2ML NPLs). Furthermore, anion exchange reactions have been performed by mixing pre-synthesized LHP NPLs with counter halide seeds. The optical properties of NPLs have been affectively tuned by postsynthetic chemical reactions without changing the thickness of the NPLs. We anticipate that our new synthetic route provides further understanding of growth dynamics of LHP NPLs.