There is no lone pair on boron in $\mathrm{B}\mathrm{C}{\mathrm{l}}_3$ hence no repulsion takes place. There is a lone pair on nitrogen in $\mathrm{N}\mathrm{C}{\mathrm{l}}_3$ hence repulsion takes place. Therefore $\mathrm{B}\mathrm{C}{\mathrm{l}}_3$ is planar molecule but $\mathrm{N}\mathrm{C}{\mathrm{l}}_3$ is pyramidal molecule.