I am planning to place switching regulator on pcb top side and the inductor on pcb bottom side to save board space. Does this have any negative impact on EMI?
The switching current associated with SW node and the inductor are best handled by locating them close to each other on the same layer. When they are located on different layers connected by vias, these switching currents can contribute more radiated energy.
If you have to switch sides on the PCB in a buck or boost design, the inductor path is the best place to switch sides. Keep the hot loop and the switches (in case of buck: CIN and the IC /top and bottom switch/diode, in case of boost: Cout and the switches/diode) on one side and place the inductor and Cout on the other side. The inductor mainly carries triangular currents which are not as wide band as the hot loop currents. Hot loop in the Buck case: The Cin decoupling and the switches.
The down side will be, that you now have at least some high dV/dT trace area on both sides, which will make it harder to get good EMI than keeping all the noise on one side.
BR
Christian