I have single story house. The 550 liter water tank is located on its slab 3.15 meters above the floor. The water tank is fitted with 1.5" outlet pipe & 0.75" pipe connected with a reducer socket, but the pressure still is very low. How can I overcome this low pressure?
The distance from the top of the water level to the faucet determines the pressure at the faucet. 1 meter =1.42 psi, 2 meters=2.84 psi and 3 meters=4.26 psi (not much pressure). Remember you measure from the water level to the faucet, not to the floor. To get more pressure you need to raise the tank or add a pump system. The pipe size has little to do with the delivered pressure, it does however control the volume of flow.
At 15' your pressure would only be about 7 psi if there is not a pump, when you say low pressure is it ~7 psi or is there a pump? I would be adding a small booster pump and pressure tank to a system like this if it is only gravity fed. If there is a pump it should have a pressure switch that many times can be adjusted for a higher pressure.