Very low water pressure in house
Pressure

In a system fed from a header-tank, the pressure depends only on the height of the tank. To increase pressure you have only two or three options:


Raise the tank higher.
Install an electric pump (these are often used for showers)


If your header-tank is filled from a high-pressure municipal supply, you would probably get better pressure from that directly but in most places this would be a large job involving extensive knowledge of plumbing and local laws or codes. Your existing pipework and fittings might not be suitable for high-pressure.

Flow rate

The rate of flow is affected by 


pressure
diameter of pipe
length of pipe


Half-inch pipe is usually adequate for sinks, basins, showers etc.