I have a single story house to which I want to supply water by gravity from a 2000 litres tank. How high should the bottom of the water tank be to ensure adequate/standard pressure in the house especially the shower whose height is about 2 meters?

Good water pressure for a residence is about 50 psi, or in metric terms 350000 Pascals. Using the formula for pressure due to the "head" of water:

P = \rho g h

where \rho is the density of water, g= acceleration due to gravity and h = height of the water in the tank above the shower, we can solve for h:

h = \frac P {\rho g} = \frac {350000 \frac N {m^2}}{(1000 \frac {Kg}{m^3})( 9.8 \frac m {s^2})} = 35 m.

This ignores losses due to friction in the pipes, so actual pressure delivered to the shower head would be a bit less.

Foot Water (60°F) to Psi Conversion - Convert Foot Water (60°F) to Psi (ftAq to Psi) - Pressure Conversion (http://www.theunitconverter.com/foot-water-to-psi-conversion/)

You need a lot more than just a tank on top of your house for a gravity fed water system my friend as the pressure will decrease according to the water level as its used. Unless you have a way to keep the water level consistent, or a mechanism for compensating for usage, then you will quickly lose pressure once the levels drop.

Another variable that has to be known is the actual tank design to calculate accurately the weight of the water at different levels. You would have to use a gauge and apply your readings to formula to get what type of pressure to expect.

So without a lot more specifics or a spec of the tank it's a crap shoot figuring the height requirements.

It has to be a lot. Pressure can be ROUGHLY calculated by figuring .4 psi per foot of elevation. So, to get to 40 psi, you would need about 100 feet.

If you have electricity, a booster pump would be a better choice.