Climate change, improper use of water resources, surface waters pollution as well as increase of water requirements are the results of growing population of people in the world. It causes water deficiency in majority of countries in the world, including Poland. Due to the water pollution advanced technologies for its treatment are in demand, what leads to increase of water price. In this connection, there are more often taken actions to reduce water consumption by using rainwater to flush toilets, wash cars, do laundry or water green areas. This publication presents results of Life Cycle Cost analysis of two variants of water supply systems designed for multi-family residential building situated in Rzeszow. In line with LCC methodology the calculations were made throughout the whole life-cycle of the building considering initial investment outlays intended for construction of water supply system as well as operation and maintenance costs. In the first of analyzed variants it was assumed that the system would be fed by municipal water supply network. In the second variant rainwater harvesting system for domestic use was additionally applied. Rainwater stored in the tank would be used in sanitary installation to flush toilets, what leads to lowering the costs of municipal water purchase, reducing fees for rainwater discharge to sewage system and consequently is beneficial for financial standing of the examined building.