The constant increase in the population of cities affects the development of housing. Investors, in their activities related to the profit from the sale of flats, focus on the completion of residential buildings, which must be timely and in accordance with the budget assumptions. Therefore, there is a problem concerning the correct planning of the costs and duration of an investment. The aim of the conducted research was to determine the shape and course of the cost curves for construction projects related to the construction of residential buildings. Based on the analysis of the authors’ own studies carried out in a homogeneous research group of 11 residence buildings, an original attempt was made to determine the area of the curve, which indicates the area of correct planning of cumulative costs and the forecasting of their deviations in the financial outlays of construction projects. By knowing the planned cost and duration of a construction project, and by using the proposed 6th degree polynomial, it is possible to determine the planned monthly work and expenditure amounts, and thus correctly plan the investment costs over time. It was proven that the planned work and expenditure advancement of the housing construction sector is greater in the first stage of its implementation when compared to the actual state. The determined 6th degree polynomials describe the regularity that shows that for half of the planned duration of works, the planned work and expenditure advancement is approx. 46%, while the actual advancement is approx. 35%.