The National Institute of Statistics calculates and publishes from 1967 the monthly index of the cost of building a residential building in order to keep up with the changes in the cost of residential construction over time.

As of January 2003, this index is calculated in the new base 2000 = 100 in implementation of EC Council Regulation (EC) No 1165/98 on short-term statistics. The benchmark base update process is needed by considering the changes in construction techniques and the legislative innovations introduced in this matter and to consider a new type of construction.

The construction cost index of a residential building is a Fixed Rate Lapsers index and measures the cost variations directly attributable to the cost of making the work; so the cost of soil, design cost and profit margins are excluded. The type of building considered is a 4 story building with 24 houses, some of which have double facilities, with cellars and boxes. The average residential area per housing unit is 88 square meters, while the non-residential area is about 50 square meters. For the purposes of calculating the index, prices for three different cost components are recorded:

• Time of labor. The data are collected directly by the Institute and refer to the labor costs for each qualification: skilled worker, skilled worker and common worker.
• Cost of materials. The data refer to the prices of materials released on site and are recorded on a monthly basis through the 20 Chambers of Commerce of the regional and the autonomous provinces.
• Transport and freight costs. The data refer to the cost of transport and freight hot that comprise the fuel consumption and / or electrical energy.

Elementary indices are calculated by comparing the monthly prices recorded in each square at the average price of the year chosen as a basis. By means of weighted average arithmetic, the category indexes and the provincial group where the weights are determined according to the incidence of each cost item is passed.

National category and group indices are obtained, respectively, from provincial category and group indices.

The national index of the cost of building a residential building is obtained from national group indices.