The first stage in the development of the Lee scheme was to determine the average fall and flow of the river and the general outline and nature of the valley. From those, an estimate of the output was made, a preliminary sketch of the development prepared and the approximate cost estimated. This work was completed based on the data available from Ordnance Survey maps of the Lee catchment area, rainfall maps and a walk-over inspection of the site. The Hydrometric section did most of this work through the assistance of a senior design engineer who inspected the site. He advised on costs and type of construction.

The second stage of the development consisted of the surveying of the valley. The River Lee Survey Plan was completed in 1943. The entire valley was surveyed to determine what land would be flooded, how many houses would be submerged, and what roads and bridges would have to be replaced. All this data was updated in 1953. The Survey Plan envisaged three dams on the Lee at Inniscarra, Carrigadrohid and above Dromcarra Bridge. The possibility of a fourth dam was also investigated. A geological survey was pursued in 1943 and a report prepared. Seepage tests were made where necessary in rock and overburden. Borings and trial pits were sunk in 1944 and again in 1948. Alternative proposals for the development were sketched out and costs estimated by the design section. The lands required for the scheme were inspected and valued by the Land Purchase section. On the basis of the flow data, which had been collected since hydro-metric observations began in 1940, plans for the possible hydroelectric development of the Lee were prepared in 1947.

A report was submitted to the ESB for approval to advance the designs to the contract stage and to obtain the necessary legislative authority for construction. In 1948 it was decided that the river Lee offered an economical return. Investigations had indicated that the most favourable development consisted of the construction of two stations, one at Inniscarra, containing two 10,000 kW turbine generating sets, and another at Carrigadrohid with two 40,000 kW sets. The final design was for two sets at Inniscarra – one 15,000 kW and one 4000 kW – and one 8000 kW set at Carrigadrohid. The proposed third dam at Dromcarra was dropped. Towards the close of the year a scheme was prepared for submission to the Minister for approval. The preparatory designs for the station were drawn up, contracts finalized and specifications and bills of quantities prepared.