The construction of La Rochelle U-boat base started in april 1941. Work lasts day and night. The night, powerful projectors light the building site but they must be extinct with air the slightest warning, plunging the workmen in the darkness what entraine the fall of some of them in concrete castings. To make the building invulnerable to the bombardments, the Germans cover it with a flagstone of concrete and of steel an average thickness of 3,5 Mr. finished work, it any more but does not remain to be made jump the dam intended to drain the building site to make of them the access free to the submarines.

In November 1941, the first section of work is finished, it only remains to arrange and equip the base with cranes and pumps intended for the docks assechables. The bunker makes it possible to place 4 U-boote in 2 wet docks, and 5 others in the 5 docks assechables.

With the increase in the number of the submarines in operations, the German staff decides to increase the number of places of the U-bunker. A second section of work is undertaken in April 1942, in order to add to it 3 new boxes, including 2 assechables.

The base thus finally consists of 10 cells making it possible to maintain 13 submarines and of 10 workshops and stores. Cells 3 to 9 can be closed by tight hoppers, and be drained by powerful pumps. To supplement the device, a new lock is built in July 1942, entirely protected by a bunker and only reserved for the passage of the submarines. The defense of the base is ensured by a whole anti-aircraft and anti-submarine device. Tethered balloons disseminated in the roads emp?chent the bombardments at low altitude, whereas the positions of Flak (gun AA) and the hunting of Luftwaffe based in Laleu avoid the bombardments at higher altitude.