The present invention provides a storage system (1) comprising a storage grid structure (104) and multiple container handling vehicles (200,300), the storage grid structure comprises vertical column profiles (102) defining multiple storage columns (105), in which storage containers (106) can be stored one on top of another in vertical stacks (107), and at least one transfer column (119,120), the column profiles are interconnected at their upper ends by top rails (110,111) forming a horizontal top rail grid (108) upon which the container handling vehicles (200,300) may move in two perpendicular directions, the container handling vehicles are able to retrieve storage containers (106) from, and store storage containers in, the storage columns (105), and transport the storage containers on the storage grid structure, wherein the storage grid structure (104) comprises at least one horizontal transfer section (2); and the storage system comprises multiple container transfer vehicles (6) and transfer rails (110',111') forming a transfer rail grid (5) upon which the container transfer vehicles (6) may move in at least one horizontal direction, and the transfer section (2) is arranged at a level below the top rail grid (108) and extends from an external side (12) of the storage grid structure (104) to a position below the at least one transfer column (119,120) and comprises at least a section of the transfer rail grid (5) upon which section the container transfer vehicles (6) may pass each other and move in two perpendicular horizontal directions; and each of the container transfer vehicles comprises a container carrier (38) for carrying a storage container (106) and a wheel arrangement (32a,32b) for moving the container transfer vehicle (6) in two perpendicular directions upon the transfer rail grid (5); and wherein the at least one transfer column (119,120) extends from the top rail grid (108) to the transfer section (2), such that a storage container (106) may be transferred between the top rail grid (108) and the container carrier of one of the container transfer vehicles (6).