A storage system comprising a storage grid structure and multiple container handling vehicles (200,300), the storage grid structure comprises vertical column profiles defining multiple storage columns, in which storage containers can be stored one on top of another in vertical stacks, and at least one transfer column (119,120), each of the storage columns and the transfer column (119,120) is defined by four of the vertical column profiles, and each of the column profiles comprises four corner sections, wherein each corner section is arranged to accommodate a corner of a storage bin, and the column profiles are interconnected at their upper ends by top rails (110,111) forming a horizontal top rail grid upon which the container handling vehicles (200,300) may move in two perpendicular directions, the container handling vehicles are able to retrieve storage containers from, and store storage containers in, the storage columns, and transport the storage containers on the storage grid structure, wherein each of the container handling vehicles (200,300) comprises a lifting device for vertical transfer of a storage container, wherein the storage grid structure comprises at least one horizontal transfer section; and the storage system comprises multiple container transfer vehicles and transfer rails (110',111') forming a transfer rail grid upon which the container transfer vehicles may move in at least one horizontal direction, and - the transfer section is arranged at a level below the top rail grid and extends from an external side of the storage grid structure to a first position under a transfer port at a lower end of the at least one transfer column (119,120) and comprises at least a section of the transfer rail grid upon which section the container transfer vehicles may pass each other and move in two perpendicular horizontal directions; and - each of the container transfer vehicles comprises a container carrier for carrying a storage container and a wheel arrangement (32a,32b) for moving the container transfer vehicle in two perpendicular directions upon the transfer rail grid; and - the transfer rail grid extends from the first position to a second position external to the storage grid structure, wherein the at least one transfer column (119,120) extends from the top rail grid to the transfer section, such that a container handling vehicle (200,300) may transfer a storage container between the top rail grid and the container carrier of one of the container transfer vehicles through the transfer port by lowering said storage container from the top rail grid, via the transfer column (119,120), to a container transfer vehicle arranged below the transfer column (119,120), and the storage container may be moved by the container transfer vehicle from the first position to the second position. wherein the storage grid structure comprises at least one horizontal transfer section; and the storage system comprises multiple container transfer vehicles and transfer rails (110',111') forming a transfer rail grid upon which the container transfer vehicles may move in at least one horizontal direction, and - the transfer section is arranged at a level below the top rail grid and extends from an external side of the storage grid structure to a first position under a transfer port at a lower end of the at least one transfer column (119,120) and comprises at least a section of the transfer rail grid upon which section the container transfer vehicles may pass each other and move in two perpendicular horizontal directions; and - each of the container transfer vehicles comprises a container carrier for carrying a storage container and a wheel arrangement (32a,32b) for moving the container transfer vehicle in two perpendicular directions upon the transfer rail grid; and - the transfer rail grid extends from the first position to a second position external to the storage grid structure, - the transfer section is arranged at a level below the top rail grid and extends from an external side of the storage grid structure to a first position under a transfer port at a lower end of the at least one transfer column (119,120) and comprises at least a section of the transfer rail grid upon which section the container transfer vehicles may pass each other and move in two perpendicular horizontal directions; and - each of the container transfer vehicles comprises a container carrier for carrying a storage container and a wheel arrangement (32a,32b) for moving the container transfer vehicle in two perpendicular directions upon the transfer rail grid; and - the transfer rail grid extends from the first position to a second position external to the storage grid structure, wherein the at least one transfer column (119,120) extends from the top rail grid to the transfer section, such that a container handling vehicle (200,300) may transfer a storage container between the top rail grid and the container carrier of one of the container transfer vehicles through the transfer port by lowering said storage container from the top rail grid, via the transfer column (119,120), to a container transfer vehicle arranged below the transfer column (119,120), and the storage container may be moved by the container transfer vehicle from the first position to the second position.