Boundary wall with neighbour

The back boundary wall between my neighbour and myself is approx. 6ft high on my side. My neighbours ground level is approx. 3ft higher than the level on my side due to sloping ground and is approx. 5 feet from both houses. There is an unsightly concrete shed on my side which helps to somewhat obscure my neighbours view but I am planning on removing the shed. The boundary wall is about 7 feet long (the rest of the boundary is hedging). Am I allowed to put fencing on my side of the boundary wall at a level that will reduce my neighbours view into my back garden and spoiling my privacy? Alternatively, can I install a barna shed on my side to provide me with some privacy? TIA