We have had something similar happen to us - we rented a place with a nice big back yard, then the landlord decided to fence it off and put another house on the back section. The plans for the other house fell through and even though we were keeping the back yard tidy they still fenced it off so we had nowhere for DS to play except for a little strip out the front that he also couldn't play in because they wouldn't put in a gate.
I was told by the tenancy place that I could ask them not to put up the fence but it came under property improvements or something and I couldn't actually stop them. The best I could do was to negotiate hours with them so that the construction didn't interfere with my 'quiet enjoyment'.
They were also the kind of landlords that didn't bother to ask before getting tradies in - I came home one day to find my back door wide open and some electrician wandering around my house! And when the landlord arranged for someone to fit curtains on a day I was away and I asked him to reschedule for a time I could be home he yelled at me and did a huge burnout on the way out of the driveway lol