Some advice on fixing my mess of a lawn

Well im new to the site so hello all.

Me and my SO recently moved to a new property and finally have a garden for us to work on and make nice, unfortunately said garden is a mess. Attached are 2 pictures of what it is like all over, the grass is pretty thick in places but the soil is just stones and what i can gather builders waste, its not deep either about a half inch maybe an inch at the deepest under the soil so scaring it and getting the moss off is just dragging out stones and making it worse. The last person who lived here apparently had the same problems of the water not draining off and the ground being soft (you leave pretty deep footprins) even in nice weather and refused to even try sorting it.

Does any one have any advice on what i can do to sort it? thanks all.