It would depend on the buyer and the perceived value of the yard. If you are getting a property ready for sale it's important like curb appeal but not as much. You don't want the state of the yard to take away from the home but don't spend 100K on a yard with waterfalls and rock walls without making improvements to the kitchen and bath, which was the case of a house on Laidley St. some years back.
On Potrero you can enhance the value with a nice usable space as you described especially if the access flows from the home as opposed to having to climb down stairs to get there.
As I try to put a quantitative number here to answer your question I find I canâ€™t without more info but Iâ€™m asking myself some questions you can answer yourself to arrive at the value. Does the view of the yard enhance the overall impression of the home? Does it expand the usable living space? Are there additional features in the yard, BBQ, hot tub, conversation area, view, etc.? Finally how does it compare to the competition on the market at the time of sale. As you answer all of these questions ask yourself, as if you were a buyer looking at the property, would I pay 10, 15, 25, 30 K more for the property if it had these features.
Most important is do it for your own enjoyment if youâ€™re not doing to sell and if you are doing it sell then do a nice cosmetic presentation donâ€™t over express your individuality.