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.
I have clients who purchased a house with a huge yard a couple years ago in Miraloma Park. The yard was the size of two lots. They spent a very large amount of money completely transforming their yard, and I have to believe they could get back a sizeable amount on that investment.
One more thing: I think a yard on a single-family lot will be worth more than one in a condo/shared yard scenario. So if you have a house in Potrero, I'd recommend sprucing up the yard a bit for the ultimate appeal. There's a reason I've recommended that sellers call in gardeners/landscapers when we've gotten their house ready to sell. As for a value on landscaping--that will vary and there's really no data to support a dollar figure, unfortunately.
The landscaped yard will make your property stand out, but I don't think it would add much "value". I suppose the possible return for your investment is that your home may sell more quickly if it has an appealing yard. Good luck.