The flat is really nice, we enjoyed our stay very much. The flat is very well equiped, nice furniture, good stuff in the kitchen, comfortable beds, good showers, etc.
You cannot see the beach from the flat, but we liked the view from the terrace (top floor, exactly south facing) with parts of the city and the mountains in the background.
When you follow the street along the flat, go down some steps, one is at the concha beach within less than 10 Minutes. Also the shopping area of San Sebastian is just a few minutes walk, and a number of restaurants and bars opposite or round the corner.
We booked the private parking space, which is in the garage below the building, and there is some space there for bikes as well, which came very handy.
The communication with the agency was good, they usually answered quickly.
We would stay in the flat again.