Whenever you subdivide land (or do any major construction project, public or private, for that matter), the planners place a lot of conditions on your subdivision. One of the conditions is dedicating a portion of an area as “open space”. Open space serves many functions, mostly as a buffer so that San Diego preserves the look of San Diego, as opposed to Los Angeles where it’s just non-stop houses and buildings and no greenery or natural habitat in many places. Other primary uses for open space is to serve as a habitat preserve for wetlands and like Rustico says, a place to put loss mitigation for large projects where you can shift your open space requirements to a better more manageable place. Other open space areas serve as a animal traffic corridor for species migration. Also, some open space may be dedicated in order to fulfill environmental requirements within a subdivision and supervised by the planners in that particular area. Open space can also serve as a fire buffer, so that the fires in an area do not touch the houses (or at least a sufficient distance away to minimize damages). Of course, there is open space that is donated by rich people, and some companies also donate open space in order to get more favorable treatment from the counties. One particular open space I helped set up in Riverside helped this company save a few taxes on basically undevelopable land. So there are quite a few uses for it.
Speaking as someone who works with the planners on a regular basis, it is very difficult for a private person or corporation to re-assign open space for another function (of course, for public entities, they do whatever they want, but it is still a long and arduous process, as it should be). Getting an open space restriction removed requires an application, environmental impact statement, FEES, public hearings, yada yada. So pursuing such an action is usually a waste of time. There are better areas to pursue in terms of what you want to do.
For functional purposes, open space is basically an area that you should not be messing with. You are not allowed to grade or disturb the area. You can do things like hike through it, maybe do some walking through it, but it is supposed to be undamaged and pristine and remain in its original condition. Some open space areas get incorporated into the trail system in San Diego. If you are caught grading open space (as has happened to this one major property owner in Lakeside), the county will sue you and force you to change it back at your expense and will charge you the fees and penalties as well.
If you have more specific questions about open space, please post it. I do not purport myself to be a planner or an expert in open space, but these are a few of my observations and experiences.
