Re: Rain Gutters on My Property
In a zero lot line subdivision, such easements almost always exist. The subdivision map may have a full description of the purpose of the easements, but you may have to look to the Conditions, covenants & Restrictions (CC&Rs) document which should be filed at the County Recorder's office. The easement may be for maintenance access only, or may have additional designated uses such as for drainage.
A drainage easement is typically for surface water that travels over the surface to some manner of channel within the easement. Sometimes that will allow the adjoining lot owner to pipe water to the easement, sometimes not. I can well imagine that in such tight confines, the site engineering was such that it accounted for roof/surface runoff from the adjacent lot into the subsurface drainage piping. It would not surprise me at all that the terms of the easement specifically allow for a vertical drain pipe as you describe. Nor would it surprise me if such use were specifically not allowed. The only way to be certain is to get the document or documents that created the easement and address its intended use(s).
Your deed should have a reference to the Book (or Volume) and Page at which the map is filed. The map may have a recording reference to the CCR's if they were filed concurrently, and may also have references to easement deeds if any pertain to private or public agency utilities such as PG&E or the local Utility Services District. If the notations on the map sheets don't fully describe the purposes of the easements, there should be a reference to whatever document does.
You get the copies of maps (make sure you get the title sheet as well as the map sheets) and other recorded documents from the County recorder's Office. The purposes of easements dedicated on the map are generally described in the Owner's Statement on the Title Sheet.
I'm a surveyor, not your surveyor & not an attorney.
Advice is general survey, not legal. Hire a local professional for specific advice.