Re: Neighbor's Treehouse on Our Property
You really need a surveyor to determine that line for you. We have no idea whether the points you found are actually those shown on the maps or how skillfull you are at measuring. You may have the correct points and may measure well, or you may not. A licensed surveyor is trained at both and licensed to determine your boundary location. Any assertions by you based on your own measurements, no offense intended, will likely be met with skepticism if not dismissed out of hand. If you have a survey map in hand and stakes by a surveyor in the ground, you have something much more credible, leaving the neighbor little room for disagreement without them hiring a surveyor to verify your surveyor's results. Short of having the survey, you could approach the neighbor with your suspicion that the tree house and junk pile may be on your property and you would like them, at a minimum to remove the junk pile. But until you have the survey, you only have a suspicion of that fact.
On old subdivision maps, those prior to WWII or so, it was common to not indicate any points set at lot corners, often showing only a few major monuments at exterior subdivision corners and sometimes a few locations such as street intersection points within the subdivision. But it was also common to set redwood hubs (stakes which were typically 2"x2" by 6" to 24" long) at all lot corners even though the map did not indicate any points set. The redwood hubs were typically driven to be flush or nearly so with the ground. In some subdivisions, redwood hubs are found even 100 years or more after being set. If your map was filed in the 1920s, if redwood hubs were set, and if any still remain, they would likely be difficult to identify to the untrained eye due to 80+ years of decomposition.
Since there are fairly recent lot surveys in the area, there is probably sufficient original controlling monumentation, and perhaps original lot corners in place for a surveyor to accomplish your survey without undue difficulty. In describing the maps, your mixing terminology, so I don't know for certain which form of map the 1984 & 1994 lot surveys are depicted on. If the maps are 18"x26", they should be titled Record of Survey. If they are 8 1/2" x 11" (or 8 1/2" x 14" more likely for that time period), then they should be titled Corner Record. If they are Records of Surveys (larger map), that most likely means that one of the statutory triggers was encountered during the surveys - more work, more money. If they are Corner Records, that's good news. That means that everything seems to fit reasonably well as compared to the Subdivision Map. It also normally means that less initial effort is involved for the surveyor and so costs can be kept lower.
With encroaching uses of land by neighboring landowners, the biggest concern is usually that the landowner holding written title to the property being used may lose it to the neighbor using the land through the doctrine of Adverse Possession. In California, because of the requirement of paying taxes on the disputed land and because of how County Assessors normally assess the taxes, a claim of Adverse Possession within a platted subdivision such as yours is rarely succesful (almost, but not quite, never succesful). Nevertheless, it is a good idea to get the survey as soon as you can afford to do so, have the surveyor set a stake or two along the property boundary near the treehouse, and bring the matter to your neighbor's attention, requesting removal of the treehouse. If you cannot afford the survey right away, it is still a good idea to notice the neighbor of your suspicion (strong suspicion if you like) that their treehouse is on your lot and that once you are able to verify it by survey, you would like them to remove it.
I hope it works out for you.
I'm a surveyor, not your surveyor & not an attorney.
Advice is general survey, not legal. Hire a local professional for specific advice.