Can a neighbor claim ownership of a parcel of land by simply maintaining the property?

In NY an adjacent property owner may assert a claim of ownership based upon adverse possession. Such a claim can be raised by the neighbor by commencing an action, or in a counterclaim, if the property owner initiates an action against the neighbor (which is often based upon encroachment or ejectment).

Prior to July, 2008, the NY law governing adverse possession claims was less stringent than the amended adverse possession law enacted in July, 2008.

Generally, a claimant asserting adverse possession must show that their occupancy or possession has been adverse, under claim of right, open and notorious, continuous, exclusive, and actual, for the statutory period of 10 years. The burden of proof is on the adjacent property owner to establish these required elements to the satisfaction of the court. 

Effective July, 2008, the amended law precludes a claim of adverse possession based upon fences, hedges, shrubbery, plantings, sheds and non-structural walls, or other "de minimus" non-structural encroachments. The amended law deems "de minimus" non-structural encroachments to be permissive and non-adverse. The July, 2008 amended NY law, effectively precludes many adverse possession claims that are based upon these "de minimus" non-structural encroachments. Additionally, the new law provides that: "the acts of lawn mowing or similar  maintenance  across the boundary line of an adjoining landowner's property shall be deemed permissive and non-adverse."

To constitute adverse possession under the July, 2008 amended NY law, 2008 there must "...have been acts sufficiently open to put a reasonably diligent owner on notice", or where the land "...has been protected by a substantial enclosure".   

However, it may be possible for a neighbor to assert a claim for adverse possession, based upon a "de minimus" non-structural encroachment, where the occupancy or possession has been adverse, under claim of right, open and notorious, continuous, exclusive, and actual, for the statutory period of 10 years, if the neighbor is able to prove that the 10 year statutory period was satisfied prior to July 2008 (when the amended NY law first went into effect).

This is an admittedly complex area of real estate litigation. Hopefully this information provided you with a useful general overview.