The well is physically located on the subject property with the neighbor having an easement to use and maintain it??? Does the subject property have a 2nd well to service the subject house or is this 1 well servicing both houses???
If the well in question is physically located on the subject property, there is very little if any affect on the subject value - at least here in my market. If that well services both houses and problems arise where it needs repairs, who pays for fixing it? If the well only services the neighbor with the easement only being viable for access and maintenance, I still can't see any actual value issue - at least here in my market. Now, if the well goes dry or a new well needs to be drilled, can the neighbor do that on the easement that is located on the subject property or will they have to drill the new well on their own property? Does or did the neighbor pay for that easement and use?
Lots of questions to be answered and I probably haven't come up with all of them yet. I've done a few with shared wells and found little to no value issues - just potential legal issues that 'could' cost real dollars, but unlikely and how much is this potential going to impact typical potential buyers of the subject? As long as the well is supplying water to the subject house, most don't care... but, that does depend on the local area and how reliable water wells are in that particular area. Are there local issues regarding water available from wells? Are some wells drying up from local draught conditions making a good and deep water well a valuable commodity?
Lots of questions need to be answered, many very local in focus, before a value conclusion can be found.