Former commercial appraiser here. I never did single family homes but I know the basics. My first thought when looking at a property with four homes is that an investor is the most likely buyer. Not a lot of owner-occupants looking to buy four houses on a five-acre property unless it's going to be a family compound. If you're in a rural area, rental data and investment sales are probably going to be hard to come by so the sale of a 4-plex on a typical lot isn't completely unreasonable.
The value of a half-acre of grape vines to an investor is basically zero. To an owner-occupant, it's probably worth something, but not much. If you've got a lot of unique amenities it's going to be very hard to determine the value of those from the market. Compare 10 similar houses that sold with pools to 10 that sold without and you can extract a range of values for how much a pool is worth. If you've got a pool, a huge outdoor kitchen, a shop building, etc. it becomes much harder to extract the value of all those extra features. Also, a buyer make look at that whole package of amenities and decide it's only worth a fraction of what they would cost to build because they only care about the shop building but not the pool or outdoor kitchen.
All that being said, contact your lender and point out what you think are errors and if possible, provide better data (try those Realtors who said it's worth $900k). That means closed sales of similar properties, i.e. rural properties with multiple homes. Don't try to contact the appraiser. You are not their client, the lender is. They have no relationship with you and have a duty to maintain confidentiality with their client. The appraiser may have done a shitty job, but it sounds like a fairly difficult property to appraise and the bank may have just hired the cheapest fastest appraiser out their with no regard for their experience.
Hmm, thanks for the advice. I guess it makes sense but is unfortunate. We'll probably just sell the place at this point. A lot of things don't make sense when valuing the property as an investment as well. For instance, one of the "comps" they pulled was a triplex with three 2/2 units. Ours is a four unit property with three 2/1s and one 3/2. The appraiser added $6k for each of the three units in the triplex because they have an extra bathroom, but only subtracted $8,500 for the lack of extra rental unit! Clearly an extra house that rents for $1200/month has to be worth at least $155,000 due to his own monthy rent multiplier calculation.
It's going to come down to whether or not you want to take on this fight.
Either the appraiser is off, or all of your realtors are off, or both.
Regardless, what matters is if you want to go through the energy of trying to make your case to the bank. This sounds like one of those persistence issues where you have to work your way through the resistance of the system to get a human being to make an exception for you.
I've done it a few times with banks when I was in an ongoing situation that was unreasonable. Every time it's been a case of an "I" being dotted and a "t" being crossed, and there's no mechanism in the system to fix it. Every time I've had to escalate it to the VP level where they, a human being, assess if what I've said makes sense, and then they override the system and make a big, loud deal about how much of an exception they're making for me.
Is your appraisal unreasonable? Yeah, sounds like it
Can you prove it definitively? Perhaps not
Can you generate compelling evidence? Yeah, sounds like it
Is it worth the hassle of butting up against a bureaucratic behemoth to get a chance to make your case based on your compelling, but not definitive evidence?
Hmm...that's up to you. Sounds like it might not be.
I wish I could tell you a simple approach to getting a thing like this dealt with, but my experience tells me that it doesn't exist.