If it is NYC it will be worth millions, and if it is in Antartica not so much.
I would contact a local real estate professional to help with this answer and to save time and money.
(I'm on the Cape, but I can help you choose a local pro if you want help doing so.)
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Would you treat it as a single family home, a 2 family or an investment property as all would come into play as how the you and the appraiser would look at it. Now forget how you would look at it, how would your underwriters look at it? Although that may not matter based on the qualifications and proposed use by the prospective new buyer.
You are also wrong from an appraisal standpoint the land could contribute a great deal to value, even without it being sub dividable.
As an appraiser, I would have to determine if as you say is the land sub dividable and what is the highest and best use of the property. I would also have to see if how comp sales are in relation to lot size, if homes on an acre sell for x and homes on 5 acres sell for y and homes on 10 acres sell for z.
I would also have to look at how much rent the rental provides and is it zoned for a second and or rental property.
Not as simple as you suggest.
The most important factor besides location is the gross living area of the home, type of home, bedrooms / baths, as well as the condition. From a lending stand point, the land land will not have as much weight given to it unless it's sub-dividable and build-able, but that type of appraisal approach would be different from a regular residential appraisal. It's not that a buyer wouldn't see value in the land, but strictly from an appraisal perspective, the home size will have much more weight than the land size in most cases. I would need to know more about the home size, condition and area of town to do the proper work need to come up with an approximate value.
If you have access to MLS and can search sold homes over the last year, that will give you a range of value. If you don't have access and would like access, I can refer you to one of the listing agents that we deal with
I have some really good local real estate agents who send their buyers to us for home loans. If you are thinking of selling, I'd be happy to have an experienced agent contact you so you can get top dollar for your home.
Good Luck and Feel free to contact me for a referral or if you have further questions.
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Your property sounds amazing! I would love the opportunity to come out and give you my opinion of value!
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If you want a rough estimate based on unsupported data, you could use Zillow.com
Otherwise I recommend that you have two to three top agents in your market do a market analysis and also you can view homes in your market that are on the market with similar qualities to yours, although it may be difficult to find others with rental property also.
Last approach would be to hire a certified appraiser that will cost you between 350 - 550