Value of buying next door land?

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I'm renovating a 1930 Craftsman bungalow(BRRR) with a ARV of 200k, about 15 miles from Chapel Hill , NC(1.1acre). The next door land (1.87acre) has come up at tax auction, I believe I can get it for 15-20k. I'd like to own it to keep out a Mcmansion or trailer next door and preserve the old-growth cove my house sits in. Do you think this purchase would add value Beyond what I pay for it? Is there an equation or rule of thumb for figuring out the added value?