Recently on Rain City Guide, a local area resident asked why we don't first add an additional 2X the land value and then value the structure value, when coming up with a listing or offer price.
In the post itself, I explained the 10% rule for extras, including extra land. That rule being that when you pull the nearby comps and value the property based on it being a "normal" sized lot for the neighborhhood, you can only add 10% of that value to the total to account for extra lot size, a pool, a tennis court and any other extras that don't value out such as additional square footage of the home, an addtional garage, bedroom or bathroom, etc.
Earlier today I tested my theory using a local neighborhood with wide variances between the lot sizes that replicate the 3X land question and also addressing whether or not the land is taxed on a price per square foot basis, or deemed an "extra" of little to no value by the tax assessor.
Turned out that the assessor gave even less value to the extra lot square footage than the market. The market allows 10%. The tax assessor gave little or no value to the extra land. See data and details in comment #35 of the above linked post.
I see that the resident who raised the question has come back in comment #36. I'll head back and see how he liked the proofs to my theory.