Mike - I kinda ride Wide instead of tall in the saddle myself, but if I have a horse big enough to carry me, he's too tall for me to get on.
In general, I might agree with the above folks, but here is my spin.
9 of 10 cases I get is a farm of 40 - 200 acres that only wants to take 10 acres and the house and outbuildings. Yes, almost any stable has a functional problem - lack of horsey people to buy them, except in real horse country. As a ten acre tract, I feel like the value is even less, as a 20 horse stable needs more than 10 acres unless you drylot the whole bunch.
Boeckh Cost guides are pretty good to calculate RCN. That is a starting point, not just for the property but for the comps. Even if you have to go back 5 years or more, find some horsey comps. Then using the correct multiplier for the date of sale, start abstracting. 2 assumptions. Land appraised as if vacant. House appraised as if no horsey barns are there..i.e.-is not impacted by the barns. If I have a sales price of $600,000, Land worth 100,000, house worth 350,000, then the barns contribute 150,000. I then attempt to allocate to those barns based upon the effective age, total life, and RCN. If you have as many as 3 or 4 such sales, a pattern emerges...The more buildings the lower per SF contribution allocated to non-wear and tear. i.e. - if there is a 30 x 40 building on the site, it exhibits very little functional obsolescence. If there are 2 hay barns and a 40 x 100 stable, then there is a lot...and if there is (like the last one we done) 20,000 SF indoor arena (18' wall, dirt floor, red iron construction); A 40 x 100 stable, (2) steel truss hay barns, AND a manf. home,, expect the Functional obsol. to be 2/3rd or more. Of the $250,000 the owner spent, we allocated less than 1/2 to these barns AND those sales I mention do suggest a marketing time of 1 year or more. There are simply going to be a dearth of buyers in addition to a high discount.
Ter