looking for the cheapest way to build just a one 10X12 horse stall l have a mini and a qh the mini has a barn and my qh has one but his keeps getting flooded so bad

l would say, just build a three sided shelter and put a gate on the open side.

Well if u can put it on higher gound that would help. if u can, get a yard+ of sand/fill, the kind that does not get mushy when wet & spread it 12x14. then l would get stone dust, about the same or just a yard might do. stamp it down, & place shed on top. building ur stall/shed with pressure treated lumber at the base will help it from rotting, & l would use plywood for the shell. l would use the half inch thick 4x6' rubber mats for the floor on top of the stone dust, make sure to cut them to fit tight & so they do not overlap. l would also make the roof hangs over sides by a foot so water dripping off roof does not errode the footing around the outside away. u can go to a lumber mill & get rough cut planks for the inside for l use 1x8'' & they work good, use screws tho, it will hold tighter & if u have to replace one its easier to get off than nails.
The other thought if u want is to put down stone dustas a base, pour a 5 to 6''concrete slab just slightly bigger than the shed base & then put the shed on top of that with the rubber mats inside. make sure to put a slant in front of the door as to not make a step issue for the horse, like avoiding chipped hooves. good luck, too much water & mud is a pain with horses, l hate it

l know this isnt practical or helpful, but a new mini arrived at my stables called ''womble'' and with his previous owner he used to live in a windy house! so sweet, even though im sure it wasnt the best for him.

is there no other stall free? if not you could just let them live out? l dont know where you live but alot of horses are quite happy outside aslong as they have some sort of shelter
good luck x

Unless you want like a tack room or other things like that, I'd just do what the first poster said and build a run in shelter that you can close in with a gate if needed. A decent sized shelter might only cost you $1500, which is pretty cheap relative to an actual barn.

You can probably work on the ground too in order to help it from getting too wet.

l would say if you use ply wood for the sides and you might be able to find some used tin for the roof. You can get 2x4 is pretty cheap.
Or if you can get a roof up, you can use a tarp for the sides if you need to.

My family recently built a barn/ chicken coop out of a carport. No lie. Carports r extremely popular in my area to use as livestock shelters. Ours has 2 sides & the roof in one solid unit with the front & back open. So, we walled up the back with boards & put in a door, then we have a four foot wide tack room that is connected to the chicken coop to one side & a gate set up on the front. The nice thing about the carport is u have a shell & can stick whatever you'd like in it :) We spread sawdust on the floor & change the dirty out with clean, but if u use sawdust do not use the exetremely fine kind or black walnut (it will mess them up really bad.) My horses do not usually live in our barn, we keep them there if necessary, but they run around the field like they like to & come in for shelter.
Best of luck!