Yes, there are people on the internet who say that raising one pig is just fine, but I know from my herd that pigs have friends and buddies, and social groups, and given the choice, are almost never by themselves. They're always in a group on the pasture, doing something with some other pig or pigs. So my vote and strong recommendation is that you consider two pigs to be your minimum number.
How much space?
The real issue is smell. The more space you give your pigs, the less the pen will smell. two pigs will do very well in a 20x20 pen with a plywood shack or calf dome for shelter. In the winter it'll be pretty muddy, and if you allow them, in the summer it'll be muddy, too. Pigs use mud for sunscreen and temperature control and insect repellent and as a sauce for whatever it is they're eating. My pigs seem to think that everything tastes better with a coating of mud.
Raising pigs with the least amount of work
Your biggest daily issue is food and water. So arrange it so that your bulk food supplies (see "cost of raising pig" in this blog) are very close to your pen. So putting your pen next to a driveway or road or other easy vehicle accessible location is a good idea. With water, it's worth spending the time to buy a nipple (check with your local feed stores). A nipple allows you to put a hose to the edge of their pen, and the pigs can get clean water whenever they want. Pretty soon they'll learn to run the water through their mouths so that they can make the mud that is so necessary to their happiness, and that's a good thing. Summary: Spend some time thinking about food and water supplies, and put your pen and food storage close together.
With respect to food, it's worth considering buying a grain feeder. Local prices are about $200 in western Washington, but they last forever, keep the food in good condition, and mean that you only have to feed your pigs every third or fourth day instead of every day.
Zoning and land use regulations
Check with your local county agriculture folks to see what the zoning is. In King County, Washington you are required to locate pig pens more than 90' from all property boundaries. In Snohomish county it's 20'. So check your local county regs for any restrictions.
How much shelter?
Pigs need shade in the summer, and protection from wet and wind. A simple plywood roof will work great. If you enclose it on three sides it makes a great winter shelter. A few bales of hay and they'll be comfortable (and they'll eat some of the hay, too, making their meat sweeter in the process!) You can buy a pig shelter if you want, but for your first pigs, even the simplest shelter will work. Plan on them eating the shelter at some point. they'll chew on anything.
How much time?
If you put your pigs on full-feed -- allow them to eat as much as they want. you can expect to slaughter them 5-6 months after you take them home.
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