Planning Where to Put Your Koi Pond
1) How much sun will the pond get?
If you want waterlilies or other aquatic plants your koi pond will need at least six hours of direct sunlight. If your koi pond is going to be in direct, intense sunlight all day, your water may need extra oxygenation. Intense sun could also become a problem if your koi pond is very shallow or is smaller than 1000 gallons. But both those situations would be bad for your koi for many other reasons, so you wouldn't be doing that anyway.
Ideally, letting the pond get a bit of shade in the afternoon is a good compromise - then there's enough light for plants, but your fish are protected from the most intense afternoon sun.
2) How close is the koi pond to your house, or where you'll be spending time outside.
Site your koi pond where you can comfortably spend some time with it. After all, there's a bit of work to having a nice koi pond - you'll certainly want to enjoy the fruits of your labors. This means putting a koi pond at the far end of the garden (where you hardly ever go) may not be a great idea. Putting the pond next to your patio, or even at the entrance to your house will assure that you get to see it all the time. This is also good for pond maintenance - if you see the pond up close at least once a day, you'll identify problems before they become serious.
Another benefit to siting the pond very close to your house is the sound of running water. A pond with even a small waterfall within ten to twenty feet of your house windows will fill your house with the soothing sound of running water throughout the warmer months. It's a wonderful sound to fall asleep to, and could help create “white noise” if you have a small noise pollution problem.
3) Will the pond be under trees?
In most places in the US, trees drop all their leaves in the fall. And even if you're among the lucky few living in a place where there is no cold weather, your trees are going to drop their leaves, and their fruit, throughout the year, too. Having a koi pond under a tree can be done, but you'll substantially add to your pond maintenance - definitely consider buying a pond vacuum.
The other serious issue with having a pond close to a tree is roots. It is inevitable that the roots of a large tree are going to create issues with a pond dug too close. If at all possible, at least site the pond outside of the tree's dripline (the outside “ring” of the leaves).
Even having the pond close to a tree can increase the maintenance. Winds blow, and leaves drift. Still, a few leaves in the pond is hardly a disaster. If you haven't overstocked your pond and have a good filtration system, a few extra leaves are no big deal.
4) Unforeseen situations.
It's a good idea to go out and sit in the spot you want your pond to be, and try to think of every possible thing that could go wrong with having the pond in that spot. Be creative… life is.
Don't put the pond in a place where the water from your roof might run off. Don't put the pond right up next to your neighbor's fence, where their vigorously digging dog might decide to dig until he reached your pond liner. Don't put the pond under an electrical wire where the birds like to sit.
And of course, if you have small children, your pond needs a fence around it, and the latch should be absolutely unopenable by little hands. For very small children who tend to wander, even having two sets of fences is not unreasonable.
Now, after all that worst-case scenario thinking, relax. You've put your koi pond site through the wringer, and you've eliminated a dozen serious problems and saved yourself a ton of work with just a little bit of smart planning.