There are some giant sized misconceptions about koi.
A lot of people think of koi when they think about a pond. A lot of those same people will go buy a plastic pond from Home Depot, throw it in the ground, and think that now they are ready for koi. Very bad move. Koi are very messy fish, and like to eat a lot. They need good filtration.
Our water gardens will always be built with enough filtration and circulation to accommodate several small Koi. But that is it. If you are looking to become a dedicated koi hobbyist, and would like to have a "Koi pond" built to accommodate a dozen or more Koi, then there are more requirements then just having a water garden with a skimmer box and a smaller biological filter. Otherwise you will end up with a lot more maintenance, and more than likely a murky pond that is hazardous to your Koi.
We can discuss in person what your unique desires are for your koi pond. Because of technological advances in the Koi pond industry there are many great options for properly building your Koi pond, depending on your budget.