I really really think it would be worth getting in a good plumber who knows how to correctly do high loop installations. The Miele doesn't have a "built-in airgap". They use a high loop instead. Functionally, it's equivalent, but since it was installed with an airgap, you need to make sure the high loop installation is correct before you abandon the airgap. It's important for keeping crud from getting in.

watermelonprincess you say you "know Miele's have their own built in airgap and the recommend not installing one at the sink" but I doubt you could find that in a Miele user manual or even get that statement repeated to you on the phone by any Miele support person. In other words, it may be an urban legend, or a lie.

The loop of the hose is not rocket science. But based on your questions, I think you may need to hire a handyman.

In a kitchen design forum you will not find plumbers. You will also not find many people able to explain in precise terms what parts to buy and where to attach them. You could try again in the plumbing forum.

Miele user manuals are all available on the Miele web site. If you do the homework you will be able to quote what Miele writes about their drain hose.

We had the dishwasher installed by a "certified" miele plumber. They put a hose extender on the existing miele drain hose that is 5/8" which from what I understand is incorrect. This hose that is 5/8" attaches to the airgap and then there is a hose from the airgap to the sink drain that is 7/8". Shouldn't the hose extender also be 7/8"?

I've seen the language in the Miele manual mentioning air gaps. The language artfully dodges around the question instead of saying that an air gap or high loop is not necessary.

I've seen discussions about malfunctioning Miele DW's one way check valve.

So, a high loop is a very good thing to have. It costs nothing. It is the same hose configured to go high and low, in a loop.

Oh! Thanks Davidro1! I guess that's why I absorbed that you need a high loop with a Miele! Sorry, Watermelon, for not getting that quite right, and thanks to Antss for the correction.

California is big state with a large populace and many, many governing bodies. Some locales and inspectors are amenable to other notions. Some absolutely require what is absolutely in the book. I ran into both with the same inspector. There was part of the plumbing that was done most neatly and correctly by a master plumber, which he required be redone to be actually less well (though still perfectly functionally) installed, just to fit with his notions, but he didn't mind that the required lightswitch by the patio door was inside of a cabinet (which would have changed into a bookcase for awhile otherwise). You can't know. Some people paste on an airgap cover with plasticine and never get caught. Others do as Watermelonprincess intends and put the airgap in for the inspectors delectation and change them out later. I had a spare hole in my sink deck anyway, so just had them put in the airgap rather than a high loop. I also figure that the airgap gives one some warning that there's a blockage about to create a big problem. :)

yes, it is a good place and time to raise this, plllog. An air gap gives a better warning signal than a high loop. If any drain blockage occurred (or backup), the dishwasher is the first place where contaminated water would pool, unless there's a hose high-looped or a physical air gap. For the sake of brevity I'll leave out many of the potentially lethal consequences and possible scenarios. In other places on the internet, you can read a few sentences that deal with unlikely occurrences, that might shock you. A cheap valve in a Miele dishwasher will not prevent contamination from seeping in, if things back up. A hose going straight from the dishwasher floor to the sink drain connection would be the first place for contaminated water to flow through, (because it is the lowest point), but not if your hose were raised to "loop to go high" (because it is no longer the lowest point). The hose being raised is a physical separation. That AND a flapper valve are good to have, as a team of two safeguards, far more guaranteed to do the job than a flapper valve alone. When calamities happen, you want your drain system _not_ to be a vector carrying bacteria to your plates cutlery and glasses. When pandemics occur, the community's common sewer and drain systems carry bacteria to new households. After you go away on vacation, when you come back your dishwasher might look clean but be contaminated : between the high loop method and the air gap method, the latter will give you more warning signals. Other advantages apply. So, in the many most-unlikely scenarios, air gaps are better.

Just thought I would post a follow-up since I called Miele technical support. They stated that we could plumb the dishwasher directly to the sink drain and there is no need to put in an air gap or a high loop because of the check valve that the dishwasher has. I got this statement directly from the technician but I don't think a high loop is a bad idea. It is another layer of protection.

I know nothing about codes/ air gaps etc. I do know I live in Califoria and have had a Miele dishwasher installed for over 11 years. I never asked to have the air gap removed but I believe it was a Miele technician who just took it out saying I don't need it (I think I needed a hose replaced or something.) Best dishwasher I've ever had BTW
Living dangerously in California

"For the sake of brevity I'll leave out many of the potentially lethal consequences and possible scenarios."

Do find some.
With good documentation, nit "i heard..." and a chain that never ends, or even runs in circles with no real data.

Every time I have poked at these terrible reports they have turned out to be unfounded.

"I heard" of one that involved a black water backup through a high loop that then drained away and left no signs (anyone that has seen a black water backup is already rolling their eyes here).

Without any hunting at all I came across a thread about air gaps this evening.

The last line as of now is that a check valve "... will not provide for backflow protection or prevent cross contamination. "
http://www.terrylove.com/forums/showthread.php?45270-Air-Gap-Overflows

Here is a link that might be useful: Air-Gap-Overflows at terrylove plumber forums