I did a DIY kitchen in our basement using IKEA cabinets. I didn’t know to allow “Filler” space against walls so that doors would fully open. Consequences was handles hit adjacent walls and pullouts were not possible.

I didn't make this mistake, but many don't realize that drawer cabinets are a lot more useful/practical for storage than standard cabinet doors.

Most builder designed homes have standard cabinet doors in the kitchens, with very few drawers, because the doors are the cheaper option and maximizes their profit.

I didn't design or install my kitchen, but what I wish would have been done is to allow enough room between the uppers flanking the range area to allow for a range hood that is 6" wider than the range. As it stands, I'm stuck with a 30" hood fan, when in reality it would be far better to have a 36" hood fan over my 30" range.

I thought I was being sooo careful, and I worked with a talented designer. But there are 3 things I'd change:

1. I measured all my large pots, skillets, and baking sheets to make sure they'd fit. In my vertical storage, I forgot to account for the fact that I could store my baking sheets sideways because it's a deep cabinet. So that section, which is above my MW, is very tall, and the shelf above it is hard for me to reach (and I'm about 5'10".) That is the only fixed shelf in my kitchen and it's just sooo high. I don't use it much.

2. I wish I had planned for a pulldown over my fridge. That storage is big and roomy, but I need to stand on a chair to reach it. Yes, I could have a pulldown installed now, but my fridge is paneled and I'm worried that installing anything now might hit the cabinet panels. (I probably would have worried about that if I had it installed from the beginning, but I think at that point the cabinet maker could have advised me about options and they would have done the installation of the pulldown.)

3. I wish I had done a pullout or U shaped drawer under my main kitchen sink. It's kind of a black hole even though the space is useful. All my other lowers are drawers (except the cabinet under my coffee bar sink, but that's less important storage because it's not in the main part of the kitchen.)

I think #1 and 2 weren't noticeable to me during the design / plan phase because it was hard to visualize in the drawings. Our ceilings are 10' so the height of those shelves didn't stand out to me as being so high when I looked at the elevations.