What height are Ikea base units? From recollection their doors are higher than the standard. Standard have 720mm base carcases with 715mm doors. If you have ‘standard’ sizes would that fit in under your window?

80mm is small. Too small to add anything to fill it other than a filler piece to fill the gap unless you can alter the size of an adjoining cabinet, make a towel carcase and rail near the sink and amend the run accordingly.

Independent kitchen suppliers can offer made to order size carcases and doors, so if you looked around you would be able to fill this with a different supplier.

My Ikea base units are 800mm high @Daisy

I agree with Sarah L, better to have a bit more "elbow room" by the wall next to the tall unit.

Hi- I have an IKEA kitchen and used a good independent fitter who was willing to work with me on creative “bespoke” changes to make use of every centimetre. Here is how I might put that space to good use for storage :

1. Consider your kitchen items that can be stored in a deep narrow space…. Rolling pins, baking sheets, platters, trays come to mind, but also cling film and foil boxes, Nespresso capsule boxes, a stick blender, a vacuum sealer, etc
2. Determine the best layout for shelves to suit..
3. Have your fitter drill shelf support holes at the required height on the sides of the adjacent cabinets before they put the cabinets in… they won’t be able to do this afterwards
4. Consider whether you want a door….if you do, get an 80 x 10cm drawer front from IKEA in the style of your selected doors. It is likely to be a plain slab, and you will turn it on it’s side to use as the door. Have your fitter cut it down to suit the space, and paint the cut edge in colour matched paint. The fitter can then attach it via hinges to the adjacent cabinet.

If you have a door, then you can buy Ikea’s normal 20cm wide x 60cm deep shelves and have the builder cut them down to size to fit.

If you don’t want a door, then buy matching 10cm x 60cm drawer fronts to use as shelves, and again have your builder cut them to size. The cut edge will not be visible so no need to paint, however you will likely want to either paint the white adjacent cabinet sides to match or perhaps cover them with an adhesive vinyl.

Good luck!

Pull out rail for a yea towel given it is next to the sink?