How to add height to a kitchen table to turn into an island

To support the extra instability of the raised legs, and the extra weight of the overhanging countertop, a base will have to be built, and the legs firmly anchored to the base.

As well, some kind of connecting supports will be needed. These can be in the form of either cross bars, or walls.

You will need the right tools to ensure that the base is absolutely level, and the pieces are cut at squared 45 degree angles. If not, the new "island" will not sit right and will be wobbily.

Can you have new legs made? Maybe use spindle posts cut to size or use hairpin legs?

I would use new legs to raise the height. Check Home Depot, Lowe's or even a local lumber yard to see what they have to offer. Take pictures and ask for advice.

There are many examples of add on legs on Ebay, Amazon, and local box stores for home improvement.

You could raise the island or table by taking off the original legs,

adding a shelf or another table the same size upside down , and paint or stain to match your cabinets. You could use the extra shelf for small appliances.

I would put my toaster, coffee maker, mixer, anything that would free up counter space.

Build a base for the table top and attach the table top to it, you'll get more storage that way. Save the legs for another project.

This is how we turned our kitchen table into an island in one afternoon. https://www.exquisitelyunremarkable.com/2013/06/decorating-cheats-farmhouse-table.html