Overview

What is an Island Check?

In short, it is any location disconnected topologically.

In or around a house, it is any element or area somehow cut off from a supply of water or electricity, or other feeds or outlets one would want: telephone, speakers, sewer, etc.

Islands can be an area on the floor, on a wall, or even on the ceiling. But in general, an island is a place cut off from services, feeds, etc.

Islands can also be odd spaces between other spaces, for example:

Hallway wall between two doors - Perhaps you want a socket here for a table lamp. Imagine placing a half-round table between those spaces. On it you want a lamp. Great, but is there a socket there? There might be code (2m/6ft from an opening), but that does not ensure a socket where you need it.
A pillar or even a mailbox - Many separated features are afterthoughts, but you might eventually want light, a camera, or some such.
Planter - Or, for example, in a circular driveway, the centre area, sometimes just a planter when small, might be devoid of sprinklers, lighting, etc.; it is an island. A planter out in the middle of nowhere: one needs to consider how water gets to the planter, but also how electricity gets there. And perhaps how water gets out of the planter (proper drainage). In fact, in general with planters and walkways, it is always best to just bury a large-diameter plastic pipe, which can be used to retrofit future runs of wire for future projects.
Kitchen Island - Does it already have an electrical feed? What about hot and cold water, and thus, a drain line? What about a gas line? If you wish to update this island to have a sink or a stove, these would be required. A nice trick I do with my kitchen Islands is to run a dedicated filtered water line from the Island to the fridge. Now I have a single filter cartridge under the Island sink for drinking water, and this also serves the fridge. More so, I ran that line from the back of the fridge upstairs to a small sink in a bedroom for local clean water.
Under and inside cabinets, and shelves - this is an often-forgotten Island condition. Mostly because Architects fail to consider decor/furniture as part of the house (amazingly). Thinking in terms of Islands mitigates this. Having outlets under all kitchen and bathroom cabinets (above and below) ensures easy access for lights, fans, water picks, blow dryers, and any device that requires charging (shavers, kitchen stirring devices, etc.).
Shed - at the back of your house. How wonderful to always have power, and perhaps water to wash your hands, or go a step further and install a toilet for yourself, or for workers.

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