To install a kitchen island with a granite countertop you may or may not need a professional; it depends on the model you purchase. Mobile kitchen islands or flat-pack models can be assembled DIY-style, as they are lightweight and typically come with all the tools and hardware you need.
However, larger kitchen islands may require electrical hookups and plumbing for under counter fridges, sinks, and dishwashers, which need a professional contractor.
Some kitchen island warranties are also void unless the unit is installed by a professional. Always check the manufacturer’s warranty before attempting to install a kitchen island with a granite countertop yourself.