In the UK, it is an implicit contract. Normally you have a written contract, or at least a verbal contract, but if that contractor just turns up at your doorstep by mistake instead of let's say your neighbours home, starts working, and you don't stop them, then you have an implicit contract and owe them money for the work.
This has actually happened when neighbour A called a landscaping company to landscape their garden while A went on holiday, the landscaping company went by mistake to neighbour B who didn't stop them, and did their garden. B had to pay. It would have been different if B had been on holiday as well.
