Ownership of a website is more complicated in practice than it appears – paying the invoice does not automatically mean the website and its code belong to you. Copyright in the work arises with the programmer and passes to the client only to the extent set out in the contract. If the contract is silent on this, you may end up with a licence to use it, not the right to modify the code, transfer it to another supplier, or use it for a further project. The contract should therefore expressly specify the scope of the licence or the transfer of economic rights, the right to modify the work, handover of the source code and database in a usable form, access to the hosting and to a domain registered under your own company ID, and technical documentation. Pay particular attention to a website built on a supplier's proprietary platform – it often remains that supplier's property and you are effectively paying rent. Consult a lawyer on the specific wording of the contract.
See also: Ownership of advertising accounts, Digital Assets of a Company, Website migration without losing rankings.