When someone builds a website for you, make sure you retain ownership of the source code and anything else that is required to make your website work.
The reason this is important is that if you tie yourself to just one provider you will no longer be able to ensure you are getting the best price or service and if you ever want to move to another provider you will have to start again from scratch.
One way to ensure you have the freedom to move is to make it a requirement that all aspects of your website are built on open source applications. Open source applications are widely available and, in many, cases provide superior performance over proprietary solutions.
Ask your web developer for a full copy of the source code for your website along with any working files, a snapshot of the database (if any) and any other code that may be required to run and maintain your website. If they are reluctant or will only provide a 'dumbed-down' version of your website you need to ask yourself if this is the type of long-term business relationship you need.
Consider also whether you would be happy to buy a car from a manufacturer that didn't allow you to buy fuel from any petrol station - the analogy holds for a web developer who forces you to host your website with them.