How can I congratulate you on handing over the house and getting the key?

You can say "Congratulations!" Give it to the person who delivered the key.

When someone gives a key, you can use "Congratulations" to express your congratulations. This is a polite and common expression, which can express your appreciation and blessing for the achievements or progress made by others.

Besides "congratulations", there are other more formal or solemn congratulations, such as "congratulations" and "congratulations".