A common question: do clients need to give permission to advertise their site? The basic answer is no; However, there is a bigger picture that is important to understand. Let’s walk through the basic two steps of setting up a new client for Google AdWords:

AGREEMENT

Sometimes consultants getting starting say their agreement verbally or through email; we advise putting it in a written agreement. This doesn’t mean investing in a huge legal document, but having a signed document talking about the service you’re providing, the goals, intentions and limitations is best. Discuss what happens if it doesn’t work? Can they ask you for the money back? Put it in writing and have a signed document to ensure everything is clear upfront. As your business grows and expands, we advise investing in a proper legal documentation.
As budget is something that can change, we suggest avoiding including it in the original agreement; However, you should still have it written in an email as confirmation.

BILLING INFO IN ACCOUNT

If this account is theirs and they have access to it, then their billing info should be in their account. Usually, clients pay for their own advertising accounts so once the billing is in the account, they are agreeing to paying for it, which backs up the agreement.
Those are the two most basic elements of getting up a client for Google AdWords. Going back to the original question; technically, you can open a site tomorrow and start advertising for it without consent. Google won’t stop you as it doesn’t know what happens outside of AdWords and won’t know if you and the website have an advertising agreement or not; they assume it’s legit; However, if you’re doing something wrong or shady, Google has access to the billing address and credit card information, so getting banned from Google advertising altogether is a possibility. Have an agreement and an email confirming the budget to avoid these complications.