Learn English – Can we say “Yes, you could” when someone asks “Could I wear jeans at work?”

Before addressing your questions specifically, let me digress briefly.

There have been lots of debates, here and elsewhere, on the use of may versus can.

While may is used in the sense of asking for permission, part of the problem with comparing it to can is that can has two different common senses.

Can I do this?

1. Yes, you are allowed to do that.
2. Yes, you are able to do that.

In the one sense, can is also about asking permission. But in the other sense, it's about determining the capability of doing something.

Now for your question:

Could I wear jeans to work?

Similarly to can, there is an ambiguous meaning—but it's slightly different.

No, I can't let you do that.
Yes, you could. But it's against the rules.
Sure, that's not a problem. Well, you could. But I'm not sure if you should.

Generally speaking, the final sense (one of recommendation or suggestion) isn't used with may or can—which are only about yes or no. But with could, questions and answers can have this additional nuance.

Also, we normally qualify the questions we ask when we use could:

Could I wear jeans to work if I wanted to?

This means you're implicitly saying that you're able to wear jeans to work but what you're really after is if it's permissible to do so or not.

Could I wear these skinny jeans to work if I went on a diet?

This means you're implicitly saying you have difficulty physically fitting into the jeans, and are asking not for permission but for the likelihood of changing that situation with certain behaviour.

 . . . it breaks the law that we can not use "could" to give permission . . .

There is no such "law" that I'm aware of, and there are certainly no guidelines of syntax or semantics against it.

"May I be allowed in?"
"No, you need to wait at the end of the line."
"What if I gave you fifty dollars? Could I be allowed in then?"
"Why, yes, you could be."

As I mention, could is less of a black-and-white word than may or can, but it still has those meanings as part of it—especially if questions are qualified in order to make one of those meanings more explicit.

Also, while may and can are used to express something about the present, could is used to express something about the future (or the past when it's used in the negative).

Update: It's true that in formal English, there is guidance against using the word could for giving permission. In a blog post, Oxford Dictionaries says that "when giving (or refusing) permission, only can (or can’t) and may (or may not) are acceptable."

But idiomatically, in informal conversation, I believe it is something that is used and taken to be meant as a way of expressing permission in the right context.

Consider the following sets of dialogue:

"Could I please take this pen?"
"Yes, you could."

"Could I please take this pen?"
"Yes, you may."

The second may be the formally correct way of expressing permission, but the first seems at least as natural an exchange—if not more so because people have a tendency when they repeat something back to somebody to repeat it back just as it was originally said rather than to change something.