The sooner you ask the better.
It sounds like you are already at the offer stage, so it might be a little late. It is going to depend entirely on the company itself. Here are a couple of ways the company might see this issue:
They hire for the team. It's possible the company has considered you to be a good fit for a specific team, and there would be almost no translation to any other office.
In this case, they will probably not be able to do anything for you besides start you over in the interview process with the new location, with the equivalent of a good recommendation. (Maybe they could also skip phone interviews.)
Specific locations have specific hiring practices. Even though the company may hire more generally, they also might have different hiring practices in general locations, enough that one location is not willing to bring you on based on another's interview. In this case, the situation would probably be the same as the above.
Generalized hiring practices. If the company has really good universal hiring practices, you could be considered directly for another location. They would still, at a minimum, need to figure out whether the team has a need for people with your specific skillset. And, the compensation portion of your current offer may become useless (and they may not be too happy about that). (See below)
So how do you know which of the following types of companies this company fits into?
- Large office in Silicon Valley is a good start. That probably means they hire a lot.
- Research online. Find out how univeral the company's hiring practice.
- If you've asked about "internal mobility" at all, this is the best hint you can get. If the company is fully internally mobile — in other words, an internal transfer is just a discussion away — then you are possibly in luck — if those transfers don't require a manager's recommendation. But, if internal transfers require internal interviews and negotiations, you are probably out of luck.
With all of that said, I think "really good universal hiring practices" are highly unusual. I don't think you're going to find what you are hoping for, but I could be wrong.
Every multi-state company that I know of has different compensation scales for different regions. So, yes, the offer would be different, but that would be both in your favor and not — yes, it means you'd get more money, but it also means that if you already have an offer in hand, even if they are thrilled to have you consider their other location, they will have to go back through the salary step of the offer, which may be a negative.
If you are going to bring up your other offers, my personal opinion is that you are going to be in tricky territory, as you're already asking them to undo some of the work they've done in getting you the offer where you are at. I'm not sure I have any specific negotiation advice, but be careful not to overextend your ask.
