I'm here in MA and Yes, it is common to seek a financing extension ... of course depending on if the original time was not adequate. They key to you as a buyer is to make sure you have a very clear financing commitment before letting your protection lapse... any bank will provide a commit with a million exemptions and caveats... you want to take the time they need to issue a clean commitment or even better a clear to close, before you risk your deposit.
It's not uncommon anywhere. Many lenders are swamped with the low rates. Others are overwhelmed with short sales and foreclosures. You don't mention why you are requesting an extension, but if it's for financing itâ€™s not uncommon.
If there are other reasons, you can ask the seller for one, but they may request you do something for them in return. This could be a release of earnest money or paying the seller's daily carrying costs.
Remember the seller is trying to make plans too. They may need the proceeds of their sale to purchase their next house; your extension will translate to them needing one and perhaps their seller's too.