Buying a House with Extension that doesnt have permit

[Mitoshi Fubari]

Hi,  
I'm planning to buy a house that was listed for $800k, after dropping from $822k after being on the market for two-ish months.

I got my offer accepted for $781k right away, no counter offer.

Then the listing agent disclosed that the extension in the back is slightly bigger than allowed, its 70 sq ft, but I dont know what the allowed amount is. The extension room consists of small room that has 2 washer and dryer machines, and another bathroom.

He said the house was bought by the seller like this, and its probably been there since 2010 or earlier. And the seller will be keeping the extension as-is, and will not remove it. The agent said not to worry about it since it's been there for so many years, the city wont care, and the seller bought it like this, so its obviously fine.
But obviously I cant take the seller agent at their word.

I checked the Department of Buildings, there is no permit done for this extension, and there's no violations on the property.
Does anyone know if I'd receive a fine, or be forced to remove the extension?

So I want to ask for advice on what the risks are for proceeding with the purchase, and what you guys suggest I do to get the best out of this situation, whether its renegotiating the price or something else? I just don't want a situation later where the city inspects or something, and either forces me to get expensive permits, or to take the whole thing down.

Thanks!