Best/cheapest way to find out size of buried drain pipe

Hi there --
I want to put a second bathroom in my home. The location already has plumbing and a drain as it used to be where the washer and dryer were located.
The drain pipe goes from the spot where I want to put the bathroom down into the garage and then into the concrete floor of the garage. From there, it goes underground about 75 feet to the main drainage line at the street.
I've discussed this with a contractor (not a plumber, but he also does plumbing), and he says that the drain pipe that is visible is too small for toilet drainage, though it is fine for shower, sink, and bath drainage, and that the only way he can tell if the main drain pipe buried beneath the garage floor is larger is to jackhammer the concrete up to inspect the pipe.
Is there another way to determine the size of this pipe, such as by looking at city records or something?
This house was built in 1962, if that helps at all.
I am also wondering if anyone has a ballpark idea of what it will cost (in the Seattle area suburbs) to replace the drain pipe if it is not large enough. This would involve tearing up the asphalt driveway and the concrete floor of the garage, putting in the new pipe and then fixing the garage floor and driveway,
Thanks for any advice you can offer!
FA
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Drainpipes under floors are usually cast iron and usually 3-4" and reduced for drain stacks which don't require that large diameter.
The drain for your laundry certianly leads to what is known as the "house trap," it does not directly lead to the sewer on it's own.
If the pipe is visable in the garage, it probably meets the housetrap somewhere just inside or outside the garage wall. Look for a vent in the exterior garage wall, or stubbed up outside the garage.
snipped-for-privacy@aol.com
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All of the drain cleaning companies have cameras they can run down a drain they should be able to tell how big the pipe is pretty easily
Wayne

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That's a great idea, thanks Wayne!
(And thanks to HaHaHa for the information, too.)
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Check this out:
http://mrrooter.com/seattle/services/trenchless.aspx
Don't know the cost but it's got to be less than tearing up a garage floor and driveway.
-Frank
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(email: change out to in)

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wrote:

That is a great tip, thanks Frank!
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