Tips on how to check if this guest house in ADU was built with permits?

Question

I am about to buy a house in Santa Clara, CA county that has an ADU. As of today ADU has guest house and an operational garage.

Seller is convinced that 990 sqft guest house in the ADU was built legally together with the main house under the same permit. Hence seller is counting guest house square footage in legal livable area.

However, I called county assessor and he told me that total living area for the property is only 4,180 sqft. This number exactly matches the main house and nothing else. Assessor also told me that property has 1,976 sqft garage. He said that they receive information from building department and permits while possible get seldom lost.

So, seller agent sent me this permit back from 1985, which I believe was issued at the time main house was built. He claims that this permit proves that 990 sqft guest house is permitted, because

1. "residence" in "residence+att garage" in his opinion refers to guest house and not to the main home.
2. seller claims that bathroom in ADU shows up on blueprints. And there would not be sole bathroom without guesthouse.

However, I am still unconvinced that guest house was introduced under this permit, because:

1. "residence" on this permit could actually still refer to the main home.
2. assessor does not have any information about this guest house at all.
3. it feels strange that guest house was introduced together with the main house but assessor did not know anything about living area beyond the 4,180 sqft.
4. if guest house is permitted then on this permit I expected to see number ~990 sq ft. OR
5. if I would subtract garage size (1,976 sqft as per assessor) from what I believe is the total ADU size (2,683 sqft), then I am left with only 707 sqft. So how could 990 sqft guest house be squeezed in a 707 sqft area?

How can I make sure that guest house is permitted? Does this permit in any way prove whether guest house was built with permit?

Update #1: Saw blueprints and it clearly shows that what they call as "Guest house" is actually labeled as "Work shop". If my understanding is correct, then Workshop does not count towards "legal livable area", but yet they still included it.

Answer 1

Very easy to resolve. Have seller put in writing that the ADU was legally permitted when built. He has put you on notice with "seller is 'convinced' that it was legally built." If you accept that statement, you are making the purchase "as is" based on what he believes, not factual evidence.

Only if he puts it in writing that the ADU is legally built would he bear responsibility for misleading you if you later have an issue with the "ADU".

Ways to validate if ADU was built with a permit.

Go to the planning department and request a copy of the blueprint. The permits would not have been issued without the drawings. Also seller, claims the ADU is on the plan, so ask the seller for the plan showing the ADU.

Edited based on additional information:

The "guest house" was originally built as a "workshop". Later converted into livable space, “ADU”, conversion most likely done without a permit. So your underlying questions has been answer.

Per your added comment, “If my understanding is correct, then Workshop does not count towards "legal livable area", but yet they still included it.”.

I do not believe there is a California law that regulates how a home's square footage must be calculated. Livable space needs to meet certain requirements, i.e., heat, ceiling height, etc.”, The bottom line is you are purchasing the “ADU” feature “As Is”. Whether you believe the seller should include it as part of the "livable" space is your opinion. Even if the seller removes the footage as part of the livable space, the price would still be the same.

Answer 2

This seems to be more of a legal problem than a "DIY" problem. My advice would be to have your offer to purchase contingent on seller providing clear and unassailable proof that the converted garage is fully permitted - whether from historic records or by going thru a new permitting request.
And, as comments say, get yourself an attorney experienced in real estate law to make sure you do not get financially committed to anything unapproved.

My opinion is that any real estate agent who failed to get this cleaned up before listing the property in the first place should lose his licence (or at the very least specifically state in the listing that the out- building in question may not be legal to occupy).