VBP,
Some people hire a civil engineer to actually map a house and see if that particular home is high enough so that even if art of the property may be in the flood zone the house is high enough to be taken out of the flood zone. This usually occurs further away from the bay rather than closer. It cost between $300-$400 dollars, but if your house is high enough it means you do not have to pay for flood insurance and can probably get more leeway from the city for an expansion.
Marcy

Vbp

It is not a matter of how bad a flood will be - it is more about what the City regulations are.

I believe it was around 1990 when San Francisquito Creek flooded and there was about 3 feet of water on 101 near Embarcadero. So it does happen. I believe very few homes were damaged but in a townhouse development where I used to live the water came up to the front door threshold but did not go inside the townhouse.

Thanks Arn & Marcy. It's hard to believe it's almost not possible to add on to the houses in flood zone. That is to add a second floor without tearing down the whole house. I have never heard about flood in the news.. Will it really be that bad?

Vbp,
The flood zone in Palo Alto is a complicated issue. You can build small additions without having to raise the entire house but as Arn said you need to talk to the city about the specific requirements. If you add mor than the allowed amount then not only do you have to raise the addition above flood level, but you also have to raise the rest of the house. When this happens it is often easier to just build a whole new house above the flood level. The area of Palo Alto west of 101 down to about Ross and south of Oregon is subject to bay water flooding. This is a very rare occurance, but when it does it is more severe than just creek flooding. North of Oregon and west of 101 is subject to San Francisquito Creek flooding. this happens much more frequently, but is not as severe. The city has a we site where you can type in an address and it will tell you if the house is in a flood zone.
Marcy

PS
You will also need to buy flood insurance due ot lender requirements - figure this will cost you about $1000 per year. Again contact your insurance agent for a more precise quote.

Vbp:
Generally the flood zone in Palo Alto is located in the areas immediately to the "west" of Highway 101.
Maps are easily available to determine specific location.
You are correct any additions or new construction need to be elevated 8 feet above the flood plain.
This applies whether you build a new house or add onto an existing house.
So it doesn't matter whether you "rebuild" and do "addition" - no difference.
You will not be able to "get around" the flood zone rules in any case.
Essentially any "new" building whether an "addition" or "all new" needs to be built above the flood zone.
I suggest you contact the City directly - building, planning, public works departments and they will be able to give you a detailed answer to your question. The City also probably has a handbook that will answer all your questions regarding residential construction. This info might also be available on their web site.
Good luck.