How to estimate 'taxable value' for home addition

[DA Nguyen]

Hi BP,

I know that CA does blended assessment but what I'm trying to understand is how to estimate the 'taxable value' of the new addition. 

For example, let's say:

- the house is worth 2,000 sqft @ $500 / sqft = $1,000,000

- You add another 2,000sqft @ $200 / sqft cost = $400,000 total cost. 

- similar 4,000 sqft houses are sold for $1,800,000.

So would the taxable value of the new addition be $400,000 or $800,000? I think it's the former since land is already factored into the existing assessment, but I just want to confirm.