Thank you for your question:
As the others pointed out the value of the new improvements will be added to the current assessed value of the property. For all practical purposes, in most cases, the added, assessed value of the improvements will be equal to the cost of the improvements.
However, in conversations that I have had with The Santa Clara County Assessor's Office, the County Assessor's Office is not limited to the actual cost of the improvements and The County Assessor's Office may choose to use another number, such as what The County Assessors's Office determines to be the actual value of those improvements, rather than the actual cost of the improvements.
This is particularly true in those cases where the homeowner did the work him or herself at a much lower cost than would have been charged by a General Contractor.
Fortunately, in my opinion, The Santa Clara County Assessor's Office is staffed by some very knowledgeable and very professional people. In my opinion, and experience, The Santa County Assessor's Office is one of the better County Assessor's Offices in The State of California.
Essentially, the additional assessed value will be equal to the cost of the improvements in most cases, however The Santa Clara County Assessor's Office says that they are not limited to that, and that they may come up with another number in some cases, if they determine that another number is more appropriate.
Fortunately, if you do not agree with that number, you can appeal the decision. If you make a reasonable case, and you have documented your case well, my experience with appeals to The Santa Clara County Assessor's office is that you will usually win an appeal, if your appeal is reasonable and well documented.
Charles Butterfield MBA
Real Estate Broker/REALTOR
Cell Phone: (408)509-6218
Email Address: firstname.lastname@example.org