How are reassessments of property tax calculated when adding sqft or rebuilding a home?

Let’s say you have a 1500 sqft home and want to add 1000 sqft. Your options are to add it to your existing home or to knock it down and rebuild a 2500 sqft home. How will the county reassess your property taxes in each scenario given prop13 and other factors?

Let’s say the home is worth $1.5M now and your property tax docs says $1M for land and 500k for improvements today. And let’s say you live in Santa Clara County, and the cost for the additional 1000sqft is $500k whereas a rebuild is $1.25M (costs are the same per sqft between addition and rebuild).