county tax calculation in magento for california

Hi, We need to setup a custom tax extension for the US State of California, which calculates taxes based on city and county (NOT zip code). This extension is required for a magento based shopping cart project. California taxes will be calculated by a custom extension that you will build, that works as follows: a. If a user selects "California" as state for SHIPPING address, then a new drop down will appear that says, "County." Based on that selection, taxes will be calculated for that order based on spreadsheet data imported. b. There will be a backend Magento tool that client can upload a CSV file that contains the latest tax information.