Mello-Roos tax is a special additional tax to theproperty tax to pay for new development for public use such as park, road, new school site to the new development areas. Often this kind of tax is short term and temporary. ( they might end up at the end of 5 years) You can negotiate with the builder to pay for this kind of tax as a buying incentive. Here is the website to give you more details in Mello-Roos tax. For better correct and correct answer you can contact the Santa Clara Assessor Office to ask.