DE 三十年没有重新估量房产，仅仅是猜测征税估价

http://rodelde.org/9-questions-about-property-reassessment-in-delaware/

The latest reassessment of property value was done in 1986. As of 2010, Delaware was one of nine states that did not have state provisions for when reassessments take place. Most states follow an annual to five-year schedule.

reassessment is to bring the valuation of properties up to market value for the purpose of tax.  

So yes, old homes have higher market value but low assessed value. But your new home's assessed value is the same as market value, so you pay more tax than older homes.