Ah ha... there, my friend, is the trick of New Castle County. Assessments on new construction are based solely on smoke and mirrors.
First, there is no calculation that
you can use, therefore you cannot estimate what your assessment should be with any real accuracy. Theoretically, the assessment of new construction in the county is what the value of that construction "would have" been if it were there in its current state and condition on July 1, 1983. So, the County is left with assessors who are assigning a market value of the improvements 23 years prior to their construction. Are they right? usually not. I myself have appealed the assesments of 5 or 6 highrise office buildings in the city, and in each case their assessment was reduced substantially. The reason is two fold. They don't have access to the records prepared by the appraisal group who performed the mass re-val in 1985 for the 1983 assessment figure, and 2. No one who works in the assessment office has the knowledge or qualifications to value a significant property. Particularly 23 years retrospective.
However, on a relatively small property, they've pretty much got you as the cost of formally appealing typically outweighs the potential savings. Appeals must be submitted by March 1st, and are not retroactive.