I appealed my assessment in 2012 and was successful–got it lowered by $40K. But I had to go all the way to the 2nd level administrative hearing. The 1st level call with the inspector was useless. Basically he just said he disagreed with my argument, and that was that.
My only advice is to make sure you are well armed with the facts, and make sure they all come from DC’s own assessment database. When my inspector, who was at the hearing, responded to my spreadsheets by saying, “…how can we trust your figures?” I responded, “These are DC’s stats…not mine. To the extent that there are errors, they are *your* errors.”
Good luck. I know it sounds like a lot of work, but I found the experience very satisfying. Will it have a future impact on your assessment? (In other words…will DC retaliate?) Not that I’ve seen.