@BldrDad, are you saying he has NO equity in these properties? From googling, we know the following:
He has two rowhouses in Park Slope
One rowhouse has doubled in value from $700,000 in 2006 to $1.4 million in 2014. (if the two together are worth $2.8 million then, the other one is also worth $1.4 million).
One news article said he had refinanced one rowhouse to a >$500,000 mortgage (http://www.nydailynews.com/new-york/mayor-de-blasio-assets-meager-retirement-fund-report-article-1.1819224)
With a >$500,000 mortgage, he has anywhere between $750K to $1 million equity in one rowhouse.