In March, a firm that has built some of Lincoln Park’s biggest homes posted a new high-water mark: a contract to build a $16-million house of up to 25,000 square feet on three lots on the 1800 block of Howe Street. BGD&C corporation, the custom designer/builder, did not return calls asking for details, but on the listing sheet it supplied to the Multiple Listing Service of Northern Illinois, it said that the site-83 feet wide by 125 feet deep-could accommodate a three-story house of more than 25,000 square feet. In April 2006, BGD&C listed a base price of $12 million for the property; the buyers-who used a land trust and so could not be identified-clearly had more lavish tastes, paying $4 million above the original price. Construction is under way.