Jim Carlson knows his dream house is close at hand, if he can land the right deal.It’s not a down payment that he’s negotiating, but rather the price of his land to sell. He bets his P-shaped 7 acres, with a long driveway that widens around an unfinished home and an old horse training arena, is worth about $500,000 today.Someday soon the land could triple in value. That is, if Clark County decides to lift a zoning overlay that has rendered the land in his neighborhood practically undevelopable.“If I can’t sell it as developable property, what does 7 acres and an old farmhouse sell for in that area?” Carlson asked. So he said he plans to wait. “We’re not going to leave $1 million sitting on the table.”Commercial interests are eyeing the Fairgrounds neighborhood, but the land is placed under what is called “urban holding.” The overlay curtails development in select places so buzzing projects — like business parks and housing subdivisions that have cropped up around Clark County — don’t overwhelm older roads and infrastructure.Good news arrived in 2015 for landowners like Carlson, private developers and economic development officials, when the state awarded $50 million to rebuild the Interstate 5 interchange near 179th Street.