Seattle is on the verge of making some controversial land-use changes that advocates say will make this increasingly expensive city more livable for people who aren’t wealthy. The city may soon allow taller buildings in the cores of many neighborhoods and ease restrictions on mother-in-law apartments and backyard cottages. But change is hard: Those moves have encountered legal challenges. Minneapolis this month took a much more dramatic step on density meant to ease its real-estate crunch and address its history of racial segregation: The City Council there voted to end single-family zoning altogether. Moving ahead, the Midwest city will allow...