When most people think about hiring an interior designer, they zero in on aesthetics: wall colors, window treatments, pillow patterns. But that's just scratching the surface of what a designer can add to a home. These pros go beyond cosmetic concerns to ensure that a space feels harmonious through and through, from its floor plan and architectural envelope to the last nailhead, tassel and tuft. Many states have a certification process for interior designers. Tiffany Eastman I