Is it a skirt? Is it a dress? No it's a Bum Cosy by Shireen Nadir designed to be worn with leggings, tights or jeggings to keep your bum/hip area warm you can buy the knitting pattern from various sites inc.
New Look Pattern 6447 Misses Dresses. Misses' dress with full or slim skirt, each in two lengths - above the knee or knee length. Shaped waistband can be made in contrasting fabric. Back v-neckline and zipper.