This is how I added both growth room and protection from snow filled boots to KS2793, one of my favorite patterns.
1. Sew both in and out seams.
2. Sew the waterproof fabric facing seam.
3. Sew 1" casing along one edge of facing.
4. Sew the bottom edges of the legs to the other edge of the facing.
5. Turn the facing to the inside and topstitch around the leg 6" from the bottom.
6. Insert 1/2" elastic into casing.
With the snow guard part of the cuff inserted into a snug boot, no snow can reach the leg. The snow guard can blouse around the ankle when the cuff is turned up. When the cuff is turned down to add 3" to the leg length the snow guard will be only 1" shorter than the pant leg.
I used this technique on toddler overalls, but I don't see why it shouldn't work on any size pant for anyone who likes (or needs) to spend time in the snow. Might even prevent wet cuffs when puddle jumping.